EL2EE3 Prac Sheet - prac PDF

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University of KwaZulu-Natal SCHOOL OF ELECTRICAL, ELECTRONIC & COMPUTER ENGINEERING Practical No

:

EL2EE3

Course

:

Electrical & Electronic Engineering

Code

:

ENEL2EEH1

Title

:

MEASUREMENTS AND INSTRUMENTATION

1.

Introduction

The purpose of this practical is to introduce the student to the equipment provided at each workstation in the Second Year Laboratory in the School. The student will note the measurement capability provided by each piece of equipment and tests will be performed so the student is familiar with all the controls and be able to verify the instruments’ accuracies. 2.

Pre-practical

Read the practical notes carefully. No pre-practical report is needed but the student is encouraged to make some written notes in order to develop a basic understanding of the practical. 3.

Workstation

Each workstation is provided with the following equipment: 3.1

Oscilloscope

ISO-Tech ISR622 20 MHz Oscilloscope with two channels. An abbreviated specification is given in Table 1 in the Appendix. 3.2

Function Generator

Topward 8110 function generator capable of producing sine, square and triangular waves of variable amplitude and frequency. An abbreviated specification is given in Table 2 in the Appendix. 3.3

Digital Multimeter

Digital Multimeter ISO-TECH IDM 203 capable of measuring ac and dc voltages and currents, as well as resistance, capacitance and frequency. An abbreviated specification is given in Table 3 in the Appendix. 3.4

Trainer

The trainer may be used to design and test a wide range of electronic circuits. It consists of the following units. Breadboard / Superstrip This device enables resistors, capacitors, diodes, transistors, etc. to be connected together without the need for soldering the connections between the components. Components are simply inserted into the holes in

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the breadboard. The interconnection of all the holes is shown above. The two top rows are normally reserved for the positive dc supply rails i.e. +12 V and +5 V, while the two rails at the bottom of the breadboard are normally reserved for the 0 V rail and the negative rails i.e. –12 V or –5 V. Power supplies The following regulated dc supplies are provided. +12 V dc for currents of 0 to 200mA +5 V dc for currents of 0 to 200mA -5 V dc for currents of 0 to 200mA -12 V dc for currents of 0 to 200mA

4.

Using the oscilloscope and function generator

4.1

Checking the calibration of the oscilloscope

Connect CH1 of the oscilloscope to the CAL terminal on the oscilloscope using a BNC to croc clip lead. Adjust the controls of the oscilloscope to display the square wave. (Settings: Mode CH1, Trigger Source CH1, Auto pressed & Lock pressed.) Check that the amplitude is 2 Vp-p. Repeat for CH2. 4.2

Displaying a sine wave, square wave and triangular wave

Connect the output of the function generator to CH1 of the oscilloscope using a BNC to BNC cable. Display a sine wave with a frequency of 1 kHz and amplitude of 1 Vp-p. Investigate the other waveforms produced by the function generator, i.e. triangular wave and square waves. Investigate the effect of varying the RAMP/PULSE control of the function generator while displaying a square wave. Measure the extent of the duty cycle variation and confirm that it is within specification. Investigate the effect of the OFFSET control of the function generator and confirm that it is within specification. (Remember to have the oscilloscope in the DC mode. You can also apply 0 V to the input by switching to GND and then moving the position of the trace to be in the centre of the screen. Now switch back to DC mode.) Investigate the effect of the attenuator button (ATT) of the function generator. Measure the attenuation L and compare your measurement with the specification. Note:

L 20 log

V0 pre ssed Vo notpre ssed

2

dB

4.3

Measuring voltage of a sine wave using an oscilloscope and a digital multimeter

Set the controls of the function generator to produce a sine wave with a frequency of 100 Hz with an amplitude of your choice. Then using the oscilloscope measure the peak-peak voltage and peak voltage. Then calculate the rms voltage of the waveform. Now use the digital multimeter to measure the rms voltage of the waveform using a BNC to croc clip lead. Use the specifications to determine the accuracy ( x%) of the measurement. If the frequency of the waveform was 100 kHz instead of 100 Hz, which instrument would give the most accurate reading of the rms voltage?

