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ELECTRICAL TESTING

EQUIPMENT (ETE-5)

Learner Guide

TABLE OF CONTENTS

ABOUT THIS PROGRAMME PAGE 1 INSTRUCTIONS FOR SELF-STUDY PAGE 2 PART 1: SET-UP AND ADJUSTMENT PROCEDURES SECTION 1:USER CONTROLS PAGE 3 SELF-TEST 1 PAGE 10 SECTION 2: PROBE FREQUENCY RESPONSE PAGE 12 PART 2: SINGLE CHANNEL MEASUREMENTS SECTION 1: VOLTAGE CALIBRATION PAGE 16 SELF-TEST 2 PAGE 22 SECTION 2: AC & DC SIGNALS (AMPLITUDE) PAGE 23 SELF-TEST 3 PAGE 28 SECTION 3: AC & DC SIGNALS (TIME PERIOD) PAGE 31 SELF-TEST 4 PAGE 37 SECTION 4: MARK-SPACE PAGE 38 SELF-TEST 5 PAGE 43 PART 3: DUAL CHANNEL OPERATION OF THE OSCILLOSCOPE. SECTION 1: PURPOSE OF USER CONTROLS PAGE 44 SELF-TEST 6 PAGE 48 SECTION 2: PHASE DISPLACEMENT PAGE 50 SECTION 3: FUNCTION OF USER CONTROLS PAGE 56 SELF-TEST 7 PAGE 61 SECTION 4: SCOPE & EQUIPMENT PAGE 63 SELF-TEST 8 PAGE 67 MODEL ANSWERS SELF-TEST 1 PAGE 70 SELF-TEST 2 PAGE 72 SELF-TEST 3 PAGE 73 SELF-TEST 4 PAGE 74 SELF-TEST 5 PAGE 75 SELF-TEST 6 PAGE 76 SELF-TEST 7 PAGE 77 SELF-TEST 8 PAGE 78

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ABOUT THIS PROGRAMME This series on the Analogue Oscilloscope has been designed to assist you, as the learner Electronics Technician to do the following: Identify and describe the purpose of all adjustable controls. Prepare an oscilloscope for practical applications. Use the scope to accurately measure the voltage, period and phase difference of

various types of waveforms. Identify hazardous measuring situations. Basic care of the oscilloscope.

The programme consists of three video modules namely: ETE-5 Part 1 Set-up and adjustments of the oscilloscope and probe. ETE-5 Part 2 Single channel operation including the measurement of amplitude,

period, pulse width and duty cycle. ETE-5 Part 3 Dual channel operation including the measurement of phase,

triggering techniques, safety hints when measuring and care of the instrument.

Accompanying the video programme is the WORKBOOK which is intended to guide you through each section of the programme. Please read through the workbook before viewing the videos as this will assist you to fully understand the subjects discussed.


INSTRUCTIONS FOR SELF STUDY The general learning procedure for effective Learner Controlled Instruction is: To read through the "resource notes". To follow any instructions given at the foot of each page in this workbook. View the relevant video programme. Have your Course Controller assess your competency.

This is usually done by way of a Criterion Test, devised by your training department, or the governing body for your particular trade.

You may view the videos as many times as you feel necessary in order that you fully understand or "see" how a particular task is performed.


ETE-5 PART 1 SECTION 1 SET-UP & ADJUSTMENT PROCEDURES

OBJECTIVE At the end of this section you will be able to identify by name and describe the

purpose of all the user controls on a typical 20 MHz oscilloscope of popular make and design.

Prepare the instrument for practical applications by: o Adjustment of the mains voltage selector. o Adjustment of operator controls before "switching on" the scope. o Adjustment of operator controls after the scope has been powered up.

WHAT RESOURCE WILL YOU NEED?

o This workbook. o The video programme No. ETE-5 Part 1. o A 20 MHz oscilloscope. o A suitable 220 V AC power source.

HOW WELL MUST YOU PERFORM? Evidence that you have understood the subject matter to the required standard is obtained by:

o Achieving a YES response / correct answer to all the criteria set out on a criterion mark sheet.

o Approval from your Facilitator or Course Controller.

PROCEED BY READING THROUGH THE NOTES ON THE FOLLOWING PAGE.


SET-UP & ADJUSTMENT PROCEDURES

1.1 INTRODUCTION TO THE ANALOGUE OSCILLOSCOPE The oscilloscope is a precision measuring instrument used extensively in the design, testing and fault-finding of most types of electrical and electronic equipment. Unlike an analogue or digital multi-meter, the oscilloscope or "scope" as it is often called, will actually trace out a picture of the voltage under test by rapidly plotting the changes in voltage over a pre-set period of time.

By examining the behaviour of the "patterns" or "waveforms" displayed on the cathode ray screen, calculations can be made concerning the signals under test. When you can read and understand the patterns displayed on the screen, your oscilloscope becomes an invaluable piece of test and measuring equipment. The most obvious feature of an oscilloscope is the cathode ray screen. It is on this screen that the visual wave patterns are displayed. You need not be concerned how the cathode ray screen works for you to be able to use the instrument.

o The screen is divided into squares by vertical and horizontal lines. o The distance between any two lines is called a division, and each division is

sub-divided into five smaller divisions.


o There are ten horizontal divisions from left to right and eight vertical divisions from top to bottom of the screen.

o The square divisions collectively, are known as the graticule.

The graticule provides visual reference points to indicate voltage or amplitude variations on the up/down axis, and measurements of time or period on the horizontal or left-to-right axis.

1.2 MAINS VOLTAGE SELECTOR Most oscilloscopes are mains powered and it is most important to ensure that the voltage selector is correctly set to match the system supply voltage. In our demonstration, the "voltage selector" has been set to for a 220 volt AC supply which is acceptable for equipment operating in the RSA.

1.3 CONTROL PANEL ADJUSTMENTS An understanding of the function and set-up of these controls is essential for the correct operation of the scope.


The front panel is divided up into four groups of controls, namely: o The screen controls. o The vertical deflection or input amplifier controls. o The trigger controls, and o The horizontal deflection or time-base controls.

Let's explain the basic function of these controls. PRE-SETTING THE SCREEN CONTROL GROUP

The "power switch" controls the power supply to the scope, set it to the "off'

position. the "intensity" or brightness control sets the intensity of the electron scanning beam

as it moves across the screen, turn it fully counter-clockwise to the minimum position.

the "focus" control sets the sharpness of the beam, set the control to its mid-position.

and the "trace rotation" control sets the "tilt" of the beam, We'll adjust this control at a later stage in the set-up routine.

PRE-SETTING THE INPUT AMPLIFIER CONTROL GROUP OR "VERTICAL" GROUP With this group of controls, we are able to adjust the movement of the beam "up and down" the screen.


Most modern scopes feature two input channels, so you will notice that the vertical group consists of two identical input amplifiers called "channel 1" and "channel 2". To position the beam in the centre of the screen, set both "vertical position"

controls to mid-position. Initially we wish to view only one trace (of the beam) therefore, set the "mode

control" to the "channel 1" position. No magnification of the time scale is required at this stage, therefore set the "X1,

X10" magnification switch to the "X1" position. Set the "normal / invert" switch to the "normal" position.

To pre-set the voltage range of the input circuits, set both "volts/division" controls

fully counter-clockwise, and ensure that both "variable volts/division" controls are turned fully clockwise to the "calibrate" position.

Controls must be calibrated to ensure accuracy of readings. Finally, set the "input coupling" switch on both channels to the "AC" position. This

control sets the type of coupling introduced between the scope and the signal to be measured.

