*MODERN ACADEMY FOR ENGINEERING & TECHNOLOGYIN MAADI

*ELECTRONIC MEASUREMENTS

Text BooksLarry D. Jones, A. Foster Chin, Electronic Instrumentation & Measurements, 2nd Ed., Prentice-Hall Inc., 1991.David A. Bell, A. Foster Chin, Electronic Instrumentation & Measurements, 2nd Ed., Prentice-Hall Inc., 1997

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Grading PolicySemester Assignments 10 (5)Mid-term Exam 20 (15)Practical Exam 30 (20)Final Exam 90 (60)Total 150 (100)

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*ELECTRONIC MEASUREMENTSCONTENTSIntroductionCathode Ray OscilloscopeSpecial Types of OscilloscopesAnalog VoltmetersDigital VoltmetersFrequency metersSpectrum Analyzers

INTRODUCTIONLife in the 21st century relies heavily on precision measurement, it is at the heart of many critical experiences like:

Satellite navigation systems that depend on ultra stable clocks, as any small error in timing can throw navigation a long way off course.

Medical and food industry where elementary components ordered from different suppliers and interact together .*

Principles for Good Measurements 1. Right tools Measurements should be made using equipment and methods that have been demonstrated to be fit for purpose2. Right people Measurement staff should be competent, properly qualified, and well informed.*

3. Right procedures Well-defined procedures consistent with national or international standards should be in place for all measurements4. Regular review There should be both internal and independent assessment of the technical performance of all measurement facilities and procedures.

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Measurements QualityWhen talking about measurement quality, it is important to understand the following concepts:Precision is about how close measurements are to one another. Thus precision is represented by a cluster of consistent measurements, with no guarantee that they are accurate .

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Accuracy is about how close measurements are to the true value. In reality, it is not possible to know the true value and so we introduce the concept of uncertainty to help quantify how wrong our value might be.Uncertainty is the quantification of the doubt about the measurement results and tells use insight about quality.

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Error is the difference between the measured value and the true value of the variable being measuredTrue value is the value that would be obtained by theoretically perfect measurements .

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*CATHOD RAY OSCILLOSCOPECATHOD RAY TUBEDEFLECTION AMPLIFIERSSWEEP GENERATORAUTOMATIC TIME BASEDUAL TRACS OSCILLOSCOPECONTROLS AND MESURMENTSOSCILL SPECS&PERFORMANCE

*CRO BLOCK DIAGRAM MAIN ITEMS:Power SupplyVertical Deflection AmplifierHorizontal Deflection AmplifierSweep GeneratorCRT

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*Main items of CRT

Electron gun sectionTriode SectionFocusing SectionDeflection SectionPost Deflection Acceleration sectionScreen

*Electrostatic Deflection

*Deflection Amplifier

*Q2&Q3 form an emitter-coupled (differential) amplifier .Q1&Q4 are emitter follower (common collector) transistors that provide high input resistance .R10 DC Shift control resistor ( potentiometer)S1 has three positions: AC,GND & DCS2 is adjusted according to scale of measured signal ( Volt/Div)

*When the input voltage to the attenuator is zero, the base of Q1 is at ground level& Q4 base is also adjusted to ground levelQ2 and Q3 bases are both at the same negative potential with respect to ground (-VB2 = -VB3)Also, IC2 = IC3, and the voltage drops across R3 and R6 set the collectors of Q2 and Q3 at ground level.

*Q2 and Q3 collectors are the amplifier outputs, and they are connected directly to the deflection platesA positive-going input produces a positive-going voltage at Q2 base, and causes IC2 to increase and IC3 to decreaseThe IC2 increase causes output VC2 to fall below its normal ground level, and the IC3 decrease makes VC3 increase above normal ground level If the change in VC2 = -1V, then VC3= +1 V

*The deflection amplifier operation principles are the same for both vertical and horizontal amplifiersThe adjustment potentiometer R10 is used to adjust the DC level for the displayed pattern on the screen as desired by the operator .

*Simple sweep generatorBasic sweep GeneratorAutomatic time base

Oscilloscope Time Base

*Simple sweep generator

*During the sweep time TS the transistor is cut off and the capacitor C is charged through the resistor R, and

Vo= Vcc ( 1 - Exp (- t/RC))

*When the input pulse Vi is applied the base of the transistor it operates on saturation mode.The capacitor C will discharge during the retrace time TrThe output voltage will be :

Vo =Vo Exp (-t /RCEC)Where RCE is the collector-emitter resistor

*Basic sweep generator

*UTP, LTP= Upper, Lower trigger point of Schmitt circuitUTP,LTP = ( Vsat) R6/R5 Vsat (Op Amp saturation Voltage) = (Vcc-1V)

The collector currentI1 ( VB1-VBE) / R3VB1= Base voltageR3, emitter resistor

The peak-to-peak voltage of ramp (2 x UTP or LTP)V1 = I1T/ C1

*Example:

For the sweep generator circuit has R3 = 4.2 K , C1=0.25 F, VB1= 4.9 V, and the trigger levels for the Schmitt are 2 V. Calculate the peak-to-peak amplitude and time period of the ramp waveform. Solution:

