Delay Line

Trigger

Amp Display

VertAmp

HorizAmp

Copyright © 2000 Tektronix, Inc. All rights reserved.

XYZs of OscilloscopesPrimer

Analog Oscilloscope

AcquisitionMemory

Amp DisplayA/D DeMux µP DisplayMemory

Digital Storage Oscilloscope

AcquisitionRasterizer

Amp DisplayA/D

µP

DisplayMemory

DigitalPhosphor

DPX

Digital Phosphor Oscilloscope

i

X YZs of Osc i lloscopes

The oscilloscope is an essential tool if you plan to design or repair electronicequipment. It lets you “see” electrical signals.

Energy, vibrating particles, and other invisible forces are everywhere in ourphysical universe. Sensors can convert these forces into electrical signals thatyou can observe and study with an oscilloscope. Oscilloscopes let you “see”events that occur in a split second.

Why Read This Book? If you are a scientist, engineer, technician, or electronics hobbyist, you shouldknow how to use an oscilloscope. The concepts presented here provide youwith a good starting point.

If you are using an oscilloscope for the first time, read this book to get a solidunderstanding of oscilloscope basics. Then, read the manual provided withyour oscilloscope to learn specific information about how to use it in yourwork. After reading this book, you will be able to: • Describe how oscilloscopes work • Describe the difference between analog, digital storage, and digital phosphor

oscilloscopes• Describe electrical waveform types • Understand basic oscilloscope controls • Take simple measurements

If you come across an unfamiliar term in this book, check the glossary in theback for a definition.

This book serves as a useful classroom aid. It includes vocabulary and multiplechoice written exercises on oscilloscope theory and controls. You do not needany mathematical or electronics knowledge. This book emphasizes teaching youabout oscilloscopes – how they work, how to choose the right one, and and howto make it work for you.

Introduction

ii

Contents

Introduction· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · iWhy Read This Book? · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · i

The Oscilloscope · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 1What Can You Do With an Oscilloscope? · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 2Analog, Digital Storage, and Digital Phosphor Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 2How Oscilloscopes Work · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 3

Analog Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 3Digital Storage Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 4Digital Phosphor Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 4Sampling Methods · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 5Real-Time Sampling with Interpolation · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 5

Oscilloscope Terminology · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7Measurement Terms · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7Types of Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7

Sine Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 7Square and Rectangular Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8Sawtooth and Triangle Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8Step and Pulse Shape · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8Complex Waves · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 8

Waveform Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9Frequency and Period · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9Voltage · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9Phase · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9

Performance Terms · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9Bandwidth · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9Rise Time · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9Effective Bits · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9Frequency Response · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10Vertical Sensitivity · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10Sweep Speed · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10Gain Accuracy · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10Time Base or Horizontal Accuracy · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10Sample Rate · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10ADC Resolution (or Vertical Resolution) · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10Record Length · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10Waveform Capture Rate · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 10

Setting Up · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11Grounding · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11

Ground the Oscilloscope · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11Ground Yourself · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11Setting the Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 11

Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 12“Intelligent” Probe Interfaces · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 12Using Passive Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 12Using Active Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 13Using Current Probes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 13Where to Clip the Ground Clip · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 13

Compensating the Probe · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 14

iii

XYZs of Oscilloscopes

iii

iv

The Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15Display Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15Vertical Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15

Position and Volts per Division · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15Input Coupling · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 15Bandwidth Limit · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 16Alternate and Chop Display · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 16Math Operations · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17

Horizontal Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17Position and Seconds per Division · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17Time Base Selections · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17Trigger Position · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 17Zoom · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18XY Mode · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18The Z Axis · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18XYZ Mode · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18

Trigger Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 18Trigger Level and Slope · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 19Trigger Sources · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 19Trigger Modes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 19Trigger Coupling · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 20Trigger Holdoff · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 20Digitizing Oscilloscope Triggers · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 20

Acquisition Controls for Digitizing Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 21Acquisition Modes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 21Stopping and Starting the Acquisition System · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 22Sampling Methods · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 22

Other Controls · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 22

Measurement Techniques · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 23The Display · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 23Voltage Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 23Time and Frequency Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 24Pulse and Rise Time Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 24Phase Shift Measurements · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 25Waveform Measurements with Digitizing Oscilloscopes · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 26What’s Next? · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 26

Written Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 27Part I Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 28Part II Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 30Answers to Written Exercises · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 34

Glossary · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 35

iv

What is an oscilloscope, what can you do with it,and how does it work? This section answers thesefundamental questions.

The oscilloscope is basically a graph-displayingdevice – it draws a graph of an electrical signal (seeFigure 1). In most applications the graph shows howsignals change over time: the vertical (Y) axis repre-sents voltage and the horizontal (X) axis representstime. The intensity or brightness of the display issometimes called the Z axis. This simple graph cantell you many things about a signal. Here are a few: • You can determine the time and voltage values of

a signal• You can calculate the frequency of an oscillating

signal• You can see the “moving parts” of a circuit repre-

sented by the signal

• You can tell how often a particular portion of thesignal is occurring relative to other portions

• You can tell if a malfunctioning component isdistorting the signal

• You can find out how much of a signal is directcurrent (DC) or alternating current (AC)

• You can tell how much of the signal is noise andwhether the noise is changing with time

An oscilloscope’s front panel includes a displayscreen and the knobs, buttons, switches, and indica-tors used to control signal acquisition and display.Front-panel controls normally are divided intoVertical, Horizontal, and Trigger sections, and inaddition, there are display controls and inputconnectors. See if you can locate these front-panelsections in Figures 2 and 3 as well as on youroscilloscope.

1

Figure 2. The TAS 465 Analog Oscilloscope front panel.

Figure 1. X, Y, and Z Components of a displayed waveform.

The Oscilloscope

What Can You Do With anOscilloscope? Oscilloscopes are used by everyone from televisionrepair technicians to physicists. They are indispens-able for anyone designing or repairing electronicequipment.

The usefulness of an oscilloscope is not limited tothe world of electronics. With the proper transducer,an oscilloscope can measure all kinds of phenomena.A transducer is a device that creates an electricalsignal in response to physical stimuli, such assound, mechanical stress, pressure, light, or heat. Forexample, a microphone is a transducer that convertssound to an electrical signal.

An automotive engineer uses an oscilloscope tomeasure engine vibrations. A medical researcheruses an oscilloscope to measure brain waves. Thepossibilities are endless.

Analog, Digital Storage, and DigitalPhosphor Oscilloscopes Electronic equipment can be divided into two types:analog and digital. Analog equipment works withcontinuously variable voltages, while digital equip-ment works with discrete binary numbers that mayrepresent voltage samples. For example, a conven-

tional phonograph is an analog device, while acompact disc player is a digital device.

Oscilloscopes also come in analog and digitizingtypes (see Figure 5). Fundamentally an analog oscil-loscope works by applying the measured signalvoltage directly to an electron beam moving acrossthe oscilloscope screen (usually a cathode-ray tube,CRT). The back side of the screen is treated with acoating that phosphoresces wherever the electronbeam hits it. The signal voltage deflects the beam upand down proportionally, tracing the waveform onthe screen. The more frequently the beam hits aparticular screen location, the more brightly it glows.This gives an immediate picture of the waveform.

The range of frequencies an analog scope can displayis limited by the CRT. At very low frequencies, thesignal appears as a bright, slow-moving dot that’sdifficult to distinguish as a waveform. At highfrequencies, the CRT’s “writing speed” defines thelimit. When the signal frequency exceeds the CRT’swriting speed, the display becomes too dim to see.The fastest analog scopes can display frequencies upto about 1 GHz.

In contrast, a digitizing oscilloscope uses an analog-to-digital converter (ADC) to convert the voltagebeing measured into digital information. The digi-tizing scope acquires the waveform as a series of

2

Figure 3. The TDS 784D Digital Phosphor Oscilloscope front panel.

Figure 4. An example of scientific data gathered by an oscilloscope.

samples. It stores these samples until it accumulatesenough samples to describe a waveform, and then re-assembles the waveform for viewing on the screen.The conventional digitizing scope is known as aDSO – Digital Storage Oscilloscope. Its displaydoesn’t rely on luminous phosphor; instead, it uses araster-type screen.

Recently a third major oscilloscope architecture hasemerged: the Digital Phosphor Oscilloscope (DPO).The DPO is a digitizing scope that faithfullyemulates the best display attributes of the analogscope and provides the benefits of digital acquisitionand processing as well. Like the DSO, the DPO uses araster screen. But instead of a phosphor, it employsspecial parallel processing circuitry that delivers acrisp, intensity-graded trace.

For both DSOs and DPOs, the digital approachmeans that the scope can display any frequencywithin its range with equal stability, brightness, andclarity. The digitizing oscilloscope’s frequency rangeis determined by its sample rate, assuming that itsprobes and vertical sections are adequate for thetask.

For many applications either an analog or digitizingoscilloscope will do. However, each type has uniquecharacteristics that may make it more or less suitablefor specific tasks.

People often prefer analog oscilloscopes when it’simportant to display rapidly varying signals in “realtime” (as they occur). The analog scope’s chemicalphosphor-based display has a characteristic knownas “intensity grading” which makes the trace brighterwherever the signal features occur most often. Thismakes it easy to distinguish signal details just bylooking at the trace’s intensity levels.

Digital storage oscilloscopes allow you to captureand view events that may happen only once – “tran-sient” events. Because the waveform information isin digital form (a series of stored binary values), itcan be analyzed, archived, printed, and otherwiseprocessed within the scope itself or by an externalcomputer. The waveform doesn’t need to be contin-uous; even when the signal disappears, it can bedisplayed. However, DSOs have no real-time inten-sity grading; therefore they cannot express varyinglevels of intensity in the live signal.

The Digital Phosphor Oscilloscope breaks down thebarrier between analog and digitizing scope tech-nologies. It’s equally suitable for viewing highfrequencies or low, repetitive waveforms, transients,and signal variations in real time. Among digitizingscopes, only the DPO provides the Z (intensity) axisthat’s missing from conventional DSOs.