NEXT THE "TRIGGER" CONTROL SECTION Owing to the complexity of the trigger controls, we'll deal with them in more detail later.

For the moment: Set the trigger "level" control to its mid-position. Set the trigger "mode" selector switch to the "auto" position. Set the trigger "slope" selector button to the "positive slope" position.


And finally set the trigger “source" selector switch to the "internal / channel 1" position.

THE HORIZONTAL DEFLECTION OR TIME-BASE CONTROL GROUP This group of controls sets the movement of the beam from "left to right" across the face of the screen.

Set the "magnification" switch to the "X1" position. Set the "horizontal position" control to mid-position. Set the "calibrate/variable" switch to the "calibrate" position. Set the "variable" control to its mid-position. And finally, set the "time / division" switch to the "5 millisecond" position.

ADJUSTMENTS TO BE MADE AFTER POWER UP Once we have finished with these basic adjustments, the scope is ready to be connected to the mains supply. Ensure that the connection is made using the manufacturers recommended power lead.

o Once connected, push the "power on / off” switch and check that the pilot light is glowing.


o Wait for about 30 seconds for the scope to "warm-up", and then adjust the "intensity" control turning it clockwise until the trace is clearly visible, but not too intense as a highly intensified trace could damage the internal surface of the screen.

o The next step is to adjust the "focus control" to obtain a sharp, clear trace. o Now using the "channel 1 vertical position control", position the trace so that it is

superimposed on the horizontal centre line of the graticule. o Adjust the “horizontal position” control so that the beginning of the trace aligns

with the left hand edge of the left-hand-side graticule. o If, after having made this adjustment, the trace is not parallel with the horizontal

centre line of the graticule, turn the “trace rotation control” until the trace is parallel to the centre line.

o When making this adjustment, be sure to use the correct adjustment tool so as not to damage the control.

THIS COMPLETES THE CONTROL PANEL ADJUSTMENTS You are now ready to watch the video up to Review Break No. 1. Please ensure that you are in possession of Tech AV video entitled ETE-5 / PART 1. When you have watched the video up to review break No.1,practice the set-ups and adjustments you have learnt using a typical 20 MHz scope. You may use the workbook to help you remember the sequence of adjustments. Before continuing with the next section of the programme, complete the Self-Test Exercise No. 1 over the page.

TURN OVER THE PAGE FOR SELF-TEST EXERCISE NO. 1.



SELF TEST EXERCISE NO. 1

QUESTION YES NO

1. Complete this sentence. Unlike an analogue or digital multi-meter, the oscilloscope or "_______________" as it is often called, will actually _________________ out a picture of the voltage under test by rapidly plotting the changes in ________________ over a pre-set period of __________________.

2. Complete this sentence. The most obvious feature of an oscilloscope is the _____________________. It is on this screen that the visual wave patterns are displayed.

3. The screen is divided into squares by horizontal and vertical lines. What do these divisions represent? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

4. The main divisions are sub-divided. How many sub-divisions in one main division? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

5. What is the value of one division? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

6. What is the value of each sub-division? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________


7. Complete the sentence. The graticule provides visual reference points to indicate voltage or amplitude variations on the _____________, and measurements of time or period on the ______________________ or left-to-right axis.

8. Draw the graticule which is attached to the face of the cathode ray screen.

9. Why must the oscilloscope controls be calibrated before taking a measurement? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________


ETE-5 PART 1 SECTION 2 PROBE FREQUENCY RESPONSE / COMPENSATION ADJUSTMENT

OBJECTIVE At the end of this section, you will be able to identify by name and describe the "make-up" of a typical oscilloscope measuring probe with internal attenuator. WHAT RESOURCE WILL YOU NEED?

o This workbook. o The video programme No. ETE-5 Part 2. o A 20 MHz oscilloscope and a measuring probe.




PROCEED NO BY READING THROUGH THE NOTES BEGINNING ON THE NEXT PAGE.


PROBE FREQUENCY RESPONSE / COMPENSATION ADJUSTMENT

1.5 INTRODUCTION TO THE MEASURING PROBE The probe enables us to couple the signal to be investigated / measured to the input of the scope. A typical oscilloscope measuring probe consists of a length of coaxial or "screened" cable designed to shield the test signal from electrical interference. The cable is terminated at one end with a BNC connector and at the other end by the probe itself. The tip of the probe has a spring loaded retractable hook tip which attaches it to the test or measuring point. Attached to the body of the probe is a "ground" lead terminated with a "crocodile" clip. This clip is connected to the "ground" or "earth" side of the equipment under test (special precautions are required for "hot chassis" equipment -see later).

THE "X1 / X10 PROBE ATTENUATOR CIRCUIT The probe assembly also incorporates a built in switchable X1-X10attenuator circuit which when selected, reduces the amplitude of the input signal by a factor of 10. How do we utilise this facility? Well, let's look at this example. The oscilloscope manufacturer tells us that the maximum applied voltage presented to the input circuit of the scope must not exceed 250 volts peak to peak, however we wish to measure a signal with an amplitude in the order of 500 volts p/p. By setting the attenuator on the probe to "X10", the signal applied to the scope is effectively reduced to 500/10 = 50 volts peak to peak (complies with manufacturers recommendations). Remember these three important points when using a probe.

When the magnitude of the voltage to be measured is unknown, always set the attenuator to the "X10" position.


Ideally the performance of a circuit to be measured should not be affected when the measuring probe is connected to it. Use the probe with the "X10" facility enabled to prevent the scope input circuits from "loading" the circuit under test. When calculating the voltage actually measured, it will be ten times greater than the voltage indicated by settings on the scope controls when the probe is set to "X10".

1.6 SETIING THE PROBE FREQUENCY COMPENSATION The probe must faithfully transfer the signal under test to the input circuits of the scope without distortion. To ensure this condition is met, always check the frequency response and calibration of the probe. Most scopes have a "calibration" facility for this purpose, By feeding this "square wave" test signal into the probe and then viewing the output on the scope, we can ensure that the probe is correctly set-up, PROCEDURE

o Begin by connecting the BNC connector to the "channel 1" input of the scope. o Connect the tip of the probe to the "probe adjusts" test point located within the

horizontal time-base section of the control panel. o Set the attenuator on the probe to "X10". o Set the "input coupling" switch to "DC". o Set the "channel 1 volts/division" control to the 1 0 millivolt position, set the "time /

division" control to the 0,2 millisecond position, and adjust the "trigger level" control if necessary to stabilise the trace.

o The picture or waveform on your scope should resemble a "square wave". o If the tops and the bottoms of the waveform are tilted or peaked, a frequency

compensating adjustment must be made to the probe.

Using a suitable insulated trimming tool, adjust the "capacitance correction trimmer" located inside the body of the BNC connector, until the top and bottom edges of the square wave are flat.


This completes the set-up and adjustments. You are now ready to watch the video to the end of ETE-5 Part 1. When you have watched the video to the end, practice the set-ups and adjustments you have learnt using the scope and measuring probe. You may use the workbook to help you remember the sequence of adjustments.

END OF ETE-S PART 1.



ETE-5 PART 2 SECTION 1 SINGLE CHANNEL MEASUREMENTS

OBJECTIVE At the end of this section you will be able to check the calibration of the voltage

attenuator in the measuring probe using the "probe adjust" calibration test signal from the scope.

Correctly measure the amplitude of an instantaneous peak-to-peak voltage. WHAT RESOURCES WILL YOU NEED?

o This workbook. o The video programme No. ETE-5 Part 2. o A 20 MHz oscilloscope. o A measuring probe. o Sine / square / triangular signal generator.