V1 = 2 x (UTP,LTP) = 2 x 2 V(upper trigger point, Lower trigger point) = 4 V p-to-p

I1 = (VB1 VBE)/ R3= ( 4.9-0.7)/4.2k = 1 mA T = (V1x C1)/ IC1 = (4Vx 0.25F)/1mA = 1ms

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*Automatic time baseThe block diagram consists of:1. Trigger amplifier2. Schmitt circuit3. Differentiator4. Positive Clipper5. Hold-Off circuit6. Sweep generator7. Blanking circuit

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*The input signal is applied to vertical amplifier and to the trigger amplifierThe output of trigger amplifier is an approximate rectangle waveThe output from the Schmitt circuit is a square waveform exactly in anti-phase with the input wave to be displayed. This square wave is applied to a differentiating circuit. The output produced by the differentiator is proportional to the rate of change of the square wave.

*During the times that the square wave is at its constant positive level or at its constant negative level, its rate of change is zero. At the positive-going edge of the square wave, the rate of change is a large positive quantity. At the negative-going edge, the rate of change is a large negative quantity.the differentiated square wave is a series of positive spikes coinciding with the positive-going edges of the square wave, and negative spikes coinciding with the negative-going edges

*Hold-Off Circuit

*The positive clipper clips the positive spikesIt is seen that the train of negative spikes causes the ramp output of the sweep generator to be synchronized with the input waveform that is to be displayed. The hold-off circuit is a spike-suppressing circuit controlled by the level of the ramp output from the sweep generator. Once a negative spike has synchronized the time base, no more spikes are allowed to pass to the sync input terminal until the ramp output of the sweep generator approaches its maximum amplitude

*Note that the sweep generator output is fed back to the hold-off circuit as well as to the horizontal amplifier.With all spikes suppressed until the ramp approaches its peak level, the ramp is not reset to zero until the electron beam has been swept horizontally to the right-hand side of the oscilloscope screen.Any number of waveform cycles can now be displayed on the oscilloscope screen.

*DC Level control on Triggering Amplifier

*The triggering amplifier is similar to the deflection amplifier. Potentiometer R10 in the triggering amplifier performs a similar function of R10 of DC shift control of deflection Amplifier . By means of this potentiometer, the amplifier dc output levels can be adjusted close to the upper trigger point of the Schmitt.

*The triggering amplifier output reaches the upper trigger point approximately at the same instant that the input wave begins its cycle. Therefore, the displayed wave commences at this instant. Output of the triggering amplifier does not reach the trigger point until some time after the input wave has begun its cycle.

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The displayed wave now commences at this point. It is seen that potentiometer

R 10 functions as a trigger level control, to adjust the instant in time at which the displayed wave commences on the oscilloscope screen.

*Fly Back

*The output from the sweep generator is fed to a blanking circuit . This is the pulse waveform output (V2) from the Schmitt circuit of Basic Sweep Generator The pulse wave is inverted and capacitor coupled to convert it to a train of positive pulses during the sweep time, and negative pulses that occur each time the ramp wave falls from its maximum positive level to its maximum negative level.

*These pulses are fed to the grid of the CRT ,the negative pulses (known as blanking pulses) drive the grid sufficiently negative to suppress the electron beam completely. This means that no electrons strike the screen while the ramp is going from its maximum positive to its maximum negative level.

*If the electrons were not suppressed during this time, every displayed wave would have a horizontal line traced by the electron beam during the fly back time from the right -hand side of the screen to the left . The positive pulses are termed un-blanking pulses, and it is applied to the CRT grid cause electrons to travel from the cathode to the screen.

*Dual Trace Oscilloscope

Dual Trace oscilloscope could be double- beam or single-beam CRT A single-beam CRT, shown in the next slid, has Two input amplifiers (channels A & B) , and only One vertical deflection amplifier feeding one set of vertical deflection plates.The input to the vertical deflection amplifier is alternately switched between channels A and B, and the switching frequency is controlled by the time base circuit.

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*ALT Mode :

*In ALT mode, the CRT alternately displaying one input wave and then the other. In the pervious figure, the input to channel A is a sine wave with time period T, and that to channel B is a triangular wave also having a time period T The two waveforms are in synchronism. Note that channel A input has a dc offset, which puts it above ground level, while channel B is offset below ground.

*Channel A input is switched to the vertical deflection amplifier and traced on the oscilloscope screen during time 0 to t1. Channel B input is next applied to the vertical deflection amplifier and traced on the oscilloscope screen during time t1 to t2 The dc offsets on the inputs cause the waveform on channel A to be traced on the top half of the oscilloscope screen and that on channel B to be traced on the bottom half of the screen.

*During the next cycle of the time base channel A input is again traced on the screen, followed by channel B input once again thus, the two inputs are alternately and repeatedly traced on the screen. The repetition frequency is usually so high that the waveforms appear to be displayed simultaneously. When the method described above is employed to display two waveforms, the oscilloscope is said to be operating in alternate (ALT) mode.