How Oscilloscopes Work To better understand the oscilloscope’s many uses,you need to know a little more about how oscillo-scopes display a signal. Although analog oscillo-scopes work somewhat differently than digitizingoscilloscopes, some of the internal systems aresimilar. Analog oscilloscopes are simpler in conceptand are described first, followed by a description ofdigitizing oscilloscopes.

Analog Oscilloscopes When you connect an oscilloscope probe to a circuit,the voltage signal travels through the probe to thevertical system of the oscilloscope. Figure 6 is asimple block diagram that shows how an analogoscilloscope displays a measured signal.

Depending on how you set the vertical scale(volts/div control), an attenuator reduces the signalvoltage or an amplifier increases the signal voltage.

Next, the signal travels directly to the vertical deflec-tion plates of the cathode ray tube (CRT). Voltageapplied to these deflection plates causes a glowingdot to move. (An electron beam hitting the phosphorinside the CRT creates the glowing dot.) A positivevoltage causes the dot to move up while a negativevoltage causes the dot to move down.

The signal also travels to the trigger system to start ortrigger a “horizontal sweep.” Horizontal sweep is aterm referring to the action of the horizontal systemcausing the glowing dot to move across the screen.Triggering the horizontal system causes the hori-zontal time base to move the glowing dot across thescreen from left to right within a specific timeinterval. Many sweeps in rapid sequence cause themovement of the glowing dot to blend into a solidline. At higher speeds, the dot may sweep across thescreen up to 500,000 times each second.

Together, the horizontal sweeping action and thevertical deflection action traces a graph of the signal

3

Figure 5. Analog and digitizing oscilloscopes display waveforms.

Figure 6. Analog oscilloscope block diagram.

Delay Line

Trigger

Amp Display

VertAmp

HorizAmp

on the screen. The trigger is necessary to stabilize arepeating signal. It ensures that the sweep begins atthe same point of a repeating signal, resulting in aclear picture as shown in Figure 7.

In summary, when using an analog oscilloscope (orany type of oscilloscope), you need to adjust threebasic settings to accommodate an incoming signal: • The attenuation or amplification of the signal.

Use the volts/div control to adjust the amplitudeof the signal to the desired measurement range

• The time base. Use the sec/div control to set theamount of time per division represented horizon-tally across the screen

• The triggering of the oscilloscope. Use the triggerlevel to stabilize a repeating signal, or for trig-gering on a single event

In addition, analog scopes have focus and intensitycontrols that can be adjusted to create a sharp,legible display.

Digital Storage Oscilloscopes Some of the systems that make up DSOs are the sameas those in analog oscilloscopes; however, digitizingoscilloscopes contain additional data processingsystems (see Figure 8). With the added systems, thedigitizing oscilloscope collects data for the entirewaveform and then displays it.

The first (input) stage of a DSO is a vertical amplifier,just like the analog oscilloscope’s. Vertical attenua-tion controls allow you to adjust the amplitude rangeof this stage.

Next, the analog-to-digital converter (ADC) in theacquisition system samples the signal at discretepoints in time and converts the signal’s voltage atthese points to digital values called sample points.The horizontal system’s sample clock determineshow often the ADC takes a sample. The rate at which

the clock “ticks” is called the sample rate and isexpressed in samples per second.

The sample points from the ADC are stored inmemory as waveform points. More than one samplepoint may make up one waveform point.

Together, the waveform points make up one wave-form record. The number of waveform points used tomake a waveform record is called the record length.The trigger system determines the start and stoppoints of the record. The display receives theserecord points after being stored in memory.

Depending on the capabilities of your oscilloscope,additional processing of the sample points may takeplace, enhancing the display. Pretrigger may beavailable, allowing you to see events before thetrigger point.

Note that the DSO’s signal path includes a micro-processor. The measured signal passes through thisdevice on its way to the display. In addition toprocessing the signal, the microprocessor coordi-nates display activities, manages the front-panelcontrols, and more. This is known as a “serialprocessing” architecture.

Digital Phosphor Oscilloscopes The Digital Phosphor Oscilloscope (DPO) offers anew approach to oscilloscope architecture. Like theanalog oscilloscope, its first stage is a vertical ampli-fier; like the DSO, its second stage is an ADC. Butafter the analog-to-digital conversion, the DPO looksquite different from the DSO. It has special featuresdesigned to recreate the intensity grading of ananalog CRT.

Rather than relying on a chemical phosphor as ananalog scope does, the DPO has a purely electronicDigital Phosphor that’s actually a continuouslyupdated data base. This data base has a separate

4

Figure 7. Triggering stabilizes a repeating waveform.

Figure 8. Digital storage oscilloscope block diagram – “serial processing.”

AcquisitionMemory

Amp DisplayA/D DeMux µP DisplayMemory

“cell” of information for every single pixel in thescope’s display. Each time a waveform is captured(in other words, every time the scope triggers), it ismapped into the Digital Phosphor database’s cells.Each cell representing a screen location that istouched by the waveform gets reinforced with inten-sity information. Others do not. Thus intensity infor-mation builds up in cells where the waveform passesmost often.

When the Digital Phosphor database is fed to theoscilloscope’s display, the display reveals intensifiedwaveform areas, in proportion to the signal’sfrequency of occurrence at each point – much likethe intensity grading characteristics of an analogoscilloscope (unlike an analog scope, though, theDPO allows the varying levels to be expressed incontrasting colors if you wish). With a DPO, it’s easyto see the difference between a waveform that occurson almost every trigger and one that occurs, say,every 100th trigger.

Importantly, the DPO uses a parallel processingarchitecture to achieve all this manipulation withoutslowing down the whole acquisition process. Likethe DSO, the DPO uses a microprocessor for displaymanagement, measurement automation, andanalysis. But the DPO’s microprocessor is outside theacquisition/display signal path (see Figure 9), whereit doesn’t affect the acquisition speed.

Sampling Methods Digitizing oscilloscopes – DSO or DPO – can useeither real-time, interpolated real-time, or equiva-lent-time sampling to collect sample points. Real-time sampling is ideal for signals whose frequency isless than half the scope’s maximum sample rate.Here, the oscilloscope can acquire more than enoughpoints in one “sweep” of the waveform to constructan accurate picture (see Figure 10). Note that real-time sampling is the only way to capture single-shottransient signals with a digitizing scope.

When measuring high-frequency signals, the oscillo-scope may not be able to collect enough samples in

one sweep. There are two solutions for accuratelyacquiring signals whose frequency exceeds half theoscilloscope’s sample rate: • Collect a few sample points of the signal in a

single pass (in real-time mode) and use interpola-tion to fill in the gaps. Interpolation is aprocessing technique to estimate what the wave-form looks like based on a few points

• Build a picture of the waveform by acquiringsamples from successive cycles of the waveform,assuming the signal repeats itself (equivalent-timesampling mode)

Real-Time Sampling with Interpolation Digitizing oscilloscopes take discrete samples of thesignal which can be displayed. However, it can bedifficult to visualize the signal represented as dots,especially because there may be only a few dotsrepresenting high-frequency portions of the signal.To aid in the visualization of signals, digitizing oscil-loscopes typically have interpolation display modes.

In simple terms, interpolation “connects the dots.”Using this process, a signal that is sampled only afew times in each cycle can be accurately displayed.However, for accurate representation of the signal,the sample rate should be at least four times thebandwidth of the signal.

Linear interpolation connects sample points withstraight lines. This approach is limited to recon-

5

Figure 9. Digital phosphor oscilloscope block diagram – “Parallel Processing.”

AcquisitionRasterizer

Amp DisplayA/D

µP

DisplayMemory

DigitalPhosphor

DPX

Figure 10. Real-time sampling.

structing straight-edged signals such as squarewaves.

The more versatile sin x/x interpolation connectssample points with curves (see Figure 11). Sin x/xinterpolation is a mathematical process in whichpoints are calculated to fill in the time between thereal samples.

This form of interpolation lends itself to curved andirregular signal shapes, which are far more commonin the real world than pure square waves and pulses.Consequently, sin x /x interpolation is the preferredmethod for most applications.

Some digitizing oscilloscopes can use equivalent-time sampling to capture very fast repeating signals.Equivalent-time sampling constructs a picture of arepetitive signal by capturing a little bit of informa-tion from each repetition (see Figure 12). The wave-form slowly builds up like a string of lights going onone-by-one. With sequential sampling, the pointsappear from left to right in sequence; with randomsampling, the points appear randomly along thewaveform.

6

Figure 12. Equivalent-time sampling.

Figure 11. Linear and sine interpolation.

Learning a new skill often involves learning a newvocabulary. This idea holds true for learning how touse an oscilloscope. This section describes someuseful measurement and oscilloscope performanceterms.

Measurement Terms The generic term for a pattern that repeats over timeis a wave – sound waves, brain waves, ocean waves,and voltage waves are all repeating patterns. Anoscilloscope measures voltage waves. One cycle of awave is the portion of the wave that repeats. A wave-form is a graphic representation of a wave. A voltagewaveform shows time on the horizontal axis andvoltage on the vertical axis.

Waveform shapes tell you a great deal about a signal.Any time you see a change in the height of the wave-form, you know the voltage has changed. Any timethere’s a flat horizontal line, you know that there’sno change for that length of time. Straight diagonallines mean a linear change – rise or fall of voltage ata steady rate. Sharp angles on a waveform meansudden change. Figure 13 shows some common

waveforms and Figure 14 shows some commonsources of waveforms.

Types of Waves You can classify most waves into these types: • Sine waves• Square and rectangular waves • Triangle and sawtooth waves • Step and pulse shapes • Complex waves

Sine Waves The sine wave is the fundamental wave shape forseveral reasons. It has harmonious mathematicalproperties – it’s the same sine shape you may havestudied in high school trigonometry class. Thepower line voltage at your wall outlet varies as a sinewave. Test signals produced by the oscillator circuitof a signal generator are often sine waves. Most ACpower sources produce sine waves. (AC stands foralternating current, although the voltage alternatestoo. DC stands for direct current, which means a

7

Oscilloscope Terminology

Figure 13. Common waveforms. Figure 14. Sources of common waveforms.

steady current and voltage, such as a batteryproduces.)