PROCEED NOW BY READING THROUGH THE NOTES BEGINNING ON THE FOLLOWING PAGE.


SINGLE CHANNEL MEASUREMENTS

HOW TO CHECK THE VOLTAGE CALIBRATION OF THE PROBE In this next section of the programme, we will discuss trace patterns and the measurement of amplitude using the scope as a single channel measuring device. Remember that the amplitude of a signal is displayed by a deflection of the trace up-and-down the screen and is expressed as a Voltage.

Most oscilloscopes feature an internal voltage generator (square wave of 0.5 volts p/p at a frequency of 1000 Hz) to check / set-up the performance of the probe. We'll use this signal source called the "probe adjust" facility to check the accuracy of the voltage attenuator of the probe.

PROCEDURE In preparation to measure the "calibrate" voltage, switch the input coupling switch

to "ground". This will aid us in establishing a ground reference point. Attach the probe to the "probe adjust" test point and the ground lead to the

adjacent "earth" point. Set the attenuator on the probe to "X10". Adjust the "vertical position" control so that the trace is superimposed on the

second lowest horizontal graticule line (our point of reference i.e. zero volts). Now we can measure the actual voltage applied to the scope probe.

Switch the "input coupling" switch to "DC" and count the number of divisions from the bottom to the top edge of the square wave.

Multiply the number of divisions by the "volts / division" setting to calculate the voltage of the square wave (the setting is 10mV/div.).


o The calculation is as follows: 5 divisions X 10 mV/div. = 50 mV.

o Your answer should be 50 millivolts, but as the probe attenuator is set to "X10" mode, the actual voltage measured is 500 millivolts or 0,5 of a volt. Since the "probe adjust" test voltage is specified as 0,5 volts, the performance of the probe acceptable.

NOW TO TEST THE X1 FACILITY OF THE PROBE Switch the probe attenuator to "X1" and adjust the "volts / division" control until

the waveform fills the screen again. Confirm that the signal voltage reading is 0,5 volts peak to peak. The calculation is:

5 divisions X 0.1 volt / div. =0.5 volts. Remember when taking accurate voltage measurements, ensure that the "variable volts / division" control is in the "calibrate" position, otherwise the readings will be inaccurate. TO SUMMARISE The "probe adjust" facility is a signal generator of known voltage. We connect the probe to this voltage source and visually display the amplitude of

this voltage on the cathode ray screen. We accurately measure the voltage and compare input to output levels with and

without the attenuator. In each case, the scope probe faithfully transmits the "test voltage" without

distortion or level change. Let's continue as we look at two general types of voltage measurement, namely:

o "Instantaneous peak-to-peak" values where the exact voltage is measured from each and every point with respect to a ground reference, or


o “Peak-to-peak" values where the total amplitude between two extremes is measured without regard to a polarity reference.

2.2 TO MEASURE THE INSTANTANEOUS PEAK-TO-PEAK VOLTAGE Let's look at the procedure.

The scope is fed with a square wave signal derived from the test signal generator.

Switch the "input coupling" switch to "ground" and adjust the "vertical position" control so that the trace is superimposed on a suitable horizontal graticule line. This sets our zero volts reference!

Switch the coupling switch to "DC" and using the "volts/division" control, adjust the vertical deflection so that the complete waveform fills the screen.

Adjust the "time / division" control to display 2 - 3 cycles of the waveform. Adjust the "horizontal position" control so that the upper trace intersects

with the vertical centre line. Now count the number of divisions, and sub divisions between the upper

and lower edges of the square wave. Calculate the instantaneous voltage of the square wave by multiplying the setting of the "volts / division" control by the number of divisions counted.


The calculation is: 6 divisions x 2 volts / division =12 volts.

o Owing to the regular repetitive pattern of this square wave, the instantaneous value of the waveform is either 0 volts or 12 volts.

o Complex waveforms will have many different instantaneous voltage values as this diagram shows.

TO SUMMARISE The instantaneous peak to peak voltage is the exact voltage measured from any point on the waveform to a ground reference. This completes the probe set-up and measurement of instantaneous peak-to-peak voltage. You are now ready to watch the video up to Review Break No. 1. Please ensure that you are in possession of Tech AV video entitled ETE-5 / PART 2. When you have watched the video up to review break No. 1, practice the measurement procedures you have learnt using a typical 20 MHz scope, probe and signal generator. You may use the workbook to help you remember the sequence of adjustments.



Before continuing with the next section of the programme, complete the Self-Test Exercise No. 2 over the page.

TURN TO THE NEXT PAGE FOR SELF-TEST EXERCISE NO. 2.



QUESTIONS YES NO

1. Why is it necessary to check the voltage calibration of the probe? ANS: _________________________________________________ _____________________________________________________

2. Where would you derive a suitable test signal for checking and setting-up the measuring probe? ANS: _________________________________________________ _____________________________________________________

3. Why is the "input coupling" switch set to "ground" when preparing to take a measurement? ANS: _________________________________________________ _____________________________________________________

4. Why is it necessary to ensure that the "variable volts/division" control is in the "calibrate" position. ANS: _________________________________________________ _____________________________________________________

5. To measure the amplitude of a voltage from the screen display we must follow two steps -what are they? ANS: _________________________________________________ _____________________________________________________



OBJECTIVE At the end of this section you will be able to identify various AC and DC signals and determine how their amplitude (expressed as a voltage) is measured using the oscilloscope i.e.: Measuring the peak -peak voltage. Measuring the peak only value. Calculating the root mean square value. Calculating the average value. Measuring voltages with both an AC and a DC component.

WHAT RESOURCES WILL YOU NEED?

o This workbook. o The video programme No. ETE-5 Part 2. o A 20 MHz oscilloscope. o A measuring probe. o A sine wave signal generator.




PROCEED BY READING THROUGH THE NOTES ON THE NEXT PAGE.


SINGLE CHANNEL MEASUREMENTS 2.3 MEASURING THE PEAK TO PEAK VOLTAGE OF A SINE WAVE We continue with voltage measurements, but this time we will measure the amplitude (expressed as a voltage) of a sine wave derived from the signal generator.

To prepare the scope to read a peak-to-peak voltage, switch the "input coupling" switch to "ground" and superimpose the trace on the horizontal centre line using the "vertical position" control. This sets the reference position for the trace.

Switch to "input coupling" switch to the "AC" position and adjust the "volts / division" control so that the entire waveform fills the screen.

Note that when AC coupled, the waveform will be symmetrical about the reference position.

Now switch the coupling switch to "DC" and observe the waveform. Notice that the waveform remains symmetrical about the horizontal centre line. This tells us that the waveform is an AC voltage with no "offset" or DC component.

Here is the procedure to measure the peak to peak voltage.

To measure the voltage:

Adjust the "time / division" control to display 2 to 3 cycles of the waveform. Adjust the "vertical position" control to position the negative peak of the

sine wave on a convenient horizontal graticule line. Adjust the "horizontal position" control so that the vertical centre line of the

graticule passes through one of the positive peaks nearest the centre line. Now count the number of divisions, and sub-divisions between the negative

and positive peaks of the sine wave.

To calculate the peak-to-peak amplitude, multiply the setting on the "volts / division control" by the number of divisions counted. The calculation is: 4 divisions x 0.5 volts/division = 2,0 volts peak-to-peak.


