Oscillscope Professional Report

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Steven SmithCommunications for Technicians Student Number

BTEC National Assessment

Date 2nd February 2009

Introduction ToOscilloscopes

Theory And Practice Of An Oscilloscope For

Trainee Engineers
http://images.google.co.uk/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Oscilloscope_Triangle_Wave.jpg/740px-Oscilloscope_Triangle_Wave.jpg&imgrefurl=http://mujtabachang.blogspot.com/2008/01/discovering-wonder-oscillos%20


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Introduction To OscilloscopesTheory And Practice Of An Oscilloscope For Trainee Engineers

Table Of Contents

Table Of Contents ........................................................................................................................... 1

Background of Oscilloscopes ........................................................................................................... 1

What Does An Oscilloscope Do? .................................................................................................... .. 2

Explanation And Instructions On How To Use An Oscilloscope .......................................................... 3

Descriptions And Functions Of The Various Controls .................................................................. 3

How To Set Up The Time-Base controls ................................................................... ..... ..... ........ 5

How To Set Up The Amplitude Controls .............................................................................. ........ 6

Example Of Measuring An AC Signal ......................................................................................... 6

How To Connect A Signal Source ................................................................................................... .. 8

Calibration .................................................................................................................................. ..... 9

Calibration Of Test Probes .......................................................................................................... 9

Bibliography / References .............................................................................................................. 10


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Introduction To OscilloscopesTheory And Practice Of An Oscilloscope For Trainee Engineers

Background of Oscilloscopes

Oscilloscopes are widely used in many areas from simple television repair to complexindustries electronics circuitry design. It is a good measuring tools that helps electronicsengineers to perform troubleshooting or research work.

The word Oscilloscope is an etymological hybrid. The first part derives from the Latinoscillare, to swing backwards and forwards; this in turn is from, oscillum, a little mask of Bacchus hung from the trees, especially in vineyards, and thus easily moved by the

wind. The second part comes from the Classical Greek skopein, to observe, aim at,examine, from which developed the Latin ending scopium, which has been used to formnames for instruments that enable the eye or ear to make observations. [ B1 ].

The oscilloscope is available in both analogue and digital formats, the most commonfound in the work place is the traditional analogue oscilloscope, and this report will cover the analogue type. This reports aims to explain the basic functions and use of anoscilloscope to the companys trainee engineers

An oscilloscope looks a lot like a small television set, except that it has a grid drawn onits screen and more controls than a television. An oscilloscope is basically a graph-displaying device - it shows electrical signals in a graphical form. In most applicationsthe graph shows how the applied signals change over time: the vertical (Y) axisrepresents voltage and the horizontal (X) axis represents time. The front panel of anoscilloscope consists of control sections divided into Vertical, Horizontal, and Trigger sections. Also present are the display controls and input connectors all of which aredetailed later.

The heart of the oscilloscope is display screen itself, the CRT. The CRT is a glass bulbwhich has had the air removed and then been sealed with a vacuum inside. At the front isa flat glass screen, which is coated inside with a phosphor material. This phosphor willglow when struck by the fast moving electronics and produce light, emitted from thefront and forming the spot and hence the trace. The rear of the CRT contains the electron

gun assembly. A small heater element is contained within a cylinder of metal called thecathode. When the heater is activated by applying a voltage across it, the cathodetemperature rises and it then emits a stream of electrons. [ B2 ].


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What Does An Oscilloscope Do?

The oscilloscope is a handy piece of kit and is useful for making measurements in boththe A.C. and D.C. .It allows the engineer to see a visual display of the electrical signals inthe circuit being tested, allowing a greater understanding of whats is happening. Theoscilloscope can easily investigate a fault in any electronic system by checking thesignals at the input and output stages. By checking each stage the engineer can quicklytell if it is operating as expected and the signals are correctly linked to the next stage?With practice, an engineer can quickly find and correct faults with ease.

The oscilloscope is an extremely simple device. Its main function is to draw a Voltageover time (V / t) graph, which is produced, on a small, TV like screen. The vertical or Y-axis displays the voltage and the horizontal or X-axis displays the time.

An example of the screen is shown below. It normally has 8 squares or divisions on thevertical axis, and 10 squares or divisions on the horizontal axis. Typically they are 1 cmin each direction.

Figure 1

The numerous controls on the front of the oscilloscope allow the engineer to change thevertical or horizontal scales of the Voltage over time graph. This allows a clear picture of the signal being investigated to be displayed.

'Dual trace' (two waveform signals) oscilloscopes have the ability to display two graphsat the same time. This allows simultaneous signals from different sources in the system to

be compared.

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The graph displayed by the oscilloscope can tell the engineer many things about thesignals. 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 tell if the signal is being distorted.

You can find out how much of a signal is direct current or alternating current.

You can tell how much of the signal is noise. ( B3 )

The purpose of the oscilloscope is not limited to testing electronic circuits. With the proper transducer, an oscilloscope can be used in all kinds of circumstances. A transducer is a device that produces an electrical signal in response to physical response, such assound, pressure, light, or heat. For example, a microphone is a transducer.