The damped sine wave is a special case you may seein a circuit that oscillates but winds down over time.

Figure 15 shows examples of sine and damped sinewaves.

Square and Rectangular Waves The square wave is another common wave shape.Basically, a square wave is a voltage that turns onand off (or goes high and low) at regular intervals. It’sa standard wave for testing amplifiers – good ampli-fiers increase the amplitude of a square wave withminimum distortion. Television, radio, andcomputer circuitry often use square waves for timingsignals.

The rectangular wave is like the square wave exceptthat the high and low time intervals are not of equallength. It is particularly important when analyzingdigital circuitry.

Figure 16 shows examples of square and rectangularwaves.

Sawtooth and Triangle Waves Sawtooth and triangle waves result from circuitsdesigned to control voltages linearly, such as thehorizontal sweep of an analog oscilloscope or theraster scan of a television. The transitions betweenvoltage levels of these waves change at a constantrate. These transitions are called ramps.

Figure 17 shows examples of sawtooth and trianglewaves.

Step and Pulse Shape Signals such as steps and pulses that only occuronce are called single-shot or transient signals. Thestep indicates a sudden change in voltage, like whatyou would see if you turned on a power switch. Thepulse indicates what you would see if you turned apower switch on and then off again. It might repre-sent one bit of information traveling through acomputer circuit or it might be a glitch (a defect) in acircuit.

A collection of pulses travelling together creates apulse train. Digital components in a computercommunicate with each other using pulses. Pulsesare also common in x-ray and communicationsequipment.

Figure 18 shows examples of step and pulse shapesand a pulse train.

Complex Waves Some waveforms combine the characteristics ofsines, squares, steps, and pulses to produce a wave-shape that challenges many oscilloscopes. The signalinformation may be embedded in the form of ampli-tude, phase, and/or frequency variations. Forexample, look at Figure 19 – although it’s an ordi-nary composite video signal, it is made up of manycycles of higher-frequency waveforms embedded in alower-frequency “envelope.” In this example it’susually most important to understand the relativelevels and timing relationships of the steps. What’sneeded to view this signal is an oscilloscope thatcaptures the low-frequency envelope and blends inthe higher-frequency waves in an intensity-gradedfashion so you can see their overall level.

Analog instruments and DPOs are most suited toviewing complex waves such as video signals. Theirdisplays provide the necessary intensity grading.Often, the frequency-of-occurrence information thattheir displays express is essential to understandingwhat the waveform is really doing.

8

Figure 15. Sine and damped sine waves.

Figure 16. Square and rectangular waves.

Figure 17. Sawtooth and triangle waves.

Figure 18. Step, pulse, and pulse train shapes.

Figure 19. Complex wave (NTSC composite video signal).

Waveform Measurements You use many terms to describe the types ofmeasurements that you take with your oscilloscope.This section describes some of the most commonmeasurements and terms.

Frequency and Period If a signal repeats, it has a frequency. The frequencyis measured in Hertz (Hz) and equals the number oftimes the signal repeats itself in one second (thecycles per second). A repeating signal also has aperiod – this is the amount of time it takes the signalto complete one cycle. Period and frequency are reci-procals of each other, so that 1/period equals thefrequency and 1/frequency equals the period. So, forexample, the sine wave in Figure 20 has a frequencyof 3 Hz and a period of 1/3 second.

Voltage Voltage is the amount of electric potential (a kind ofsignal strength) between two points in a circuit.Usually one of these points is ground (zero volts) butnot always – you may want to measure the voltagefrom the maximum peak to the minimum peak of awaveform, referred to as the peak-to-peak voltage.The word amplitude commonly refers to themaximum voltage of a signal measured from groundor zero volts. The waveform shown in Figure 21 hasan amplitude of one volt and a peak-to-peak voltageof two volts.

Phase Phase is best explained by looking at a sine wave.The voltage level of sine waves is based on circularmotion, and a circle has 360 degrees (°). One cycle ofa sine wave has 360°, as shown in Figure 21. Usingdegrees, you can refer to the phase angle of a sine

wave when you want to describe how much of theperiod has elapsed.

Phase shift describes the difference in timingbetween two otherwise similar signals. In Figure 22,the waveform labeled “current” is said to be 90° outof phase with the waveform labeled “voltage,” sincethe waves reach similar points in their cycles exactly1/4 of a cycle apart (360°/4 = 90°). Phase shifts arecommon in electronics.

Performance Terms The terms described in this section may come up inyour discussions about oscilloscope performance.Understanding these terms will help you evaluateand compare your oscilloscope with other models.

Bandwidth The bandwidth specification tells you the frequencyrange the oscilloscope accurately measures.

As signal frequency increases, the capability of theoscilloscope to accurately respond decreases. Byconvention, the bandwidth tells you the frequency atwhich the displayed signal reduces to 70.7% of theapplied sine wave signal. (This 70.7% point isreferred to as the “–3 dB point” – a term based on alogarithmic scale.)

Rise Time Rise time is another way of describing the usefulfrequency range of an oscilloscope. Rise time may bea more appropriate performance consideration whenyou expect to measure pulses and steps. An oscillo-scope cannot accurately display pulses with risetimes faster than the specified rise time of the oscil-loscope.

Effective Bits Effective bits is a measure of a digitizing oscillo-scope’s ability to accurately reconstruct a signal byconsidering the quality of the oscilloscope’s ADCand amplifiers. This measurement compares theoscilloscope’s actual error to that of an ideal digi-tizer. Because the actual errors include noise anddistortion, the frequency and amplitude of the signalas well as the bandwidth of the instrument must bespecified.

9

Figure 21. Sine wave degrees. Figure 22. Phase shift.

Figure 20. Frequency and period.

Frequency Response Bandwidth alone is not enough to ensure that anoscilloscope can accurately capture a high frequencysignal. The goal of oscilloscope design is to haveMaximally Flat Envelope Delay (MFED). Afrequency response of this type has excellent pulsefidelity with minimum overshoot and ringing. Sincea digitizing oscilloscope is composed of real ampli-fiers, attenuators, ADCs, interconnect and relays, theMFED response is a goal which can only beapproached. Pulse fidelity varies considerably withmodel and manufacturer.

Vertical Sensitivity The vertical sensitivity indicates how much thevertical amplifier can amplify a weak signal. Verticalsensitivity is usually given in millivolts (mV) perdivision. The smallest voltage a general purposeoscilloscope can detect is typically about 1 mV pervertical screen division.

Sweep Speed For analog oscilloscopes, this specification indicateshow fast the trace can sweep across the screen,allowing you to see fine details. The fastest sweepspeed of an oscilloscope is usually given in nanosec-onds/div.

Gain Accuracy The gain accuracy indicates how accurately thevertical system attenuates or amplifies a signal. Thisis usually listed as a percentage error.

Time Base or Horizontal Accuracy The time base or horizontal accuracy indicates howaccurately the horizontal system displays the timingof a signal. This is usually listed as a percentageerror.

Sample Rate On digitizing oscilloscopes, the sample rate indicateshow many samples per second the ADC (and there-fore the oscilloscope) can acquire. Maximum samplerates are usually given in megasamples per second(MS/s). The faster the oscilloscope can sample, themore accurately it can represent fine details in a fastsignal. The minimum sample rate may also beimportant if you need to look at slowly changing

signals over long periods of time. Typically, thesample rate changes with changes made to thevertical sensitivity control to maintain a constantnumber of waveform points in the waveform record.

ADC Resolution (or Vertical Resolution) The resolution, in bits, of the ADC (and therefore thedigitizing oscilloscope) indicates how precisely itcan turn input voltages into digital values.Calculation techniques can improve the effectiveresolution.

Record Length The record length of a digitizing oscilloscope indi-cates how many waveform points the oscilloscope isable to acquire for one waveform record. Some digi-tizing oscilloscopes let you adjust the record length.The maximum record length depends on the amountof memory in your oscilloscope and its ability tocombine memory length from unused channels.Since the oscilloscope can only store a finite numberof waveform points, there is a trade-off betweenrecord detail and record length. You can acquireeither a detailed picture of a signal for a short periodof time (the oscilloscope “fills up” on waveformpoints quickly) or a less detailed picture for a longerperiod of time. Some oscilloscopes let you add morememory to increase the record length for specialapplications.

Waveform Capture Rate Waveform capture rate is the rate at which an oscil-loscope triggers, acquires, and displays waveforms.On DSOs, the rate is a few hundred times per secondat the most, due to their serial processing architec-ture. All in all, most DSOs sample about 1% of thetotal time the signal is available to them. The limita-tion of this approach is that signal activity continueseven though the oscilloscope isn’t sampling veryoften. And an important waveform aberration mightoccur during that lapse. A new digitizing oscillo-scope architecture, the DPO, has emerged to solvethis problem. On a DPO, signal acquisition isrepeated hundreds of thousands of times per second– as fast as an analog oscilloscope. The DPO’sextremely high waveform capture rate (as well astheir digital phosphor technology) makes it possibleto view rare, erratic signal events.

10

This section briefly describes how to set up and startusing an oscilloscope – specifically, how to groundthe oscilloscope, set the controls in standard posi-tions, and compensate the probe.

Proper grounding is an important step when settingup to take measurements or work on a circuit.Properly grounding the oscilloscope protects youfrom a hazardous shock and grounding yourselfprotects your circuits from damage.

Grounding

Ground the Oscilloscope Grounding the oscilloscope is necessary for safety. Ifa high voltage contacts the case of an ungroundedoscilloscope, any part of the case including knobsthat appear insulated, can give you a shock.However, with a properly grounded oscilloscope, thecurrent travels through the grounding path to earthground rather than through you to earth ground.

To ground the oscilloscope means to connect it to anelectrically neutral reference point (such as earthground). Ground your oscilloscope by plugging itsthree-pronged power cord into an outlet grounded toearth ground.

Grounding is also necessary for taking accuratemeasurements with your oscilloscope. The oscillo-scope needs to share the same ground as any circuitsyou are testing.

Some oscilloscopes do not require the separateconnection to earth ground. These oscilloscopeshave insulated cases and controls, which keeps anypossible shock hazard away from the user.