2.4 MEASURING THE PEAK ONLY VALUE It is sometimes necessary to express the voltage of a sine wave as a "peak" only value. This is done by dividing the "peak to peak" value by two. Therefore that the peak value will be half the amplitude of the peak-to-peak voltage.

o Usually a sine wave is symmetrical which allows us to divide the peak-to-peak in half, or measure the amplitude of either the positive or negative peak, to establish the "peak" value.

In this example, the peak value V peak =2/2 =1.0 volts.

2.5 CALCULATING THE ROOT MEAN SQUARE VALUE An AC voltage is often expressed as an rms. or root-mean-square value. To calculate the r.m.s. value multiply the peak value of the sine wave by the mathematically derived conversion factor of 1/ or 0,707. This simply means that the r,m,s. value is 70.7% of the peak value.

The calculation is 1.0 X 0,707 = 0,707 volts r.m.s.


o Note that the r.m.s. value of the AC voltage is equivalent to a DC voltage which will produce the same heating effect.

2.6 MEASURING THE AVERAGE VALUE OF THE SINE WAVE. Finally, the average value of the AC voltage is calculated as 63,7% of the peak value.

The calculation is: 0,637 X 1,4 = 0,9 volts.

The average value is sometimes required but usually the rms. value is more often required for circuit analysis and performance. 2.7 MEASURING VOLTAGES WITH AN AC AND A DC COMPONENT

In preparation, switch the "input coupling" switch to "ground" and superimpose the trace on the horizontal centre line of the graticule (sets reference point).

Set the "input coupling" switch to "AC" and adjust the "volts/division" control so that the waveform fills the screen.

Now switch the "input coupling" switch to "DC" and observe the waveform. Notice that it has shifted upwards and off the screen. Bring the waveform back into view by adjusting the "volts / division" control.

o A typical example of this type of waveform would be seen at the output of a

DC power supply with insufficient smoothing.

The output would indicate a DC voltage with an AC ripple signal riding on top of the DC level.


It is good measuring practice to always check for an AC and a DC component. This completes the section on voltage measurement. You are now ready to watch the video up to Review Break No. 2. Please ensure that you are in possession of Tech AV video tape entitled ETE-5 / PART 2. When you have watched the video up to review break No. 2, practice the measurement procedures you have learnt using a typical 20 MHz scope. You may use the workbook to help you remember the sequence of adjustments. Before continuing with the next section of the programme, complete the Self Test Exercise No. 3 over the page.

TURN OVER THE PAGE FOR THE SELF-TEST EXERCISE NO. 3.




QUESTIONS YES NO

1. Before taking a voltage measurement, why do we switch the "input coupling switch" to the "ground" position? ANS: ________________________________________________ _____________________________________________________

2. Complete the following sentence: To calculate the voltage of the signal being measured by the scope, _____________________ the setting on the ____________________________ by the number of divisions counted.


3. If the peak-to-peak level of a sine wave is 10 volts, what is the value of the peak only voltage? Draw the waveform.

4. If the amplitude of the signal measured is 1 volt peak-to-peak, what is the rms. value of the measured voltage? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

5. If the amplitude of the signal measured is 100 volt peak-to-peak, what is the average value of the measured voltage? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________


6. Why is it best to check for both an AC and a DC component when measuring an unknown voltage? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________



OBJECTIVE At the end of this section you will be able to identify various AC and DC signals and

determine how their time period (expressed as a frequency) is measured using the oscilloscope i.e.:

Measuring the period and frequency of a square wave. Measuring the period and frequency of a sine wave. Measuring the period and frequency of a triangular wave.


o This workbook. o The video programme No. ETE-5 Part 2. o A 20 MHz oscilloscope. o A measuring probe. o A sine / square / triangular wave signal generator.




PROCEED NOW BY READING THROUGH THE NOTES ON THE FOLLOWING PAGE.



INTRODUCTION The oscilloscope is not only able to measure the amplitude of DC and AC voltages, but also the period or repetition rate of the voltage, as each division on the horizontal axis represents a known time interval as defined by the "time / division" setting. A fixed DC voltage is one whose voltage and polarity remains unchanged within a given period of time, e.g. a battery,

whereas the voltage produced by a signal generator is continually changing in amplitude and polarity many times per second. Using the scope, we can observe and measure these changes and determine the frequency at which they occur. Let's explain the measurement procedures. 2.8 MEASURING THE PERIOD AND FREQUENCY OF A SQUARE WAVE The square wave derived from a signal generator.

To prepare the scope, switch the "input coupling" switch to "ground" and superimpose the trace on the horizontal centre line using the "vertical position" control. This sets the reference position for the trace.

Switch to "input coupling switch to "AC" and adjust the "volts / division" control so that the waveform fills the screen. The trace is now easily visible.

Set the "time / division" control to display 2 -3 cycles. Check that the horizontal time-base calibration switch is set to the "calibrate"

position. This will ensure that the reading taken from the "time / division" control is

the correct value.


Using the "horizontal position" control, position the 'leading edge" of the square wave so that it is co-incident with a left hand side vertical graticule line. This sets our reference point for the time measurement.

Count the number of divisions from here to the next "leading edge" of the

square wave, i.e. where the waveform begins to repeat itself again. Multiply the number of divisions by the setting on the "time/division"

control. The calculation is:

5 divisions X 0.2 milliseconds / division = 1 millisecond (1 mS). Therefore the "period" of this square wave is 1 milli-second which means

that the time taken to complete one full cycle of the waveform is 1 millisecond.

Having determined the period, we can calculate the frequency of this signal. The frequency of a waveform is defined as the number of full periods completed or traced out in one second and is measured in cycles / second or Hertz (Hz.). Frequency is expressed as the reciprocal of time as the formula suggests.

frequency = Hz.

Now, the time period to complete one full cycle is 1 millisecond, therefore the frequency is the reciprocal of 1000 = 1 kilo-Hertz.


It helps when working with these types of calculations to use the correct multipliers and dividers. Here are the ones you will most likely encounter: milli 10-3 kilo 103

micro 10-6 mega 106

nano 10-9 giga 1010

pico 10-12

2.9 MEASURING THE PERIOD AND FREQUENCY OF A SINE WAVE Let's look at another example where the input signal is a sine wave.

Set the scope to display 2 -3 cycles of the waveform using the adjustment procedures described in previous sequences.

Position the wave so that the positive going zero crossing point intersects with a suitable vertical graticule line.

Once again, count the number of divisions from this point to the next

positive going zero crossing point. o The horizontal displacement represents the time taken for one complete

cycle of the waveform to be traced out on the screen. Now apply the formula to calculate the frequency of the sine wave. 6

divisions X 0.1 millisecond per division = a period of 0.6 milliseconds. Frequency =the inverse of time.

frequency = Hz.

1/0,0006 cycles / second or 1666 Hertz or 1,666 kHz.

TO SUMMARISE Time measurements can be made commencing at any convenient point on

the waveform, as long as the time taken to complete one full cycle is accurately measured, and


Use the sub-divisions on the centre graticule line to assist in making an accurate measurement of time.

o When very high frequency signals are analysed, use the horizontal magnification switch to "stretch" the signal so that it is clearly visible on the screen which will consequently make it easier to accurately read the waveform.

2.10 MEASURING THE PERIOD AND TIME OF A TRIANGULAR WAVE

o The measurement of period is taken from any convenient point on the waveform to a point where the waveform repeats itself.

o Suitable measuring points exist between consecutive positive or negative peaks.