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*CHOP Mode:

The Figure below illustrates oscilloscope operation in the chop mode Channel A input is traced for a short time, t1 Channel B input is traced for time t2. Back to channel A input for t3, then channel B for t4, and so on, as illustrated. High-frequency waveforms would be displayed as dashed lines, exactly as shown in Figure below. However, the breaks in the traced waveforms are of such short duration that they become invisible when medium-and low-frequency waves are displayed.

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*Dual-trace oscilloscopes that employ channel switching normally permit selection of operation in either alternate mode or chop mode. For high-frequency inputs the alternate mode is best, because the waveform traces appear continuous rather than broken. Using the chop mode with low-frequency inputs results in both waves being displayed continuously. The breaks in each trace are so short that they cannot be seen.

*Oscilloscope Controls

*Voltage & Frequency Controls

*Voltage & Frequency Measurements

Voltage MeasurementsVP-P = (VP-P Divs/Cycle) X (Volts/Div)

For Signal AV = (4.6 Div) x 100 mV = 460 mV

For Signal BV = (2 Div) x 100 mV = 200 mV

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Frequency MeasurementsT = (Horizontal Divs/Cycle) X (Time/Div)For Signal AT = (8.8/2) Div x 0.5 ms = 2.2 msF = 1 / 2.2 ms = 455 Hz

For Signal BT = (8.8/6) Div x 0.5 ms = 0.73 msF = 1 / 0.73 ms = 1.36 kHz

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*Phase Measurements

Each wave time period (cycle) = 8 Div. 8 Div = 360 , 1 Div = 360/ 8 = 45 The time between the commencement of each cycle is 1.4 Div.Thus the phase difference is : = 1.4 x 45 = 63

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*Pulse Measurements

If the vertical sensitivity is 0.1 V/DivUpper Wave Amplitude PA = 2 X 0.1 = 0.2 VLower Wave Amplitude PA = 2.4 X 0.1 = 0.24 V

If the horizontal sensitivity is 1 s/DivUpper Wave : Pulse Width Pw = 4.5 s Space Width Sw = 3.5 sWave Period T = 8 s

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Lower Wave

Rise Time: time required for the leading edge to rise from 10% to 90% of the pulse amplitude Tr = 0.7 sFall Time: time required for the lagging edge to fall from 90% to 10% of the pulse amplitude Tf = 0.9 sDelay Time: time elapsed from the commencement of the input pulse until the output pulse reaches 10% of the pulse amplitude Td = 1 s*

*Oscilloscope 1:1 Probe

The central conductor carries the input signal, and the circular conductor is grounded to prevent unwanted noise.1:1 probe does not contain resistors to attenuate the input signalthe input impedance of the oscilloscope is typically 1 M 30 pFThe coaxial cable adds a capacitance Ccc, typically of 100 pF

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For minimum attenuation and phase shift (overloading), the total impedance of the measuring instrument should be much larger than the signal source impedance (circuit under test)At frequencies when the reactance of (Ccc + Ci) is much larger than (Rs + Ri), the capacitances have negligible effect, and the oscilloscope terminal voltage Vi = Vs (Ri / Rs +Ri) For Ri is much larger than Rs , Vi = Vs

(no attenuation)*

Oscilloscope 10:1 ProbeAttenuator probe includes 9 M resistor in series with the input terminal, there for it offers a much larger impedance for the measuring device, and attenuates the input signal by a factor of 10. 10:1 probes usually have an adjustable capacitor to compensate for the coaxial cable capacitance Ccc, and the input capacitance CiThe input resistance and input capacitance vary from one oscilloscope to another, so it is very important that every probe has to be calibrated when first use .

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X-Y and Z DisplaysWhen the oscilloscope time base is disconnected and sine waves are applied to both horizontal and vertical inputs, the resulting display depends on the relation between the two sin waves.Very simple displays occur when the wave forms are equal in frequency, and quit complex figures may be produced with different frequency waves.In all cases these are known as Lissajou figures.

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Vertical input= sine wave & Horizontal input= Zero

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Vertical input= 0 & Horizontal input= sine wave

*Two Waves having the same frequency and in phase

*Two Waves having the same frequency but anti phases

*Two Waves having the same frequency and 90 phase difference

*Two Waves having the same frequency with phase difference 0

*Two Waves having the same Phase but different frequencies (f1:f2 = 2 :1)

In case of the vertical input frequency is twice the horizontal input frequency: One cycle of the horizontal input causes the electron beam to travel from the center of the screen to the RHS, then back through the center to the LHS, then back to the center again. During this time the vertical input deflects the beam from center, up, down, back to center, then up, down, and back to center once more.The ratio of the vertical frequency (f1) to the horizontal frequency (f2) can be determined from the display as follows: f1/f2 = number of positive peaks / number of RHS peaks

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*Two Waves having the same Phase but f1:f2 = 3 :2

Some oscilloscopes have a rear connector for intensity modulation inputs.This usually termed Z-axis modulation, because its input modulates the voltage of on the oscilloscope grid.The Z input drives the grid voltage more negative during each negative half cycle of the modulation wave, therefore dims or completely blanks the trace intensity. This makes a number of gaps on the displayed figure that equal to the ratio of modulating frequency to deflecting plate frequency Fm/Fp

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