Ground Yourself If you are working with integrated circuits (ICs), youalso need to ground yourself. Integrated circuits havetiny conduction paths that can be damaged by staticelectricity that builds up on your body. You can ruinan expensive IC simply by walking across a carpet ortaking off a sweater and then touching the leads ofthe IC. To solve this problem, wear a grounding strap(see Figure 23). This strap safely sends static chargeson your body to earth ground.

Setting the Controls After plugging in the oscilloscope, take a look at thefront panel. It is divided into three main sectionslabeled Vertical, Horizontal, and Trigger (see Figure24). Your oscilloscope may have other sections,depending on the model and type (analog or digi-tizing).

Notice the input connectors on your oscilloscope.This is where you attach probes. Most oscilloscopeshave at least two input channels and each channel

11

Setting Up

Figure 23. Typical wrist type grounding strap.

Figure 24. Front-panel control sections of a typical oscilloscope.

can display a waveform on the screen. Multiplechannels are handy for comparing waveforms.

Some oscilloscopes have an AUTOSET or PRESETbutton that sets up the controls in one step to accom-modate a signal. If your oscilloscope does not havethis feature, it is helpful to set the controls to stan-dard positions before taking measurements.

Standard positions include the following: • Set the oscilloscope to display channel 1 • Set the volts/division scale to a mid-range posi-

tion • Turn off the variable volts/division • Turn off all magnification settings • Set the channel 1 input coupling to DC • Set the trigger mode to auto • Set the trigger source to channel 1 • Turn trigger holdoff to minimum or off • Set the intensity control to a nominal viewing

level • Adjust the focus control for a sharp display

These are general instructions for setting up youroscilloscope. If you are not sure how to do any ofthese steps, refer to the manual that came with youroscilloscope. The Controls section describes thecontrols in more detail.

Probes Now you are ready to connect a probe to your oscil-loscope. It’s important to use a probe designed towork with your oscilloscope. A probe is more than acable with a clip-on tip. It’s a high-quality connector,carefully designed not to pick up stray radio andpower-line noise.

Probes are designed not to influence the behavior ofthe circuit you are testing. However, no measure-ment device can act as a perfectly invisible observer.The unintentional interaction of the probe and oscil-loscope with the circuit being tested is called circuitloading. To minimize circuit loading, you will prob-ably use a 10X attenuator (passive) probe.

Your oscilloscope probably arrived with a passiveprobe as a standard accessory. Passive probesprovide you with an excellent tool for general-purpose testing and troubleshooting. For morespecific measurements or tests, many other types ofprobes exist. Two examples are active and currentprobes.

Descriptions of these probes follow, with moreemphasis given to the passive probe since this is theprobe type that allows the most flexibility of use.

“Intelligent” Probe Interfaces Many modern oscilloscopes provide special auto-mated features built into the input and mating probeconnectors. The act of connecting the probe to theinstrument notifies the oscilloscope about theprobe’s attenuation factor, which in turn scales thedisplay so that the probe’s attenuation is figured intothe readout on the screen.

Some probe interfaces also recognize the type ofprobe; that is, passive, active, or current. Lastly, theinterface may act as a DC power source for probes.Active probes have their own amplifier and buffercircuitry that requires DC power.

Using Passive Probes Most passive probes have some attenuation factor,such as 10X, 100X, and so on. By convention, attenu-ation factors, such as for the 10X attenuator probe,have the X after the factor. In contrast, magnificationfactors like X10 have the X first.

The 10X (read as “ten times”) attenuator probe mini-mizes circuit loading and is an excellent general-purpose passive probe. Circuit loading becomesmore pronounced at higher frequencies, so be sure touse this type of probe when measuring signals above5 kHz. The 10X attenuator probe improves the accu-racy of your measurements, but it also reduces theamplitude of the signal seen on the screen by a factorof 10.

Because it attenuates the signal, the 10X attenuatorprobe makes it difficult to look at signals less than 10millivolts. The 1X probe is similar to the 10X attenu-ator probe but lacks the attenuation circuitry.Without this circuitry, more interference is intro-duced into the circuit being tested. Use the 10Xattenuator probe as your standard probe, but keepthe 1X probe handy for measuring weak signals.Some probes have a convenient feature for switchingbetween 1X and 10X attenuation at the probe tip. Ifyour probe has this feature, make sure you are usingthe correct setting before taking measurements.

Many oscilloscopes can detect whether you are usinga 1X or 10X probe and adjust their screen readoutsaccordingly. However with some oscilloscopes, youmust set the type of probe you are using or read fromthe proper 1X or 10X marking on the volts/divcontrol.

The 10X attenuator probe works by balancing theprobe’s electrical properties against the oscillo-scope’s electrical properties. Before using a 10Xattenuator probe, you need to adjust this balance foryour particular oscilloscope. This adjustment iscalled compensating the probe and is further

12

described elsewhere in this document. Figure 25shows a simple diagram of the internal workings of aprobe, its adjustment, and the input of an oscillo-scope.

Figure 26 shows a typical passive probe and someaccessories to use with the probe.

Using Active Probes Active probes provide their own amplification orperform some other type of operation to process thesignal before applying it to the oscilloscope. Thesetypes of probes can solve problems such as circuitloading or can perform tests on signals, sending theresults to the oscilloscope. Active probes require apower source for their operation.

Using Current Probes Current probes enable you to directly observe andmeasure current waveforms. They are available formeasuring both AC and DC current. Current probesuse jaws that clip around the wire carrying thecurrent. This makes them unique since they are notconnected in series with the circuit; therefore, theycause little or no interference in the circuit.

Where to Clip the Ground Clip Measuring a signal requires two connections: theprobe tip connection and a ground connection.Probes come with an alligator-clip attachment forgrounding the probe to the circuit under test. Inpractice, you clip the grounding clip to a knownground point in the circuit, such as the metal chassisof a stereo you are repairing, and touch the probe tipto a test point in the circuit.

13

Figure 26. A typical passive probe with accessories.

Figure 25. Typical probe/oscilloscope 10-to-1 divider network.

Compensating the Probe Before using a passive probe, you need to compen-sate it – to balance its electrical properties to aparticular oscilloscope. You should get into the habitof compensating the probe every time you set upyour oscilloscope. A poorly adjusted probe can makeyour measurements less accurate. Figure 27 showswhat happens to measured waveforms when using aprobe that is not properly compensated.

Most oscilloscopes have a square-wave referencesignal available at a terminal on the front panelwhich can be used to compensate the probe. Youcompensate a probe by:

• Attaching the probe to an input connector • Connecting the probe tip to the probe compensa-

tion signal • Attaching the ground clip of the probe to ground • Viewing the square wave reference signal • Making the proper adjustments on the probe so

that the corners of the square wave are square

When you compensate the probe, always first attachany accessory tips you will use and connect theprobe to the vertical channel you plan to use. Thisway, the oscilloscope has the same electrical proper-ties for compensation as it does when you makemeasurements.

14

Figure 27. The effects of improper probe compensation.

This section briefly describes the basic controlsfound on analog and digitizing oscilloscopes.Remember that some controls differ between analogand digitizing oscilloscopes; your oscilloscope prob-ably has controls not discussed here.

Display Controls Display systems vary between analog oscilloscopesand digitizing scopes, including both DSOs andDigital Phosphor Oscilloscopes (DPOs). Commoncontrols include: • An intensity control to adjust the brightness of

the waveform. As you increase the sweep speedof an analog oscilloscope, you need to increasethe intensity level.

• A focus control to adjust the sharpness of thewaveform. Digitizing oscilloscopes may not havea focus control.

• A trace rotation control to align the waveformtrace with the screen’s horizontal axis. The posi-tion of your oscilloscope in the earth’s magneticfield affects waveform alignment. Digitizing oscil-loscopes may not have a trace rotation control.

• On DPOs, a contrast control.• On many DSOs and on DPOs, a color palette

control to select trace colors and intensity gradingcolor levels.

• Other display controls may let you adjust theintensity of the graticule lights and turn on or offany on-screen information (such as menus).

Vertical Controls Use the vertical controls to position and scale thewaveform vertically. Your oscilloscope also hascontrols for setting the input coupling and othersignal conditioning, described later in this section.

Figure 28 shows a typical front panel and on-screenmenus for the vertical controls.

Position and Volts per Division The vertical position control lets you move thewaveform up or down to exactly where you want iton the screen.

The volts per division (usually written volts/div)setting varies the size of the waveform on the screen.A good general purpose oscilloscope can accuratelydisplay signal levels from about 4 millivolts to 40volts.

The volts/div setting is a scale factor. For example, ifthe volts/div setting is 5 volts, then each of the eightvertical divisions represents 5 volts and the entirescreen can show 40 volts from bottom to top(assuming a graticule with eight major divisions). Ifthe setting is 0.5 volts/div, the screen can display 4volts from bottom to top, and so on. The maximumvoltage you can display on the screen is the volts/divsetting times the number of vertical divisions.(Recall that the probe you use, 1X or 10X, also influ-ences the scale factor. You must divide the volts/divscale by the attenuation factor of the probe if theoscilloscope does not do it for you.)

Often, the volts/div scale has either a variable gain ora fine gain control for scaling a displayed signal to acertain number of divisions. Use this control to takerise time measurements.

Input Coupling Coupling means the method used to connect an elec-trical signal from one circuit to another. In this case,the input coupling is the connection from your testcircuit to the oscilloscope. The coupling can be set toDC, AC, or ground. DC coupling shows all of aninput signal. AC coupling blocks the DC componentof a signal so that you see the waveform centered at

Figure 28. Typical Vertical controls.

15

The Controls

zero volts. Figure 29 illustrates this difference. TheAC coupling setting is handy when the entire signal(alternating plus constant components) is too largefor the volts/div setting.

The ground setting disconnects the input signal fromthe vertical system, which lets you see where thezero-volt level is on the screen. With grounded inputcoupling and auto trigger mode, you see a horizontalline on the screen that represents zero volts.Switching from DC to ground and back again is ahandy way of measuring signal voltage levels withrespect to ground.

Bandwidth Limit Most oscilloscopes have a circuit that limits thebandwidth of the oscilloscope. By limiting the band-width, you reduce the high-frequency noise thatsometimes appears on the displayed waveform,providing you with a more refined signal display.