To measure the period, adjust the horizontal time-base to display 2 -3 cycles. Adjust the horizontal position of the waveform so that a positive peak

coincides with a convenient vertical graticule line. The measurement of period is taken from any convenient point on the

waveform to a point where the waveform repeats itself. Suitable measuring points exist between consecutive positive or negative

peaks. Adjust the vertical position of the wave so that the tips of the positive peaks

touch the horizontal centre line. The period is determined by counting the number of divisions and

subdivisions between adjacent positive peaks.

The number of divisions counted equals 7,6.

Now we multiply the number of divisions by 10 micro seconds, the setting on the "time / division" control. This gives a period of 76 S (micro seconds) which translates to a frequency

of 13.15 kHz. This completes the section on measuring time and frequency. You are now ready to watch the video up to Review Break No.3. Please ensure that you are in possession of Tech AV video entitled ETE-5 / PART 2. When you have watched the video up to review break No.3, practice the measurement procedures you have learnt using a typical 20 MHz scope. You may use the workbook to help you remember the sequence of adjustments. Before continuing with the next section of the programme, complete the Self- Test Exercise No. 4 over the page.

TURN OVER THE PAGE FOR THE SELF-TEST EXERCISE NO. 4.




QUESTIONS YES NO

1. What do you understand when we measure the "period" of a waveform? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

2. What do you understand about the term "frequency" of a waveform? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

3. Can you write out a formula to calculate the frequency of a waveform? ANS: _________________________________________________ _____________________________________________________

4. When making accurate measurements of time using the scope, what must you ensure with regard to the time / division controls? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

5. Assuming that the scope has been set up to measure the period of a waveform, how would you determine the time taken to trace out one complete cycle? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________



OBJECTIVE At the end of this section you will be able to: Identify the mark-space timings of a square wave. Measure the duration of the "mark" and the "space". Calculate the "mark-space ratio" and "duty cycle" of the waveform. Measure and calculate the rise and fall time of a typical square wave signal.


o This workbook. o The video programme No. ETE-5 Part 2. o A 20 MHz oscilloscope. o A measuring probe. o A square wave signal generator.




PROCEED NOW BY READING THROUGH THE NOTES ON THE FOLLOWING PAGE.



INTRODUCTION In the study and design of digital electronic circuits, we find switching waveforms which are either "on" or "off" for specific periods of time. In this section we will determine these timings as well as the response of the switching signal i.e. its rise and fall time. 2.11 MEASUREMENT OF "PULSE WIDTH" AND "DUTY CYCLE"

o Here we see a typical digital waveform. Notice that for a given time during one cycle of the waveform, the pulse is "on", and for the remainder of the cycle, the pulse is "off”.

o The "on" time is called the "mark" and the "off' time is called the "space". o The relationship between the mark and the space is called the mark-space

ratio which is expressed as either a ratio or duty cycle. o Using the scope as an accurate measuring tool, we can determine this ratio

by accurate measurement of the two pulse widths. o For a test signal we'll use a square wave (DC switching pulses) derived from

the signal generator. To measure the length of the "mark", superimpose the top edge of the pulse

on the horizontal centre line. Position the leading edge of the pulse on a convenient vertical line. Count the number of divisions from the leading edge to the trailing edge of

the pulse. This is the duration of the mark.


Now measure the number of divisions between the trailing edge of one pulse to the leading edge of the next. This will be the duration of the “space".

The mark is 2 divisions and the space 6 divisions. The "mark-space" ratio is therefore 2 : 6 or more conveniently 1 : 3.

The duty cycle calculated as a percentage is: 1/3 X 100% =33.33%. This means that the pulse is "on" for 33,3% of the total time and "off' for the balance of the period (66,6%).

2.12 PROCEDURES FOR MEASURING TRANSITION (RISE & FALL) TIME When working with pulse, digital and switching circuits, knowledge of "rise" and "fall" time is often required. Collectively termed "transition" time, this measurement defines the time taken for the leading edge of the waveform to rise from 10% to 90% of its value, and, the trailing edge to fall from 90% to 10% of its value.


Let's demonstrate the measurement procedure. Set the "input coupling" switch to "AC". Adjust the "time / division" switch to display about 2 cycles of the pulse and

ensure that the horizontal time-base is calibrated -the selector switch must be in the "calibrate" position.

Centre the trace using the "vertical position" control and adjust the "volts / division" control to fill the screen.

Careful observation of the graticule will reveal two horizontal dotted lines marked 0% and 100%.

Using the "variable volts/division" control, adjust the amplitude of the signal so that the upper and lower edges of the pulses are contained within the 0% and 100% percent lines.

Switch the "horizontal magnification" control to "X10" and align the leading edge of the trace to the 10% marker using the "horizontal position" control.


Count the number of divisions from this point to a point where trace intersects with the 90% graticule line.

Multiply the number of divisions by the "time / division" setting. The calculation is 2 divisions X 1 micro-second / division =2 S, however we

are not quite finished since "X10" magnification has been applied to the time scale, which means that the "time / division" reading must be divided by 10. Therefore the rise time equals 0,2 S or 200 nano seconds (200nS).

The same procedure is used to determine the "fall time" of the waveform, but this time, the trailing edge of the signal is examined. This completes Part 2 of the programme. In Part 3 (the final part), we'll discuss procedures for dual channel operation of the oscilloscope. You are now ready to watch the video up to Review Break No.4. Please ensure that you are in possession of Tech AV video entitled ETE-5 / PART 2. When you have watched the DVD up to review break No. 3, practice the measurement procedures you have learnt using a typical 20 MHz scope. You may use the workbook to help you remember the sequence of adjustments. Before continuing with the Part 3 the programme, complete the Self-Test Exercise No. 5 over the page.

TURN THE PAGE FOR SELF TEST EXERCISE NO. 5.




QUESTIONS YES NO

1. A DC switching pulse (square wave) exists in one of two states. Name them. ANS: _________________________________________________ _____________________________________________________

2. What does the "mark" time mean? ANS: _________________________________________________ _____________________________________________________

3. What does the "space" time mean? ANS: _________________________________________________ _____________________________________________________

4. What does the mark : space ratio tell us? ANS: _________________________________________________ _____________________________________________________

5. What does the duty cycle indicate? ANS: _________________________________________________ _____________________________________________________

6. To what does the term "transition time" refer? ANS: _________________________________________________ _____________________________________________________

END OF ETE-5 PART 2.


ETE-5 PART 3 SECTION 1 DUAL CHANNEL OPERATION OF THE OSCILLOSCOPE

OBJECTIVE At the end of this section you will be able to identify by name and describe the

purpose of all the user controls for dual channel operation on a typical 20 MHz oscilloscope of popular make and design.

Use the oscilloscope to: o Display two signals using both input channels. o Add two signals together (additive mode). o Subtract one signal from the other (differential mode).


o This workbook. o The video programme No. ETE-5 Part 3. o A 20 MHz oscilloscope. o Set of two measuring probes. o Sine / square/triangular waveform generator.




PROCEED BY FIRST READING THROUGH THE NOTES BEGINNING ON THE FOLLOWING PAGE.


DUAL CHANNEL OPERATION OF THE OSCILLOSCOPE

INTRODUCTION Most modem scopes are “dual channel” devices, which simply mean that they can display two signals on the screen at the same time (simultaneously). A closer look at the "vertical section" of the control panel reveals two identical input amplifiers called "channel 1" and "channel 2" respectively, and signals are applied to these inputs via scope probes or test leads.

Setting up for dual channel operation is similar to the setup procedures for single channel operation with the following exception: Set the vertical "mode" switch to "dual" in order to display both traces.