4.4

Measuring the frequency of a sine wave using a digital multimeter and an oscilloscope

Set the controls of the function generator to produce a sine wave with a frequency and amplitude of your choice. Now measure the frequency of the waveform using the digital multimeter. Set the frequency mode by turning the rotary function switch to “Hz ADP” and pressing the blue button. Measure the period of the waveform using the oscilloscope and calculate the frequency of the sine wave. Compare your measurements. Determine the accuracy of the two measurements using the specifications of the digital multimeter and the oscilloscope. Which measurement is more accurate? 4.5

Measuring the frequency response of an oscilloscope using a BNC to croc clip lead

This experiment will demonstrate that BNC leads are only suitable for voltage measurement with an oscilloscope if the source resistance of the source is low enough to charge and discharge the capacitance of the BNC cable. The capacitance of BNC cables used in this experiment is in the order of 100 pF. Connect the function generator to CH1 of the oscilloscope using two BNC to croc clip leads clipping corresponding croc clips together (red to red and black to black). Display a sine wave with an amplitude of 1 Vp-p at 100 Hz. Now insert a 100 k resistor between the two red croc clips to simulate a voltage source of 1 Vp-p with a source resistance of 100 k . Measure the peak-to-peak voltage displayed on the oscilloscope and calculate the reduction factor. Compare your result with the theoretical reduction factor k

where Rin = 1 M

10 6

R in R in

Rs

10 6

05

.91

the input resistance of the oscilloscope and Rs = 100 k .

With the 100 k resistor in place increase the frequency and note that the displayed amplitude reduces. Determine the frequency where the amplitude is 0.707 x amplitude at 100 Hz. This is the –3 dB cut-off frequency. The frequency may be read of the dial of the function generator. (The easiest way to perform this measurement is to first adjust the amplitude of the function generator to give a convenient number of peak-to-peak divisions on the screen, such as six. Then determine the frequency at which the amplitude is 6 x 0.7 = 4.2 divisions peak-to-peak.) The -3dB frequency may be predicted using the formula

f

1 3 dB

2

Rs Rin C lead R s R in

3

C in )

where Clead = capacitance of the BNC to croc clip lead and Cin = 25 pF the input capacitance of the oscilloscope. The equivalent circuit is shown in Figure 1.

Rs Vs Clead

Source

BNC lead

R in

Cin

Oscilloscope

Figure 1 Equivalent circuit of a BNC to croc clip lead connected to an oscilloscope

5.

The 10:1 oscilloscope probe

5.1

Adjusting the probe

To measure high frequency signals it is normal to use a 10:1 probe instead of a BNC to croc clip lead. Connect the probe to CH1 of the oscilloscope. Before the probe can be used it should be calibrated by connecting the tip of the probe to the CAL terminal. A perfect square wave should be displayed with an amplitude of 1/10 x 2 Vp-p. A small adjustable capacitor (1.5 – 11.5pF) is fitted in the probe with a small screw to vary the capacitance. With the assistance of a Laboratory Demonstrator who has a trimmer screwdriver, adjust the capacitor noting its effect. If the capacitance is too large the wave will rise too slowly and be rounded in shape. If the capacitance is too small the wave will rise too rapidly and overshoot. If adjusted correctly the waveform will be perfectly square. Sketch the three waveforms described. 5.2

Amplitude correction factor

An equivalent circuit of a 10:1 probe connected to an oscilloscope is shown in Figure 2.

Rp Probe

Cp Cadj

Clead

Cin

Rin

Croc clip 10:1 Probe

Oscilloscope

Figure 2 Equivalent circuit of a 10:1 probe connected to an oscilloscope

Connect the function generator to the oscilloscope using the 10:1 probe and a BNC to croc clip lead with the settings as in Section 4.5 (sine wave with an amplitude of 1 Vp-p at 100 Hz). Note that the displayed amplitude is now 100 mVp-p instead of 1 Vp-p due to the 9 M resistor Rp in the tip of the probe. The signal is reduced by a factor of 1/10 due to the 1 M input resistance Rin of the oscilloscope. Note the reading.