Alternate and Chop Display On analog scopes, multiple channels are displayedusing either an alternate or chop mode. (Digitizingoscilloscopes can present multiple channels simulta-neously without the need for chop or alternatemodes.)

Alternate mode draws each channel alternately – theoscilloscope completes one sweep on channel 1,then one sweep on channel 2, a second sweep onchannel 1, and so on. Use this mode with medium-to high-speed signals, when the sec/div scale is set to0.5 ms or faster.

Chop mode causes the oscilloscope to draw smallparts of each signal by switching back and forthbetween them. The switching rate is too fast for youto notice, so the waveform looks whole. You typi-cally use this mode with slow signals requiringsweep speeds of 1 ms per division or less. Figure 30shows the difference between the two modes. It isoften useful to view the signal both ways, to makesure you have the best view.

16

Figure 29. AC and DC input coupling.

Figure 30. Multi-channel display modes.

Math Operations Your oscilloscope may also have operations to allowyou to add waveforms together, creating a new wave-form display. Analog oscilloscopes combine thesignals while digitizing oscilloscopes create newwaveforms mathematically. Subtracting waveformsis another math operation. Subtraction is possiblewith analog oscilloscopes by using the channelinvert function on one signal and then using the addoperation. Digitizing oscilloscopes typically have asubtraction operation available. Figure 31 illustratesa third waveform created by adding two differentsignals together.

Horizontal Controls Use the horizontal controls to position and scale thewaveform horizontally. Figure 32 shows a typicalfront panel and on-screen menus for the horizontalcontrols.

Using the power of their internal processors, digi-tizing oscilloscopes offer many advanced math oper-ations: multiplication, division, integration, FastFourier Transform, and more.

Position and Seconds per Division The horizontal position control moves the waveformleft and right to exactly where you want it on thescreen.

The seconds per division (usually written as sec/div)setting lets you select the rate at which the waveformis drawn across the screen (also known as the timebase setting or sweep speed). This setting is a scalefactor. For example, if the setting is 1 ms, each hori-zontal division represents 1 ms and the total screenwidth represents 10 ms (ten divisions). Changing thesec/div setting lets you look at longer or shorter timeintervals of the input signal.

As with the vertical volts/div scale, the horizontalsec/div scale may have variable timing, allowing youto set the horizontal time scale in between thediscrete settings.

Time Base Selections Your oscilloscope has a time base usually referred toas the main time base and it is probably the mostuseful. Many oscilloscopes have what is called adelayed time base – a time base sweep that startsafter a pre-determined time from the start of themain time base sweep. Using a delayed time basesweep allows you to see events more clearly or evensee events not visible with the main time base sweepalone.

The delayed time base requires the setting of a delaytime and possibly the use of delayed trigger modesand other settings not described in this book. Referto the manual supplied with your oscilloscope forinformation on how to use these features.

Trigger Position Horizontal trigger position control is only availableon digitizing oscilloscopes. The trigger positioncontrol may be located in the horizontal controlsection of your oscilloscope. It actually represents“the horizontal position of the trigger in the wave-form record.”

Varying the horizontal trigger position allows you tocapture what a signal did before a trigger event

17

Figure 31. Adding channels.

Figure 32. Typical Horizontal controls.

(called pretrigger viewing). Thus it determines thelength of the viewable signal both preceding andfollowing a trigger point.

Digitizing oscilloscopes can provide pretriggerviewing because they constantly process the inputsignal whether a trigger has been received or not. Asteady stream of data flows through the oscilloscope;the trigger merely tells the oscilloscope to save thepresent data in memory. In contrast, analog oscillo-scopes only display the signal (that is, write it on theCRT) after receiving the trigger.

Pretrigger viewing is a valuable troubleshooting aid.For example, if a problem occurs intermittently, youcan trigger on the problem, record the events that ledup to it and, possibly, find the cause.

Zoom Your oscilloscope may have special horizontalmagnification settings that let you display a magni-fied section of the waveform on-screen. On a DSO,the operation is performed on stored digitized data.

XY Mode Most analog oscilloscopes have the capability ofdisplaying a second channel signal along the X-axis

(instead of time). This is known as XY mode. The XYmode is further explained in the MeasurementTechniques section of this document.

The Z Axis The Z axis brings a third dimension – intensity – tothe traditional waveform display. One application ofthe Z-axis is to feed special timed signals into theseparate Z input to create highlighted “marker” dotsat known intervals in the waveform.

XYZ Mode DPOs can use the Z input to create an XY displaywith intensity grading. In this case, the DPO samplesthe instantaneous data value at the Z input and usesthat value to intensify a specific part of the wave-form. XYZ is especially useful for displaying thepolar patterns commonly used in testing wirelesscommunication devices.

Trigger Controls The trigger controls let you stabilize repeating wave-forms and capture single-shot waveforms. Figure 33shows a typical front panel and on-screen menus forthe trigger controls.

18

Figure 33. Typical Trigger controls.

The trigger makes repeating waveforms appear staticon the oscilloscope display by repeatedly displayingthe same portion of the input signal. Imagine thejumble on the screen that would result if each sweepstarted at a different place on the signal (see Figure34).

Trigger Level and Slope Your oscilloscope may have several different types oftriggers, such as edge, video, pulse, or logic. Edgetriggering is the basic and most common type.

For edge triggering, the trigger level and slopecontrols provide the basic trigger point definition.

The trigger circuit acts as a comparator. You selectthe slope and voltage level of one side of thecomparator. When the trigger signal matches yoursettings, the oscilloscope generates a trigger.• The slope control determines whether the trigger

point is on the rising or the falling edge of asignal. A rising edge is a positive slope and afalling edge is a negative slope.

• The level control determines where on the edgethe trigger point occurs.

Figure 35 shows the effect the trigger slope and levelsettings have on how a waveform is displayed.

Trigger Sources The oscilloscope does not necessarily have to triggeron the signal being measured. Several sources cantrigger the sweep: • Any input channel • An external source other than the signal applied

to an input channel• The power source signal • A signal internally generated by the oscilloscope

Most of the time, you can leave the oscilloscope setto trigger on the channel displayed. Many oscillo-scopes provide a trigger output that delivers thetrigger signal to another instrument.

Note that the oscilloscope can use an alternatetrigger source whether displayed or not. So you haveto be careful not to unwittingly trigger on, forexample, channel 1 while displaying channel 2.

Trigger Modes The trigger mode determines whether or not theoscilloscope draws a waveform if it does not detect atrigger. Common trigger modes include normal andauto.

In normal mode, the oscilloscope only sweeps if theinput signal reaches the set trigger point; otherwise(on an analog oscilloscope) the screen is blank or (ona digitizing oscilloscope) frozen on the last acquiredwaveform. Normal mode can be disorienting sinceyou may not see the signal at first if the level controlis not adjusted correctly.

Auto mode causes the oscilloscope to sweep, evenwithout a trigger. If no signal is present, a timer inthe oscilloscope triggers the sweep. This ensures thatthe display will not disappear if the signal drops tosmall voltages. It is also the best mode to use if youare looking at many signals and do not want tobother setting the trigger each time.

Figure 34. Untriggered display.

Figure 35. Positive and negative slope triggering.

19

In practice, you will probably use both modes:normal mode because it lets you select just the signalarea you need to see, and auto mode because itrequires less adjustment.

Some oscilloscopes also include special modes forsingle sweeps, triggering on video signals, or auto-matically setting the trigger level.

Trigger Coupling Just as you can select either AC or DC coupling forthe vertical system, you can choose the kind ofcoupling for the trigger signal.

Besides AC and DC coupling, your oscilloscope mayalso have high frequency rejection, low frequencyrejection, and noise rejection trigger coupling. Thesespecial settings are useful for eliminating noise fromthe trigger signal to prevent false triggering.

Trigger Holdoff Sometimes getting an oscilloscope to trigger on thecorrect part of a signal requires great skill. Manyoscilloscopes have special features to make this taskeasier.

Trigger holdoff is an adjustable period of time duringwhich the oscilloscope cannot trigger. This feature isuseful when you are triggering on complex wave-form shapes, so that the oscilloscope only triggers onthe first eligible trigger point. Figure 36 shows howusing trigger holdoff helps create a usable display.

Digitizing Oscilloscope Triggers In addition to the usual threshold triggering, manydigitizing oscilloscopes offer a host of specializedtrigger settings which have no equivalents on analoginstruments. These triggers respond to specificconditions in the incoming signal, making it easy todetect, for example, a pulse that is narrower than itshould be. Such a condition would be impossible todetect with a voltage threshold trigger alone.Following is a partial list of the digital triggers foundon advanced DSOs and DPOs, along with a briefdefinition of each: • Pulse Width and Glitch trigger

Detects pulses either within or exceeding speci-fied widths.

• Runt Pulse triggerDetects a pulse that crosses the lesser but not thegreater of two threshold levels.

• Logic (Boolean) triggerUses multiple oscilloscope inputs as binaryinputs that must meet logical conditions such asNAND or NOR to produce a trigger.

• Serial Data triggerDetects specific data combinations in digitaltelecom signals

• Setup and Hold Violation triggerDetects violations of digital setup and hold timewhen clock and data signals are acquired on twodifferent inputs.

20

Figure 36. Trigger holdoff.

Acquisition Controls for DigitizingOscilloscopes Digitizing oscilloscopes have settings that let youcontrol how the acquisition system processes asignal. Look over the acquisition options on yourdigitizing oscilloscope while you read this descrip-tion. Figure 37 shows an example of an acquisitionmenu.

Acquisition Modes Acquisition modes control how waveform points areproduced from sample points. Recall from the firstsection that sample points are the digital values thatcome directly out of the Analog-to-Digital-Converter(ADC). The time between sample points is called thesample interval. Waveform points are the digitalvalues that are stored in memory and displayed toform the waveform. The time value differencebetween waveform points is called the waveforminterval. The sample interval and the waveforminterval may be, but need not be, the same. This factleads to the existence of several different acquisitionmodes in which one waveform point is made upfrom several sequentially acquired sample points.Additionally, waveform points can be created from acomposite of sample points taken from multipleacquisitions, which leads to another set of acquisi-tion modes. A description of the most commonlyused acquisition modes follows. • Sample Mode: This is the simplest acquisition

mode. The oscilloscope creates a waveform pointby saving one sample point during each wave-form interval.