6.1 SIMULTANEOUS MEASUREMENT OF TWO SIGNALS Most electronic design and repair applications, call for the simultaneous examination of both the input and the output signals of a circuit. In this example of an audio circuit, channel 1 and channel 2 are the respective input and output signals.

To examine the circuit behaviour, adjust the "volts / division" and "time / division" controls as previously discussed to produce a visible, stable, trace for both channels.


With our basic knowledge of the scope, we can now measure the voltage and frequency of the input and output signals and then determine the amplification.

o NOTE: That the output signal is a function of the input signal, therefore we use channel 1 as the trigger source.

When viewing two unrelated signals, the trigger source is set to the "vertical" position to synchronise each trace independently.

6.2 ADDITIVE MIXING OF TWO SIGNALS Dual channel operation has many useful facilities, for example "additive" and "differential" operation. A control facility on the channel 2 input amplifiers allows the incoming signals to be "added" together or "subtracted" from each other.

Let's look at an example of additive mixing. An identical sine wave signal of 10 volts peak-to peak is applied to

both channel 1 and channel 2. To add the signals at channel 1 and 2, set the "vertical mode" switch

to "add" and observe the display. The amplitude of the resultant signal is displayed as 20 volts peak-to-

peak.


6.3 SUBTRACTIVE MIXING OF TWO SIGNALS

To subtract the signals, set the "normal / invert switch to "invert" and the amplitude of the resultant drops to zero; confirming that channel 1 has been subtracted from channel 2. This completes the first section of the programme. You are now ready to watch the video up to Review Break No. 1. Please ensure that you are in possession of Tech AV video entitled ETE-5 / PART 3 and that it has been fully rewound. When you have watched the video up to review break No. 1, practice the measurement procedures you have learnt using a typical 20 MHz scope and probe. You may use the workbook to help you remember the sequence of adjustments. Before continuing with the next section of the programme, complete the Self- Test Exercise No. 6 over the page.

POCEED TO THE NEXT PAGE FOR SELF TEST EXERCISE NO. 6.




QUESTION YES NO

1. What do we mean when we say the scope is "dual channel" device? ANS: ________________________________________________ _____________________________________________________

2. Complete this sentence. Setting up for dual channel operation is similar to the set-up procedures for single channel operation with the following exception: Set the vertical "mode" switch to "_______________" in order to display both traces.

3. When viewing two signals of the same shape and frequency, where would we set the trigger source control? ANS: _________________________________________________ _____________________________________________________

4. The scope allows us to "add" two signals together. What do we call this function? ANS: _________________________________________________

5. The scope allows us to "subtract" two signals from each other. What do we call this function? ANS: _________________________________________________

6. If we have two signals (25 volts peak-peak) of the same shape and frequency, and we add them together using the additive facility in the scope, what is the value of the resultant? ANS: _________________________________________________


7. If the two signals are subtracted from each other, what is the resultant? ANS: _________________________________________________

TURN OVER PAGE FOR ETE-5 PART 3 SECTION 2.



OBJECTIVE At the end of this section you will be able to use two different methods to determine

the phase displacement between two signals namely: o Dual channel method. o Lissajous patterns.


o This workbook. o The video programme No. ETE-5 Part 3. o A 20 MHz oscilloscope. o Set of two measuring probes. o Sine / square / triangular waveform generator.






DUAL CHANNEL OPERATION OF THE OSCILLOSCOPE 3.4 DUAL TRACE METHOD TO MEASURE PHASE DIFFERENCE The phase difference or phase angle between two signals can be measured using the dual channel feature of the scope or operating the scope in the "X-Y" mode. Let's begin with the dual channel method which works with any type of waveform. The method is effective for measuring large or small differences in phase at any frequency up to 20 MHz.

Apply the two signals to be investigated to the channel 1 and channel 2 inputs respectively.

Set the channel 1 "input coupling" switch to "ground". This sets the reference position for the trace.

Move the channel 2 trace off the screen. We will use it later. Using the "vertical position" control, superimpose the trace on the

horizontal centre graticule line. Switch the coupling switch to "AC" and adjust the peak-to-peak amplitude of

the signal to span exactly 6 vertical divisions using the "volts/division" control as well as the "variable volts per division" control.

Position the start of a cycle on a suitable vertical graticule line as shown.

Using the "time/division" control, together with the "variable" control, adjust the period of the signal to span exactly 7.2 horizontal divisions. This sets-up channel 1 as the "reference trace".

o NOTE: That one cycle of a waveform occurs over a 360 degree period, and we have set 7,2 divisions to equal one complete period or 360 degrees. It follows that each division then represents 50 degrees, and each sub-division is equal to 10 degrees.

Bring the "channel 2" signal back onto the screen and using the "volts / division", the "variable volts / division" and the "vertical position" controls, set the peak-to-peak amplitude to span exactly 6 vertical divisions.


Now measure the horizontal distance separating the two signals using the graduations on the horizontal centre line as an accurate measuring scale. This distance represents the phase shift between the two signals.

The calculation is as follows: 7 sub-divisions X 10 degrees per sub-division = 70 degrees shift.

3.5 LISSAJOUS PATTERN METHOD Here is another method of measuring phase difference but it only suitable with sine wave signals. Measurements are possible up to 500 kHz, but for maximum accuracy when working with small differences in phase, the frequency of the input signals should be limited to 50 kHz.

o The scope set-up is almost identical to the set-up described in the previous sequence with the exception that the horizontal time-base is switched to the "X-Y" mode by turning the "time/division" control fully clockwise.

o A trace appears on the screen, whose shape is dependent on the phase relationship between the two signals applied to the scope.

To measure the phase shift, set the channel 1 "input coupling" switch to "ground" and adjust the vertical trace to span exactly six divisions, three above and three below the horizontal centre line. This sets up channel 1 as the reference trace.

Then set the channel 2 "input coupling" switch to "ground" and adjust the beam position (which now looks like a bright green spot) to "dead centre" position on the screen.

Set both input coupling switches to "AC" and the pattern should return to the screen.

Measure the number of vertical divisions and sub-divisions enclosed by the circle. This is dimension "A".

Dimension “B” has a value of six since it was pre-set to this value. To calculate the phase shift, the formula is:

phase shift = arc sin a/b


If both signals were in phase, the pattern would look like this.

a/b = 0

arc sin a= 0 degrees shift When the phase difference between the two signals is 900 the pattern is circular.

a/b =1

arc sin 1 = 90 degrees

If the signals were 180˚ out of phase the pattern would look like this.


a/b = 0

arc sin 0 = 180 degrees shift CONCLUSION Between 0° and 180° shift, the shape will vary from a diagonal line with a positive gradient, to a "sausage" shape which will eventually become a circle when the shift is exactly 90°. Thereafter the shape will change to a "sausage" but this time sloping in the opposite direction, then to a straight line with a negative gradient when the phase difference between the two signals is 180°. X-Y OPERATION EXPLAINED The measurement procedure just described may seem confusing, so let's briefly explain what occurs during "X-Y" operation. Normally the horizontal time-base is free running, however when the "X-Y" setting is selected, the time-base stops and the horizontal trace becomes a spot. Channel 1 now becomes the "X" deflection input and channel 2 remains as the "Y" deflection input.

IMPORTANT! After completing the measurements, always switch the "time / division" control away from the "X-Y" setting to prevent the spot from burning the screen.


This completes the second section of the programme. You are now ready to watch the video up to Review Break No. 2. Please ensure that you are in possession of Tech AV video entitled ETE-5 / PART 3. When you have watched the video up to review break No. 2, practice the measurement procedures you have learnt using a typical 20 MHz scope, probes and signal generator. You may use the workbook to help you remember the sequence of adjustments. You may continue with the next section of the programme as there is no Self- Test Exercise for this section.