An automotive engineer uses an oscilloscope to measure engine vibrations. A medicalresearcher uses an oscilloscope to measure brain waves. The possibilities are endless.

Explanation And Instructions On How To Use An Oscilloscope

Descriptions And Functions Of The Various Controls

Below is a typical standard analogue oscilloscope front display; this shows the variouscontrols on the front panel. These are numbered and are used for reference through outthis report.

Figure 2

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The basic control functions vary between oscilloscopes but most common controls are:

Power

Turns oscilloscope mains power on or off (1).

Brightness

This controls the intensity of the waveform displayed. The operator should not allow thescope to be left unattended with a high brilliance, as this will certainly burn the trace intothe display. An increase in sweep speed will normally require and increase in brightnesslevel. (2)

Focus

The focus control adjusts the sharpness of the waveform and focusing the screen is bestdone whilst viewing a waveform. (3)

Trace

Selects, which trace or signal is to be displayed on the screen.

A - Selects trace A only (single channel).

B - Selects trace B only (single channel).

A+B - Selects both trace A and trace B (dual channel).

ADD - Both channel inputs are added and displayed as a single trace. (11)

Time base

The speed the spot travels across the screen of the tube can be varied by means of theTime base selector switch (9). This is calibrated in Seconds (S), Milliseconds (mS) or Microseconds (uS).

Trigger selector

Selects the trigger source. Dual beam scopes may be triggered from either Channel A or channel B. The oscilloscopes also has the facility to trigger from an external source, anexternal TRIGGER input is on the front panel. (5)

Trigger level

A trace displaying a waveform without the use of TRIGGER will roll in much the sameway as a TV with the horizontal hold set wrongly. Trigger action will stop the trace fromstarting, until a determined part of the waveform occurs.

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This will make each "sweep" of. The tube face occurs in exactly the same place and thedisplay will appear to be stationary. The trigger level control is used to select that point of

the waveform.Position control

Rotation of this knob will adjust the vertical position of the Channel 1 and Channel 2waveform on the screen (15)

Voltage Sensitivity or amplitude controls

The range on this control is from 20 volts/div (less sensitive) to 5mV/div (moresensitive). The 2 position on the Channel 1 input implies that a 2-volt signal wouldcause a deflection of 2 divisions vertically on the oscilloscope face. (14)

Sensitivity calibration

This knob is used to change the vertical scale. If it is not turned all the way clockwise, thescope will be uncalibrated and your data will be worthless. Check this knob frequently asyou take data. (13)

Channel AC-GND-DC Switch

AC -Input coupling with blocking of any DC signal component.

GND -The input signal is connected to ground. This mode is useful in determining thezero reference.

DC -Coupling, both the DC and AC components of the input signal are displayed. (16)

How To Set Up The Time-Base controls

The oscilloscope sweeps an electron beam across the display screen from left to right at aspeed set by the TIMEBASE control (9). The time base control, controls the amount of time the sweeping dot takes to move 1cm; effectively it is setting the scale on the x-axis.The time base control is normally labeled TIME/CM.

When the oscilloscope is set to slow time basesettings (such as 50ms/cm) you can see a dotmoving across the screen but at faster settings(such as 1ms/cm) the dot is moving so fast thatit appears to be a straight static line.

Figure 3

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When a signal is viewed on the scopes display, the period of the signal can be measured by counting the number of horizontal divisions that the signal "covers" on the display andmultiplying by the scale of each division.

Example, if the applied signal "covers" 4 divisions or squares across the screen at 5milliseconds per division, then the signal period is 20 milliseconds and hence thefrequency is 50Hz.

The variable time base control can fine-tune the speed of the sweep, but it must be left inthe calibrated position if you wish to take time readings from the trace drawn on thescreen.

Make sure that X-Y mode is not selected as this disables the time base.

How To Set Up The Amplitude Controls

The amplitude adjustment of the oscilloscope controls how tall the input voltage signalswill appear on the screen. The screen is marked off with horizontal lines to indicate thesignal's voltage. The absolute voltage per horizontal line is adjustable. Thus, if theamplitude is set to 1V/ then inside each block (or 1cm square) the signal is 1volt per division. The purpose of this adjustment is that you can see a very large or a very smallsignal on the same screen.

Figure 4

Example Of Measuring An AC Signal

To produce an alternating signal in this example we will use a signal generator.

Before starting ensure that the entire function generator controls are completely turnedclockwise and all buttons are in an out position.

The amplitude dial on the generator is used to adjust the peak-to-peak voltage of the produced a.c. waveform. This has a range of 0 to 25volts.In this example set this controlto 5volts.

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The function buttons set the type of waveform produced, this being a sine, square, or triangular. Again in this example set this to sine wave.

The frequency and range buttons are used in conjunction with each other for setting thefrequency of the output produced waveform. Set this to 2khz.