• Peak Detect Mode: The oscilloscope saves theminimum and maximum value sample pointstaken during two waveform intervals and usesthese samples as the two corresponding wave-form points. Digitizing oscilloscopes with peakdetect mode run the ADC at a fast sample rate,even at very slow time base settings (long wave-form interval), and are able to capture fast signalchanges that would occur between the waveformpoints if in sample mode. Peak detect mode isparticularly useful for seeing narrow pulsesspaced far apart in time.

• Hi Res Mode: Like peak detect, hi res mode is away of getting more information in cases whenthe ADC can sample faster than the time basesetting requires. In this case, multiple samplestaken within one waveform interval are averagedtogether to produce one waveform point. Theresult is a decrease in noise and an improvementin resolution for low-speed signals.

• Envelope Mode: Envelope mode is similar to peakdetect mode. However, in envelope mode, theminimum and maximum waveform points frommultiple acquisitions are combined to form awaveform that shows min/max changes overtime. Peak detect mode is usually used to acquirethe records that are combined to form the enve-lope waveform.

• Average Mode: In average mode, the oscilloscopesaves one sample point during each waveforminterval as in sample mode. However, waveformpoints from consecutive acquisitions are thenaveraged together to produce the final displayedwaveform. Average mode reduces noise withoutloss of bandwidth but requires a repeating signal.

A special note about DPO acquisition: The DigitalPhosphor Oscilloscope has a high display sampledensity and an innate ability to capture intensity(Z-axis) information. With its intensity axis, the DPOis able to provide the same type of 3-dimensional,real-time display that analog scopes are known for.As you look at the waveform trace on a DPO, you cansee brightened areas. These are the areas where thesignal occurs most often. This makes it easy todistinguish, for example, the basic signal shape froma transient that occurs only once in a while. Thebasic signal would appear much brighter.

DPOs also include all the acquisition modesdescribed above.

21

Figure 37. Example of an acquisition menu.

Stopping and Starting the Acquisition System One of the greatest advantages of digitizing oscillo-scopes is their ability to store waveforms for laterviewing. To this end, there are usually one or morebuttons on the front panel that allow you to stop andstart the acquisition system so you can analyzewaveforms at your leisure. Additionally, you maywant the oscilloscope to automatically stopacquiring after one acquisition is complete or afterone set of records has been turned into an envelopeor average waveform. This feature is commonlycalled single sweep or single sequence and itscontrols are usually found either with the otheracquisition controls or with the trigger controls.

Sampling Methods In digitizing oscilloscopes that can use either real-time sampling or equivalent-time sampling asdescribed earlier, the acquisition controls will allowyou to choose which one to use for acquiring signals.

Note that this choice makes no difference for slowtime base settings and only has an effect when theADC cannot sample fast enough to fill the recordwith waveform points in one pass.

Other Controls So far, we have described the basic controls that abeginner needs to know about. Your oscilloscopemay have other controls for various functions. Someof these may include: • Measurement cursors• Keypads for mathematical operations or data

entry • Printing capabilities • Interfaces for connecting your oscilloscope to a

computer

Look over the other options available to you andread your oscilloscope’s manual to find out moreabout these other controls.

22

This section teaches you basic measurement tech-niques. The two most basic measurements you canmake are voltage and time measurements. Just aboutevery other measurement is based on one of thesetwo fundamental techniques.

This section discusses methods for making measure-ments visually with the oscilloscope screen. This is acommon technique with analog instruments, andalso may be useful for “at-a-glance” interpretation ofDSO or DPO displays.

Note that most digitizing oscilloscopes include auto-mated measurement tools. Knowing how to makemeasurements manually as described here will helpyou understand and check the automatic measure-ments of DSOs and DPOs. Automated measurementsare explained later in this section.

The Display Take a look at the oscilloscope display. Notice thegrid markings on the screen – these markings createthe graticule. Each vertical and horizontal lineconstitutes a major division. The graticule is usuallylaid out in an 8-by-10 division pattern. Labeling onthe oscilloscope controls (such as volts/div andsec/div) always refers to major divisions. The tickmarks on the center horizontal and vertical graticulelines (see Figure 38) are called minor divisions.

Many oscilloscopes display on the screen how manyvolts each vertical division represents and howmany seconds each horizontal division represents.

Voltage Measurements Voltage is the amount of electric potential, expressedin volts, between two points in a circuit. Usually oneof these points is ground (zero volts) but not always.Voltages can also be measured from peak-to-peak –from the maximum point of a signal to its minimumpoint. You must be careful to specify which voltageyou mean.

The oscilloscope is primarily a voltage-measuringdevice. Once you have measured the voltage, otherquantities are just a calculation away. For example,Ohm’s law states that voltage between two points ina circuit equals the current times the resistance.From any two of these quantities, you can calculatethe third using the following formula:

Ohm’s Law:

Voltage = Current * Resistance

Current = Voltage

Resistance

Resistance = Voltage

Current

Power Law:

Power = Voltage * Current

Another handy formula is the power law: the powerof a DC signal equals the voltage times the current.Calculations are more complicated for AC signals,but the point here is that measuring the voltage is thefirst step toward calculating other quantities.

Figure 39 shows the voltage of one peak (Vp) and thepeak-to-peak voltage (Vp-p), which is usually twiceVp. Use the RMS (root-mean-square) voltage (VRMS)to calculate the power of an AC signal.

The most basic method of taking voltage measure-ments is to count the number of divisions a wave-form spans on the oscilloscope’s vertical scale.Adjusting the signal to cover most of the screen

23

Measurement Techniques

Figure 38. An oscilloscope graticule.

Figure 39. Voltage peak and peak-to-peak voltage.

vertically, then taking the measurement along thecenter vertical graticule line having the smaller divi-sions makes for the best voltage measurements (seeFigure 40). The more screen area you use, the moreaccurately you can read from the screen.

Many oscilloscopes have on-screen cursors that letyou take waveform measurements automatically on-screen, without having to count graticule marks. Acursor is simply a line that you can move across the

screen. Two horizontal cursor lines can be moved upand down to bracket a waveform’s amplitude forvoltage measurements, and two vertical lines moveright and left for time measurements. A readoutshows the voltage or time at the positions of thecursors.

Time and Frequency Measurements You take time measurements using the horizontalscale of the oscilloscope. Time measurementsinclude measuring the period, pulse width, andtiming of pulses. Frequency is the reciprocal of theperiod, so once you know the period, the frequencyis one divided by the period. Like voltage measure-ments, time measurements are more accurate whenyou adjust the portion of the signal to be measured tocover a large area of the screen. Taking time measure-ments along the center horizontal graticule line,having smaller divisions, makes for the best timemeasurements (see Figure 41).

Pulse and Rise Time Measurements In many applications, the details of a pulse’s shapeare important. Pulses can become distorted andcause a digital circuit to malfunction, and the timingof pulses in a pulse train is often significant.

Standard pulse measurements are pulse width andpulse rise time. Rise time is the amount of time apulse takes to go from the low to high voltage. Byconvention, the rise time is measured from 10% to90% of the full voltage of the pulse. This eliminatesany irregularities at the pulse’s transition corners.This also explains why most oscilloscopes have 10%and 90% markings on their screen. Pulse width isthe amount of time the pulse takes to go from low tohigh and back to low again. By convention, the pulsewidth is measured at 50% of full voltage. See Figure42 for these measurement points.

Pulse measurements often require fine-tuning thetriggering. To become an expert at capturing pulses,you should learn how to use trigger holdoff and howto set the digitizing oscilloscope to capture pretriggerdata, as described earlier in The Controls section.Horizontal magnification is another useful feature formeasuring pulses, since it allows you to see finedetails of a fast pulse.

24

Figure 41. Measure time on the center horizontal graticule line.

Figure 42. Rise time and pulse width measurement points.

Figure 40. Measure voltage on the center vertical graticule line.

Phase Shift MeasurementsThe horizontal control section may have an XYmode that lets you display an input signal ratherthan the time base on the horizontal axis. This modeof operation opens up a whole new area of phase-shift measurement techniques.

The phase of a wave is the amount of time thatpasses from the beginning of a cycle to the beginningof the next cycle, measured in degrees. Phase shiftdescribes the difference in timing between twootherwise identical periodic signals.

One method for measuring phase shift is to use XYmode. This involves connecting one signal to thevertical system as usual and then another signal tothe horizontal system. (This method only works ifboth signals are sinusoidal.) This set up is called anXY measurement because both the X and Y axis are

tracing voltages. The waveform resulting from thisarrangement is called a Lissajous pattern (named forFrench physicist Jules Antoine Lissajous andpronounced LEE-sa-zhoo). From the shape of theLissajous pattern, you can tell the phase differencebetween the two signals. You can also tell theirfrequency ratio. Figure 43 shows Lissajous patternsfor various frequency ratios and phase shifts.

The XY measurement mode originated with analogoscilloscopes. Due to their relatively low sampledensity, DSOs may have difficulty creating real-timeXY displays. Some DSOs create an XY image byaccumulating data points over time, then displayingthe composite. Digital Phosphor Oscilloscopes, onthe other hand, are able to acquire and display agenuine XY mode image in real-time, using a contin-uous stream of digitized data. DPOs can also displayan XYZ image with intensified areas.

25

Figure 43. Lissajous patterns.

Waveform Measurements withDigitizing Oscilloscopes Digitizing oscilloscopes have functions that makewaveform measurements easier. Modern DSOs andDPOs have front-panel buttons or screen-basedmenus from which you can select fully automatedmeasurements. These include amplitude, period,rise/fall time, and much more. Many digitizinginstruments also provide mean and RMS calcula-tions, duty cycle, and other math operations.Automated measurements appear as on-screenalphanumeric readouts. Typically these readings aremore accurate than it’s possible to obtain with directgraticule interpretation.

Following is a list of the fully automated waveformmeasurements available on the TDS 500D/700DSeries Digital Phosphor Oscilloscopes.