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Now insert the 100 k resistor into the circuit at the croc clip to simulate a source resistance of 100 k and re-measure the amplitude. The reduction should be very small indeed as the input resistance of the oscilloscope plus the 10:1 probe is now 10 M instead of 1 M . Note the reading. 5.3

Measurement capability at high frequencies

A 10:1 probe, if adjusted correctly can measure signals with frequency components far exceeding those that can be measured with a BNC lead. This is made possible by attaching a very small capacitance Cp in parallel with the 9M resistor. The value is chosen to make to make the reactance of this parallel combination of Rp and Cp equal to the reactance of the parallel combination of the lead capacitance Clead, input capacitance Cin of the oscilloscope and the input resistance Rin of the oscilloscope. A small adjustable capacitor Cadj in the probe is used to ensure that the reactance of the two networks are exactly equal at all frequencies. See Figure 2. To achieve this the two RC time constants need to be made equal. Thus

R pC p

R in ( C adj

Clead

Cin )

Typical values are Rp = 9 M , Cp = 15 pF, Rin = 1 M , Cadj = 10 pF, Clead = 100 pF and Cin = 25 pF. The input resistance at the tip of the probe is Rprobe = 1 M + 9 M = 10 M Cprobe equal to Cp in series with the sum of all the three other capacitors. So

Z in R probe //

C p (C adj Cp

Clead

C adj

Cin )

Clead

0M

Cin

with an input capacitance

14 pF

Rs Vs

Rprobe

Cprobe

Source

10:1 Probe & Oscilloscope

Figure 3 A voltage source Vs with a source resistance R s, 10:1 probe and oscilloscope

If the source resistance is low, e.g. 50 (the output resistance of the function generator) then the –3dB cut-off frequency due to the probe will be very high indeed as the following equation indicates. The oscilloscope will therefore measure up to its cut-off frequency, in this case 20 MHz.

f

3dB

1 1 2 R sC probe 2 50 x14 x10

However, if the source resistance is high, e.g. 100 k 3 dB cut-frequency given by

f

3dB

1 2 R sC probe

2

27 MHz

the probe will not give accurate readings beyond the

1 2 0 14 x10 5

5

2

14 kHz

Insert the 100 k resistor in series with the signal at the croc clip and measure the –3 dB frequency. Note the reading. Now remove the 100 k resistor and confirm that the output amplitude does not vary with frequency.

6.

Frequency Response of a digital multimeter

Investigate the frequency response of the digital multimeter by apply a sine wave of 1 V rms at 50 Hz from the function generator to the digital multimeter. Note that as the frequency is increased the digital multimeter ac voltage reading reduces. Measure the -3 dB frequency of the digital multimeter by increasing the frequency until the voltage reduces to 0.707V rms. Note the frequency.

7.

Effect of wave shape on a digital multimeter.

Digital multimeters measure the rms amplitude of a sine wave by rectifying the input signal and averaging the resultant dc voltage. Multiplying the reading by an appropriate factor F for a sine wave gives the true rms reading provided the input signal is a sine wave. If the input is sin ωt volts then the rms value is

1

= 0.707 volts.

2 The rectified output is v = sin ωt for 0 < ωt < Integrating sin ωt between 0 and

and v = -sin ωt for

and dividing by the period

< ωt < 2 .

gives the average value = 2/ volts.

The appropriate factor (form factor F) is therefore F

2

1 2

and

F

2 2

1.111

Apply a square wave of 2 Vp-p to the oscilloscope and the digital multimeter. The peak values of the wave must be +1 V and –1 V. The rms value of this waveform should be 1 V rms. (It is likely that the function generator will have a small dc offset of say +0.1 V which will mean that the square wave will have peak amplitudes of +1.1 V and –0.9 V. This will not give a rms value of 1 V. It is therefore necessary to first remove the offset using the OFFSET control on the function generator and then adjust the amplitude to be +1 V and -1 V.) Note the reading on the digital multimeter. What is the significance of the reading?

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8.

Tolerance of an attenuator

Connect an attenuator using two BNC to croc clip leads between the function generator and the digital multimeter with a gain A = Vo/Vi = 1/11 = 0.0909. Use a 1 k resistor and 10 k resistor both with tolerances of ±5% as shown in Figure 4. Calculate the minimum and maximum gain factors to account for the tolerance in the resistors. Measure A using the digital multimeter a sine wave with Vi = 1 V rms at 1 kHz from the function generator. Is A within the predicted tolerances?  10 kΩ Vi

1 kΩ

Vo

0V Figure 4 The attenuator

9.

Characterising the power supplies

Measure the dc voltages of the +12 V, +5 V, -5 V and –12 V supply rails using the digital multimeter under no load conditions. Now apply a load current of about 100 mA to each voltage rail in turn and remeasure the dc voltages. Resistors of 120 and 47 are provided for this purpose. Provided the maximum load current is not exceeded, the power supplies approximate voltage sources with very low source resistances. Determine the equivalent circuit of each supply in terms of an open-circuit voltage Voc and a source resistance Ro. (Ro = V/ I) 10.