TURN OVER PAGE FOR NEXT SECTION.




OBJECTIVE At the end of this section you will be able to: Identify by name and demonstrate the function of all the user controls in the

triggering control group. Correctly set-up the triggering controls so as to synchronise the scope to a variety of

different signals. WHAT RESOURCE WILL YOU NEED?








INTRODUCTION Triggering the horizontal time-base is one of the most difficult adjustments to perform because of the many options available and the exacting requirements of certain signals. In this programme, we have seen how signals applied to the vertical amplifiers of the scope deflect the beam "up and down" the screen whilst at the same time the horizontal time-base sweeps the trace across the screen from left to right. Successfully triggering the time-base involves extracting a trigger signal from the incoming signal to be displayed, which will inform the time-base when to begin tracing out the signal. When the scope is correctly triggered, the sweep will be synchronised to the incoming signal so producing a stable, readable waveform. 3.6 TECHNIQUES FOR CORRECTLY TRIGGERING THE OSCILLOSCOPE Let's investigate the function of the trigger controls, starting with the "trigger mode" selector.

Four options are available namely:

o "Auto" is always the best mode to begin with since the time-base is free-running and will always show a horizontal trace on the screen.

o "Normal" for example, will only trigger the time-base when a signal is present.

o For this reason, the "normal" mode is used when viewing signals with low repetition rates.


o "TV-V" and "TV-H" are used to display television related signals since this mode ensures that the time-base will always be locked to the incoming vertical or horizontal television sync signals.

The next step is to choose the origin of the trigger signal which is controlled by the "source" selector switch.

o The setting is normally "internal" which means that the trigger signal is extracted from either channel 1, channel 2 or both vertical inputs.

o Selecting "line" causes the time-base to run at the power line or mains frequency and is useful when analysing power control equipment such as DC and AC motor drive systems whose circuitry is all mains frequency related.

o The "external" position simply means that the trigger signal is derived from an external source and fed to the scope via an external input BNC connector. For example, when examining the head output signal of a DVD cassette recorder, the trigger must be externally derived from the head switching pulse circuit, since the head signal itself is unable to provide a reliable trigger signal for the horizontal time-base.

X5 MAGNIFICATION FACILITY ON CH.1INPUT

o When viewing very small amplitude signals, it is a good idea to make use of the X5 magnification facility to increase the, size of the signal.

o The actual amplitude of the signal is then five times the value indicated on the "volts / division" switch.


Having decided on the "mode" and "source" of the trigger, we must decide whether the time-base must be triggered on a positive or negative edge of the incoming signal. This is set by the "slope" switch.

Finally the "level" at which the trigger will occur is determined by the "trigger level" control and can be set to any position on the positive or negative slope of the signal.

As a rule, always choose the signal with the steepest slope and the least amount of jitter for reliable triggering.


For slow rising waveforms, we use the mid-point as this area is usually the cleanest part of the transition.

This completes this section of the programme. You are now ready to watch the video up to Review Break No. 3. Please ensure that you are in possession of Tech AV video entitled ETE-5 / PART 3B. When you have watched the video up to review break No. 3, practice the measurement procedures you have learnt using a typical 20 MHz scope and probe. You may use the workbook to help you remember the sequence of adjustments. Before continuing with the next section of the programme, complete the Self- Test Exercise No. 7 over the page.

TURN TO NEXT PAGE FOR SELF-TEST EXERCISE NO. 7.




QUESTIONS YES NO

1. Triggering the horizontal time-base is one of the most difficult adjustments to perform because of the many options available and the exacting requirements of certain signals. Now complete the following sentence: Successfully triggering the time-base involves extracting a _________________signal from the ____________________ signal to be displayed, which will inform the time-base when to begin tracing out the signal.

2. Complete this sentence: When the scope is correctly triggered, the sweep will be ____________________to the incoming signal so producing a stable, readable waveform.

3. What is the best selection to make when setting the "trigger mode" selector and why is it the best selection? ANS: _________________________________________________ _____________________________________________________

4. What is the purpose of the "trigger source selector switch? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

5. What is the function of the trigger "slope" selector? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________


6. What is the function of the trigger "level" control? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

7. What is the best position on the incoming waveform to set the trigger point? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

TURN OVER PAGE FOR ETE-5 PART 3 SECTION 4.



OBJECTIVE At the end of this the final section you will be able to: Avoid two types of hazardous connections between the scope and equipment. Install and handle the scope correctly.






PROCEED NOW BY READING THROUGH THE NOTES BEGINNING ON THE FOLLOWING PAGE.



INTRODUCTION In this the final section of the programme, we discuss some very important points to remember when using and caring for the oscilloscope. 3.7 CALIBRATION OF CONTROLS

Whenever taking voltage or time measurements, ensure that the calibration switches are set to the "calibrate" position.

In the vertical section this includes both "variable volts / division controls, and in the horizontal time-base section, the "calibrate / variable" switch.

3.8 HAZARDOUS CONNECTIONS TO AVOID Certain electronic equipment is connected directly to the mains power line and is called "transformerless" or "hot chassis" equipment.

When measuring on this type of circuitry, either disconnect the earth wire on the scope power lead or apply power the scope via a suitable isolation transformer.

o This will eliminate the possibility of an accidental short circuit being placed across the mains supply should the device under test be "cross phased".


Failure to observe these precautions will cause damage to both personnel and equipment.

3.9 COMMUNING OF PROBES Finally beware of making a connection like this. It’s called "communing" of probes. In this example we see that by connecting both "ground" leads of the scope to the circuit, an accidental short circuit, therefore use only one "ground" lead when taking measurements with both probes.

3.10 INSTALLATION AND HANDLING PROCEDURES o Always try to operate the scope in a clean, dry well ventilated environment. o Keep the scope away from excessive vibrations and strong magnetic fields

which will affect the reliability and accuracy of the instrument. o Don't exceed the maximum ratings of the input circuitry -the limit is 250

volts peak-to peak. o And no tea, coffee or soup cups put on top of the scope case.

Attention to these details will ensure accuracy and reliability from the instrument.


This completes the final section of this programme. You are now ready to watch the video up to Review Break No. 4. Please ensure that you are in possession of Tech AV video entitled ETE-5 Part 3. When you have watched the video up to the end of review break No.4, practice the measurement procedures and remember the precautions you have learnt using a typical 20 MHz scope, measuring probes and signal generator. You may use the workbook to help you remember the procedures. Before continuing with the next section of the programme, complete the Self- Test Exercise No. 8 over the page.

GO TO NEXT PAGE FOR SELF-TEST EXERCISE NO. 8.




QUESTIONS YES NO

1. When setting the user controls on the scope in preparation to take an accurate measurement, what should you check? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

2. Certain electronic equipment is connected directly to the mains power line and is called "transformerless" or "hot chassis" equipment. What two precautions should you make before attempting to use the scope on this type of equipment? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

3. What do you understand by the term "communing of the probes"? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

4. The scope input circuits have a specified "maximum rating". What should you ensure? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________

5. What should not be placed on top of the scope? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________


6. There are two main types of area that you should avoid using the scope. What are they? ANS: _________________________________________________ _____________________________________________________ _____________________________________________________


THIS CONCLUDES THE PROGRAMME. o The producers of the Tech AV training material hope that the information given in

this presentation will be of value to you as you continue to apply yourself in the fields of electrical and electronic engineering.

o This programme is one of many available in the Tech AV range available to all industrial training concerns to build and strengthen the future of our country's engineering endeavours.