Period Duty cycle + High Frequency Duty cycle – Low Width + Delay Minimum Width – Phase Maximum Rise time Burst width Overshoot + Fall time Peak-to-peak Overshoot – Amplitude Mean RMS Extinction ratio Cycle mean Cycle RMS Mean optical power Cycle area

What’s Next? This section has covered basic measurement tech-niques. Other measurement techniques involvesetting up the oscilloscope to test electrical compo-nents on an assembly line, subtracting noise from asignal, capturing elusive transient signals, and manyothers that would take too much room to list. Themeasurement techniques you will use depend onyour application, but you have learned enough to getstarted. Practice using your oscilloscope and readmore about it. Soon its operation will be secondnature to you.

26

This section contains written exercises that cover information in this book. Theexercises are divided into two parts, Part I and Part II.

Part I covers information presented in these sections: • The Oscilloscope• Oscilloscope Terminology

Part II covers information presented in sections: • Setting Up• The Controls• Measurement Techniques

27

Written Exercises

Part I ExercisesThe following exercises cover information presented in these sections:• The Oscilloscope• Oscilloscope Terminology

Check how well you have absorbed the information in these sections by doing this short self-test. Answers aregiven on page 34.

Vocabulary Exercise Write the number of the definitions in the right column next to the correct words in the left column.

Term Definition

1. ___ Acquisition A. The unit of electric potential difference.

2. ___ Analog B. A performance measurement indicating the precision of an ADC, measured in bits.

3. ___ Bandwidth C. Term used when referring to degree points of a sine wave.

4. ___ Digital Phosphor D. The number of times a signal repeats in one second.

5. ___ Frequency E. The amount of time it takes a wave to complete one cycle.

6. ___ Glitch F. A stored digital value that represents the voltage of a signal at a specific point in time.

7. ___ Period G. A common waveform shape that has a rising edge, a width, and a falling edge.

8. ___ Phase H. A performance measurement indicating the fastest edge a given oscilloscope can accurately display.

9. ___ Pulse I. Oscilloscope circuitry that controls the timing of the sweep.

10. ___ Waveform Point J. An intermittent error in a circuit.

11. ___ Rise Time K. A signal measured by an oscilloscope that only occurs once.

12. ___ Sample Point L. The oscilloscope’s process of collecting sample points from the ADC, processing them, and storing them in memory.

13. ___ Digital Storage M. Something that operates with continuous values.

14. ___ Time Base N. Digitizing Oscilloscope that captures 3 dimensions of signal information in real- time.

15. ___ Transient O. Digitizing Oscilloscope with serial processing.

16. ___ ADC Resolution P. A frequency range.

17. ___ Volt Q. The raw data from an ADC used to calculate waveform points.

28

Using Oscilloscopes Exercise Circle the best answer for each statement. Some statements have more than one right answer.

1. With an oscilloscope you can:

a. Calculate the frequency of a signal.

b. Find malfunctioning electrical components.

c. Analyze bird calls.

e. All the above.

2. The difference between analog and digitizing oscilloscopes is:

a. Analog oscilloscopes do not have on-screen menus.

b. Analog oscilloscopes apply a measurement voltage directly to the display system, while digital oscil-loscopes first convert the voltage into digital values.

c. Analog oscilloscopes measure analogs, whereas digitizing oscilloscopes measure digits.

d. Analog oscilloscopes do not have an acquisition system.

3. An oscilloscope’s vertical section does the following:

a. Acquires sample points with an ADC.

b. Starts a horizontal sweep.

c. Lets you adjust the brightness of the display.

d. Attenuates or amplifies the input signal.

4. The time base control of the oscilloscope does the following:

a. Adjusts the vertical scale.

b. Shows you the current time of day.

c. Sets the amount of time represented by the horizontal width of the screen.

d. Sends a clock pulse to the probe.

5. On an oscilloscope display:

a. Voltage is on the vertical axis and time is on the horizontal axis.

b. A straight diagonal trace means voltage is changing at a steady rate.

c. A flat horizontal trace means voltage is constant.

d. All the above.

6. All repeating waves have the following properties:

a. A frequency measured in hertz.

b. A period measured in seconds.

c. A bandwidth measured in hertz.

d. All the above.

7. If you probe inside a computer with an oscilloscope, you are likely to find the following types of signals:

a. Pulse trains.

b. Ramp waves.

c. Sine waves.

d. All the above.

8. When evaluating the performance of an analog oscilloscope, some things you might consider are:

a. The bandwidth.

b. The vertical sensitivity.

c. The ADC resolution.

d. The sweep speed.

9. The difference between digital storage oscilloscopes (DSO) and digital phosphor oscilloscopes (DPO) is:

a. The DSO has a higher bandwidth.

b. The DPO captures three dimensions of waveform information in real-time.

c. The DSO has a color display.

d. The DSO captures more signal details.

29

Part II ExercisesThe following exercises cover information presented in these sections:• Setting Up• The Controls• Measurement Techniques

Check how well you have absorbed the information in these sections by doing this short self-test. Answers aregiven on page 34.

Vocabulary Exercise Write the letter of the definitions in the right column next to the correct words in the left column.

Term Definition

1. ___ Averaging Mode A. The unintentional interaction of the probe and oscilloscope with the circuit being tested which distorts a signal.

2. ___ Circuit Loading B. A conductor that connects electrical currents to the Earth.

3. ___ Compensation C. A sampling mode in which the digital oscilloscope collects as many samples as it can as the signal occurs, then constructs a display, using interpolation if necessary.

4. ___ Coupling D. A sampling mode in which the digital oscilloscope constructs a picture of a repetitive signal by capturing a little bit of information from each repetition.

5. ___ Earth Ground E. A device that converts a specific physical quantity such as sound, pressure, strain, or light intensity into an electrical signal.

6. ___ Equivalent-Time F. A test device for injecting a signal into a circuit input.

7. ___ Graticule G. A processing technique used by digital oscilloscopes to eliminate noise in a signal.

8. ___ Interpolation H. The method of connecting two circuits together.

9. ___ Real Time I. A “connect-the-dots” processing technique to estimate what a fast waveform looks like based on only a few sampled points.

10. ___ Signal Generator J. The grid lines on a screen for measuring oscilloscope traces.

11. ___ Single Sweep K. A trigger mode that triggers the sweep once, must be reset to accept another trigger event.

12. ___ Transducer L. A probe adjustment for 10X attenuator probes that balances the electrical properties of the probe with the electrical properties of the oscilloscope.

30

Using Oscilloscopes Exercise Circle the best answer for each statement. Some statements have more than one right answer.

1. To operate an oscilloscope safely, you should:

a. Ground the oscilloscope with the proper three-pronged power cord.

b. Learn to recognize potentially dangerous electrical components.

c. Avoid touching exposed connections in a circuit being tested even if the power is off.

d. All the above.

2. Grounding an oscilloscope is necessary:

a. For safety reasons.

b. To provide a reference point for making measurements.

c. To align the trace with the screen’s horizontal axis.

d. All the above.

3. Circuit loading is caused by:

a. An input signal having too large a voltage.

b. The probe and oscilloscope interacting with the circuit being tested.

c. A 10X attenuator probe being uncompensated.

d. Putting too much weight on a circuit.

4. Compensating a probe is necessary to:

a. Balance the electrical properties of the 10X attenuator probe with the oscilloscope.

b. Prevent damaging the circuit being tested.

c. Improve the accuracy of your measurements.

d. All the above.

5. The trace rotation control is useful for:

a. Scaling waveforms on the screen.

b. Detecting sine wave signals.

c. Aligning the waveform trace with the screen’s horizontal axis on an analog oscilloscope.

d. Measuring pulse width.

6. The volts per division control is used to:

a. Scale a waveform vertically.

b. Position a waveform vertically.

c. Attenuate or amplify an input signal.

d. Set the numbers of volts each division represents.

7. Setting the vertical input coupling to ground does the following:

a. Disconnects the input signal from the oscilloscope.

b. Causes a horizontal display to appear on the screen.

c. Lets you see where zero volts is on the screen.

d. All the above.

8. The trigger is necessary to:

a. Stabilize repeating waveforms on the screen.

b. Capture single-shot waveforms.

c. Mark a particular point of an acquisition.

d. All the above.

9. The difference between auto and normal trigger mode is:

a. In normal mode the oscilloscope only sweeps once and then stops.

b. In normal mode the oscilloscope only sweeps if the input signal reaches the trigger point; otherwisethe screen is blank.

c. Auto mode makes the oscilloscope sweep continuously even without being triggered.

d. All the above.

31

10. A digital oscilloscope’s acquisition controls let you specify:

a. Whether the oscilloscope uses real-time or equivalent-time sampling to collect sample points.

b. Whether to average a collection of records to form a waveform.

c. How sample points are processed to form waveform points.

d. All the above.

11. The acquisition mode that best reduces noise in a repeating signal is:

a. Sample mode.

b. Peak detect mode.

c. Envelope mode.

d. Averaging mode.

12. The two most basic measurements you can make with an oscilloscope are:

a. Time and frequency measurements.

b. Time and voltage measurements.

c. Voltage and pulse width measurements.

d. Pulse width and phase shift measurements.

13. If the volts/division is set at 0.5, the largest signal that can fit on the screen (assuming an 8 x 10 divisionscreen) is:

a. 62.5 millivolts peak-to-peak.

b. 8 volts peak-to-peak.

c. 4 volts peak-to-peak.

d. 0.5 volts peak-to-peak.

14. If the seconds/division is set at 0.1 ms, the amount of time represented by the width of the screen is:

a. 0.1 ms.

b. 1 ms.

c. 1 second.

d. 0.1 kHz.

15. By convention, pulse width is measured:

a. At 10% of the pulse’s maximum voltage.

b. At 50% of the pulse’s maximum voltage.

c. At 90% of the pulse’s maximum voltage.

d. At 10% and 90% of the pulse’s maximum voltage.