Connect the interconnecting bnc leads from the signal generator to the oscilloscope.

Be careful to watch the polarity of the two pieces of equipment (red to red, black to black).

Then set the volts /div buttons, the vertical axis represents the number of volts.

Then set the sec /div or timbase setting (9). The numbers corresponding to the outer dialrepresent the amount of time Per Cm Square in the horizontal direction.

Use triggering level to obtain a steady trace on the display

Position the displayed wave so it lines up with a cm square line marking, count thenumber of cm squares between top and bottom of the trace.mulitpy the number of cmsquares by the volts/cm setting and this will give you the maximum peak voltage.

Position the displayed sine wave trace so that the beginning of the wave lines up with acm squares along the x-axis. Count the number of cm squares along the x-axis of onecycle of the sine wave and multiply by the sweep time. This is the frequency beingmeasured.

Remember that the signal generator is producing the voltage and frequency theoscilloscope is measuring the output.

You should have obtained an oscilloscope trace like the one shown below.

INSERT PICTURE FROM MULTISIM

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How To Connect A Signal Source

The input lead to an oscilloscope should be a co-axial lead and the diagram below showsits construction. The central wire carries the signal and the screen is connected to earth(0V) to shield the signal from electrical interference (noise).

The oscilloscope has two input ports, which are standard BNC sockets (12). The leadswith BNC sockets are connected with a push and twist action, and disconnected with atwist and pull action.

Equipment connected to the oscilloscope is connected just like a voltmeter. The screen or black connection of the input lead is connected to mains earth at the oscilloscope thismeans it must be connected to earth or 0V on the circuit being tested. It is important toremember that the oscilloscope can only take readings of voltages. This means that theoscilloscope should always be connected in parallel to the circuit in which we are tryingto find the voltage off.

To connect theoscilloscope to a signal

source such as a signalgenerator you will needa connecting lead,which is coaxial withtwo bnc connectors ateach end.

Figure 6

Normally the output of the signal generator has more than one output. We nearly always

use the 600ohm output. This is connected to the CH 1 input of the oscilloscope using theBNC to BNC lead.

Also a high-quality connector, called a probe is used as its designed not to influencetesting results of circuits by not loading the circuit being measured.

Switch on the function generator and oscilloscope and allow them to warm up a fewminutes. The rotating frequency control and range switches are used together todetermine the frequency of the output signal.

Construction of a coaxial leadFigure 5

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.

Adjust the output level to produce a visible signal on the oscilloscope screen. Adjust theTIME/DIV and VOLTS/DIV buttons to obtain a clear display.

Calibration

Calibration Of Test Probes

When using the oscilloscope it is very easy just to plug the test probes in and start usingthe scope, this is wrong .The probes have to be calibrated otherwise their response willnot be flat. To calibrate the probes there is a built in sine wave calibrator, basically a builtin signal generator that generates a low frequency square wave (~1kHz).

First of all adjust the focus and intensity to get a nice sharp line on the display. Then setthe input to ac and plug in the probe to the channel you are looking at (i.e. channel 1)Put the probe tip to the calibrator tip and probe ground clip to the oscilloscope ground. Atthis point you should be able to see the square wave displayed on the screen. Thereshould be some sort of screw for adjustments in the compensation box of the probe. Byinspecting the square wave signals in the oscilloscope you can compensate the probe.

Overcompensated or under-compensated probes may produce significant errors in risetime and amplitude.

Figure 7

Standard Probe Kit

Figure 8

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Bibliography / References

Below are books I explicitly referred to in this report all other are bibliography.

B1

B2

B3 https://www.cs.tcd.ie/courses/baict/bac/jf/labs/scope/oscilloscope.html

Figure 1 http://i.ehow.com/images/GlobalPhoto/Articles/4526145/xandyaxis_Full.jpg

Figure 2 http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/scopec.html#c39

Figure 4 http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/scopec.html#c39

Figure 5 http://www.kpsec.freeuk.com/cro.htm

Figure 6 http://www.doctronics.co.uk/scope.htm

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https://www.cs.tcd.ie/courses/baict/bac/jf/labs/scope/oscilloscope.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/scopec.html#c39http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/scopec.html#c39https://www.cs.tcd.ie/courses/baict/bac/jf/labs/scope/oscilloscope.htmlhttp://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/scopec.html#c39http://hyperphysics.phy-astr.gsu.edu/Hbase/Electronic/scopec.html#c39


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Figure 7 http://www.scienceprog.com/oscilloscope-probes-for-accurate-signal-measurements/

Figure 8 Rapid Electronics http://www.rapidonline.com/

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http://www.scienceprog.com/oscilloscope-probes-for-accurate-signal-measurements/http://www.scienceprog.com/oscilloscope-probes-for-accurate-signal-measurements/http://www.scienceprog.com/oscilloscope-probes-for-accurate-signal-measurements/

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Oscillscope Professional Report