Future programmes in this series will include: Typical applications of the scope as a precision measuring instrument. Using the basic function generator. Using AC and DC power supplies.

Operating a frequency counter.

END OF PROGRAMME.


MODEL ANSWERS SELF TEST EXERCISE NO. 1

QUESTION YES NO

1. Complete this sentence. Unlike an analogue or digital multi-meter, the oscilloscope or "scope" as it is often called, will actually trace out a picture of the voltage under test by rapidly plotting the changes in voltage over a pre-set period of time.

2. Complete this sentence. The most obvious feature of an oscilloscope is the cathode ray screen. It is on this screen that the visual wave patterns are displayed.

3. The screen is divided into squares by horizontal and vertical lines. What do these divisions represent? The graticule.

4. The main divisions are sub-divided. How many sub-divisions in one main division? Five (5).

5. What is the value of one division? 1.

6. What is the value of each sub-division? 0,2.

7. Complete the sentence. The graticule provides visual reference points to indicate voltage or amplitude variations on the up-down axis, and measurements of time or period on the horizontal or left-to-right axis.


8. Draw the graticule which is attached to the face of the cathode ray screen.

9. Why must the oscilloscope controls be calibrated before taking a measurement? To ensure that the voltage and time readings taken will be accurate.



QUESTIONS YES NO

1. Why is it necessary to check the voltage calibration of the probe? To ensure accuracy when measurements are taken using the probe.

2. Where would you derive a suitable test signal for checking and setting-up the measuring probe? From the "probe adjust" test point on the scope.

3. Why is the "input coupling" switch set to "ground" when preparing to take a measurement? To establish a reference position from which to measure the amplitude of the voltage under test.

4. Why is it necessary to ensure that the "variable volts/division" control is in the "calibrate" position. To ensure that the value of "volts / division" indicated by the control knob is the true value. Therefore readings taken from this control will be accurate.

5. To measure the amplitude of a voltage from the screen display we must follow two steps -what are they? Count the number of divisions from the top to the bottom of the signal. Multiply the number of divisions by the setting on the "volts / div." control.



QUESTIONS YES NO

1. Before taking a voltage measurement, why do we switch the "input coupling switch" to the "ground" position? To establish a reference position from which to measure the amplitude.

2. Complete the following sentence: To calculate the voltage of the signal being measured by the scope, multiply the setting on the “volts / division” control by the number of divisions counted.

3. If the peak-to-peak level of a sine wave is 10 volts, what is the value of the peak only voltage? Draw the waveform. 5 Volts peak-to-peak.

4. If the amplitude of the signal measured is 1 volt peak-to-peak, what is the rms. value of the measured voltage? 0,707 Volts rms.

5. If the amplitude of the signal measured is 100 volt peak-to-peak, what is the average value of the measured voltage? 63,7 Volts rms.

6. Why is it best to check for both an AC and a DC component when measuring an unknown voltage? If this is not established, it will leak to misleading measurements.



QUESTIONS YES NO

1. What do you understand when we measure the "period" of a waveform? The period is the time taken to trace out one complete cycle of that waveform.

2. What do you understand about the term "frequency" of a waveform? The frequency of a waveform is the number of complete cycles traced out in one second.

3. Can you write out a formula to calculate the frequency of a waveform? Frequency = 1/Time (the reciprocal time).

4. When making accurate measurements of time using the scope, what must you ensure with regard to the time/division controls? That the time / division calibrate switch is the "calibrate" position.

5. Assuming that the scope has been set up to measure the period of a waveform, how would you determine the time taken to trace out one complete cycle? Count the number of divisions and sub-divisions from the start of a cycle to the start of the next consecutive cycle.



QUESTIONS YES NO

1. A DC switching pulse (square wave) exists in one of two states. Name them. The ON state and the OFF state.

2. What does the "mark" time mean? It describes the "on "time of the signal.

3. What does the "space" time mean? The space time defines the "off" time of the switching pulse.

4. What does the mark : space ratio tell us? The ratio of the "on" time to the "off" time.

5. What does the duty cycle indicate? The duty cycle is the ratio of the “on” time to the “off” time expressed as a percentage.

6. To what does the term "transition time" refer? Transition time refers to the "rise" and "fall" time of the waveform i.e. the behaviour of the leading and trailing edge of the pulse.



QUESTION YES NO

1. What do we mean when we say the scope is "dual channel" device? It means that we can view two similar or dissimilar signals on the scope at the same time.

2. Complete this sentence. Setting up for dual channel operation is similar to the set-up procedures for single channel operation with the following exception: set the vertical "mode" switch to "dual" in order to display both traces.

3. When viewing two signals of the same shape and frequency, where would we set the trigger source control? Set the control to the channel 1 position as the output is a function of the input.

4. The scope allows us to "add" two signals together. What do we call this function? Additive mixing.

5. The scope allows us to "subtract" two signals from each other. What do we call this function? Differential of subtractive mixing.

6. If we have two signals (25 volts peak-peak) of the same shape and frequency, and we add them together using the additive facility in the scope, what is the value of the resultant? 50 Volts peak-peak.

7. If the two signals are subtracted from each other, what is the resultant? 0 Volts.



QUESTIONS YES NO

1. Triggering the horizontal time-base is one of the most difficult adjustments to perform because of the many options available and the exacting requirements of certain signals. Now complete the following sentence: Successfully triggering the time-base involves extracting a trigger signal from the incoming signal to be displayed, which will inform the time-base when to begin tracing out the signal.

2. Complete this sentence: When the scope is correctly triggered, the sweep will be synchronised to the incoming signal so producing a stable, readable waveform.

3. What is the best selection to make when setting the "trigger mode" selector and why is it the best selection? "Auto" is always the best made to being with since the time-base is free-running and will always show a horizontal trace on the screen.

4. What is the purpose of the "trigger source selector switch? This switch allows us to choose the source or origin of the trigger signal.

5. What is the function of the trigger "slope" selector? It enables us to choose either the positive or negative edge of the signal to trigger the time-base.

6. What is the function of the trigger "level" control? The level control sets the position on the selected edge of the signal where the time-base will commence its sweep.

7. What is the best position on the incoming waveform to set the trigger point? As a rule, always choose the signal with the steepest slope and the least amount of jitter for reliable triggering.


SELF TEST EXERCISE 8

QUESTIONS YES NO

1. When setting the user controls on the scope in preparation to take an accurate measurement, what should you check? Ensure that any control which features a "calibrate" option is set to the "calibrate position.

2. Certain electronic equipment is connected directly to the mains power line and is called "transformerless" or "hot chassis" equipment. What two precautions should you make before attempting to use the scope on this type of equipment? When measuring on this type of circuitry, either disconnect the earth wire on the scope power lead or apply power the scope via a suitable isolation transformer.

3. What do you understand by the term "communing of the probes"? When connecting both "ground" leads of the scope to the circuit, an accidental short circuit can arise, therefore use only one "ground" lead when taking measurements with both probes.

4. The scope input circuits have a specified "maximum rating". What should you ensure? Don't exceed the maximum ratings of the input circuitry - the limit is 250 volts peak-to peak.

5. What should not be placed on top of the scope Any type of liquid that could spill if accidently knocked over.

6. There are two main types of area that you should avoid using the scope. What are they? Where there is excessive vibration, and/or strong magnetic fields.

electrical testing equipment - techav | online technical training · 2019-12-11 · have your...

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