Investigating a current source

A current source is not as easy to visualise as a voltage source. This experiment attempts to demonstrate that current sources do exist! A constant current source can be realised in several different ways using electronic components. Such a realisation is shown in Figure 5 using a BC547 transistor. When a dc voltage source is connected to the V+ and COM terminal a constant current will flow of approximately 1 mA dc irrespective of the source voltage. It is not necessary to understand how the circuit works but useful to test its ability to supply a constant current at different dc voltages.

Figure 5 A constant current source

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The circuit shown in Figure 5 is constructed on a small printed circuit board (PCB) which is plugged into the breadboard and has two “flying leads”, one black (COM) and one red (V+). Touch these leads onto the screw terminals of the power supply and measure the resulting current that flows. The trainer can supply dc voltages of +5 V, +7 V, +10 V, +12 V, +17 V and +24 V using the different rail voltages available. Progressively increase the voltages noting the current corresponding to every applied voltage. Measure the dc current I to as many decimal places as the meter provides. Method: Connect the black (COM) “flying lead” to say 0V or GND. The red flying lead is then connected to a more positive voltage e.g. +5V via the digital multimeter (switched to dc current). The positive terminal of the digital multimeter (red crocodile clip) should be connected to say +5V, while the negative terminal of the multimeter is connected to the red “flying lead”. Model the current as an ideal current source I with a resistor R in parallel to account for the very small increase as V is increased. V 24 5 I 5V and R I24V 5V

Revised: A D Broadhurst, 28 June 2006 R Sewsunker, 02 Feb 2011

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Appendix A1. Oscilloscope Table 1. An abbreviated specification of the ISO-Tech ISR622 20MHz Oscilloscope Vertical axis

Sensitivity Sensitivity accuracy Vernier vertical sensitivity Screen size Bandwidth Coupling Rise time Input Impedance Vertical modes

Triggering Triggering source

Coupling Polarity Sensitivity Modes

Input Impedance Horizontal Axis Sweep time Sweep time accuracy Vernier sweep control Sweep magnification X-Y Mode Mode Frequency bandwidth X – Channel Y – Channel Z AXIS Sensitivity Frequency bandwidth Input resistance Calibration Voltage Waveform Frequency Output voltage Output impedance

1mV/div to 5V/div in 1-2-5 sequence 5mV/div to 5V/div 3% 2mV/Div 5% 1 to 2.5 of panel indicated value 10 div wide x 8 div high DC to 20MHz DC, GND or AC coupling – lower limit frequency 10Hz 17.5ns 1M ±2% in parallel with 25pF CH1: single channel CH2 :single channel DUAL: alternate mode chopped mode set automatically by the TIME/DIV switch ADD: CH1 and CH2 algebraic addition Triggering source – CH1, CH2, LINE (50Hz) and EXT (CH1 and CH2 can be selected only when the vertical mode is DUAL or ADD.) If the TRIG ALT switch is pushed in , it can be used for alternate triggering of two different sources. AC, HF-REJ, TV and DC + or – 0.5 div AUTO – Most useful as the trace is always displayed. NORM - No trace displayed unless a signal is applied. SINGLE – One shot sweep, to repeat press RESET. 1M ±2% in parallel with 35pF 0.1 s/div to 0.5s/div, 21 steps in 1-2-5 sequence 3% x1 to x2.5 of panel indicated value 10 times (maximum sweep time 10ns/div) X-axis CH1, Y-axis CH2 DC to 1MHz (-3dB) DC to 20MHz (-3dB) Phase difference between X and Y channels 3% at DC to 50kHz. 3Vp-p (Trace becomes brighter with negative input.) DC to 5MHz 5k Square wave 1kHz 5% 2Vp-p 2% 2k

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A2. Function Generator Table 2. An abbreviated specification of the Topward 8110 function generator Frequency range 0.1Hz to 2MHz in 7 ranges Waveforms Sine, Square, Triangular, Ramp & Pulse Amplitude Variable from 5mVp-p to 20Vp-p with no load 0dB or –30dB (Useful when a very small signal is required.) Attenuator Output Impedance 50 10% DC Offset +10V to –10V Duty cycle for triangular Pull RAMP/PULSE duty cycle continuously variable from 80:20 to and pulse waveforms 20:80. When pushed in the duty cycle is 50:50. Frequency accuracy 5% of full scale Distortion...