16. You attach a probe to your test circuit but the screen is blank. You should:

a. Check that the screen intensity is turned up.

b. Check that the oscilloscope is set to display the channel that the probe is connected to.

c. Set the trigger mode to auto since norm mode blanks the screen.

d. Set the vertical input coupling to AC and set the volts/division to its largest value since a large DCsignal may go off the top or bottom of the screen.

e. Check that the probe isn’t shorted and make sure it is properly grounded.

f. Check that the oscilloscope is set to trigger on the input channel you are using.

g. All of the above.

32

Answers to Exercises on Back

33

Answers to Written ExercisesThis section provides the answers to all written exercises in the previous sections.

Part I: Vocabulary Exercise Answers

1. L 5. D 9. G 13. O

2. M 6. J 10. F 14. I

3. P 7. E 11. H 15. K

4. N 8. C 12. Q 16. B

17. A

Part I: Oscilloscope Usage Exercise Answers

1. D 3. D 5. D 7. D

2. B, D 4. C 6. A,B 8. A, B, D

9. B

Part II: Vocabulary Exercise Answers

1. G 4. H 7. J 10. F

2. A 5. B 8. I 11. K

3. L 6. D 9. C 12. E

Part II: Oscilloscope Usage Exercise Answers

1. D 5. C 9. B,C 13. C

2. A, B 6. A, C, D 10. D 14. B

3. B 7. D 11. D 15. B

4. A, C 8. D 12. B 16. G

34

AC (Alternating Current) A signal in which the currentand voltage vary in a repeating pattern over time.

ADC (Analog-to-Digital Converter) A digital electroniccomponent that converts an electrical signal intodiscrete binary values.

Alternate Mode A display mode of operation in which the oscillo-scope completes tracing one channel before begin-ning to trace another channel.

Amplitude The magnitude of a quantity or strength of a signal.In electronics, amplitude usually refers to eithervoltage or power.

Analog Oscilloscope One of three prevalent oscilloscope architectures(the other two are DSOs and DPOs – see definitionsbelow). An instrument that creates a waveformdisplay by applying the input signal (conditionedand amplified) to an electron beam moving across aCRT screen. A chemical phosphor coating on theCRT creates a glowing trace wherever the beamhits.

Attenuation A decrease in signal voltage during its transmissionfrom one point to another.

Averaging A processing technique used by digital oscillo-scopes to eliminate noise in a signal.

Bandwidth A frequency range.

CRT (Cathode-Ray Tube) An electron-beam tube inwhich the beam can be focused on a luminescentscreen and varied in both position and intensity toproduce a visible pattern. A television picture tubeis a CRT.

Chop Mode A display mode of operation in which small partsof each channel are traced so that more than onewaveform can appear on the screen simulta-neously.

Circuit Loading The unintentional interaction of the probe andoscilloscope with the circuit being tested,distorting the signal.

Compensation A probe adjustment for 10X probes that balancesthe capacitance of the probe with the capacitanceof the oscilloscope.

Coupling The method of connecting two circuits together.Circuits connected with a wire are directlycoupled; circuits connected through a capacitor ora transformer are indirectly (or AC) coupled.

Cursor An on-screen marker that you can align with awaveform to take accurate measurements.

DC (Direct Current) A signal with a constant voltageand current.

Digital Phosphor Oscilloscope (DPO) A digitizing oscilloscope that closely models thedisplay characteristics of an analog oscilloscopewhile providing traditional digitizing oscilloscopebenefits (waveform storage, automated measure-ments, etc.). The DPO uses a parallel processingarchitecture to pass the signal to the raster-typedisplay. This provides intensity-graded viewingcharacteristics.

Digital Storage Oscilloscope (DSO) An oscilloscope that acquires signals via digitalsampling (using an analog-to-digital converter). Ituses a serial architecture that employs a singleprocessor to control acquisition, user interface, andthe raster display.

Division Measurement markings on the CRT graticule of theoscilloscope.

Earth Ground A conductor that will dissipate large electricalcurrents into the Earth.

Envelope The outline of a signal’s highest and lowest pointsacquired over many repetitions.

Equivalent-time Sampling A sampling mode in which the oscilloscopeconstructs a picture of a repetitive signal bycapturing a little bit of information from each repe-tition.

Focus The oscilloscope control that adjusts the CRT elec-tron beam to control the sharpness of the display.

Frequency The number of times a signal repeats in onesecond, measured in Hertz (cycles per second). Thefrequency equals 1/period.

Gigahertz (GHz) 1,000,000,000 Hertz; a unit of frequency.

Glitch An intermittent error in a circuit.

Graticule The grid lines on a screen for measuring oscillo-scope traces.

Ground 1. A conducting connection by which an electriccircuit or equipment is connected to the earth toestablish and maintain a reference voltage level. 2. The voltage reference point in a circuit.

Hertz (Hz) One cycle per second; the unit of frequency.

Kilohertz (kHz) 1000 Hertz; a unit of frequency.

Interpolation A “connect-the-dots” processing technique to esti-mate what a fast waveform looks like based on onlya few sampled points.

35

Glossary

Megahertz (MHz) 1,000,000 Hertz; a unit of frequency.

Megasamples per second (MS/s) A sample rate unit equal to one million samplesper second.

Microsecond (µs) A unit of time equivalent to 0.000001 seconds.

Millisecond (ms) A unit of time equivalent to 0.001 seconds.

Nanosecond (ns) A unit of time equivalent to 0.000000001 seconds.

Noise An unwanted voltage or current in an electricalcircuit.

Oscilloscope An instrument used to make voltage changesvisible over time. The word oscilloscope comesfrom “oscillate,” since oscilloscopes are often usedto measure oscillating voltages.

Peak (Vp) The maximum voltage level measured from a zeroreference point.

Peak-to-peak (Vp-p) The voltage measured from the maximum point ofa signal to its minimum point, usually twice the Vplevel.

Peak Detection An acquisition mode for digital oscilloscopes thatlets you see the extremes of a signal.

Period The amount of time it takes a wave to complete onecycle. The period equals 1/frequency.

Phase The amount of time that passes from the beginningof a cycle to the beginning of the next cycle,measured in degrees.

Probe An oscilloscope input device, usually having apointed metal tip for making electrical contact witha circuit element and a flexible cable for transmit-ting the signal to the oscilloscope.

Pulse A common waveform shape that has a fast risingedge, a width, and a fast falling edge.

RMS Root mean square.

Real-time Sampling A sampling mode in which the oscilloscopecollects as many samples as it can as the signaloccurs.

Record Length The number of waveform points used to create arecord of a signal.

Rise Time The time taken for the leading edge of a pulse torise from its minimum to its maximum values(typically measured from 10% to 90% of thesevalues).

Sample Point The raw data from an ADC used to calculate wave-form points.

Screen The surface of the CRT upon which the visiblepattern is produced – the display area.

Signal Generator A test device for injecting a signal into a circuitinput; the circuit’s output is then read by an oscil-loscope.

Sine Wave A common curved wave shape that is mathemati-cally defined.

Single Shot A signal measured by an oscilloscope that onlyoccurs once (also called a transient event).

Single Sweep A trigger mode for displaying one screen full of asignal and then stopping.

Slope On a graph or an oscilloscope screen, the ratio of avertical distance to a horizontal distance. A posi-tive slope increases from left to right, while a nega-tive slope decreases from left to right.

Square Wave A common wave shape consisting of repeatingsquare pulses.

Sweep One horizontal pass of an oscilloscope’s electronbeam from left to right across the CRT screen.

Sweep Speed A measurement of how fast the time base “sweeps”the electron beam across the CRT screen.

Time Base Oscilloscope circuitry that controls the timing ofthe sweep. The time base is set by the seconds/divi-sion control.

Trace The visible shapes drawn on a CRT by the move-ment of the electron beam.

Transducer A device that converts a specific physical quantitysuch as sound, pressure, strain, or light intensityinto an electrical signal.

Transient A signal measured by an oscilloscope that onlyoccurs once (also called a single-shot event).

Trigger The circuit that initiates a horizontal sweep on anoscilloscope and determines the beginning point ofthe waveform.

Trigger Holdoff A control that inhibits the trigger circuit fromlooking for a trigger level for some specified timeafter the end of the waveform.

Trigger Level The voltage level that a trigger source signal mustreach before the trigger circuit initiates a sweep.

Volt The unit of electric potential difference.

Voltage The difference in electric potential, expressed involts, between two points.

36

Waveform A graphic representation of a voltage varying overtime.

Waveform Point A digital value that represents the voltage of asignal at a specific point in time. Waveform pointsare calculated from sample points and stored inmemory.

Z-axis The signal in an oscilloscope that controls electron-beam brightness as the trace is formed.

37

For further information, contact Tektronix:

Worldwide Web: for the most up-to-date product information visit our web site at: www.tektronix.com/scopes/ASEAN Countries (65) 356-3900; Australia & New Zealand 61 (2) 9888-0100; Austria, Central Eastern Europe, Greece, Turkey, Malta,& Cyprus +43 2236 8092 0; Belgium +32 (2) 715 89 70; Brazil and South America 55 (11) 3741-8360; Canada 1 (800) 661-5625; Denmark +45 (44) 850 700; Finland +358 (9) 4783 400; France & North Africa +33 1 69 86 81 81; Germany + 49 (221) 94 77 400; Hong Kong (852) 2585-6688; India (91) 80-2275577; Italy +39 (2) 25086 501; Japan (Sony/Tektronix Corporation) 81 (3) 3448-3111; Mexico, Central America, & Caribbean 52 (5) 666-6333; The Netherlands +31 23 56 95555; Norway +47 22 07 07 00;People’s Republic of China 86 (10) 6235 1230; Republic of Korea 82 (2) 528-5299; South Africa (27 11)651-5222; Spain & Portugal +34 91 372 6000;Sweden +46 8 477 65 00; Switzerland +41 (41) 729 36 40; Taiwan 886 (2) 2722-9622; United Kingdom & Eire +44 (0)1344 392000; USA 1 (800) 426-2200.

From other areas, contact: Tektronix, Inc. Export Sales, P.O. Box 500, M/S 50-255, Beaverton, Oregon 97077-0001, USA 1 (503) 627-6877.

0300 TD/XBS 03W-8605-1

Copyright © 2000, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK are registered trademarks of Tektronix, Inc. All other trade names referenced are the service marks, trademarks or registered trademarks of their respective companies.