ECE 231 Laboratory Exercise 3 Oscilloscope/Function-Generator Operation ECE 231 Laboratory Exercise 3 – Oscilloscope/Function Generator Operation
Laboratory Group (Names) ______OBJECTIVES
Gain experience in using an oscilloscope to measure time varying signals. Gain experience in using a signal generator to create time varying test signals. Gain experience in properly using an oscilloscope’s controls and soft keys. Learn the frequency limitations of instruments.
EQUIPMENT REQUIRED One banana cable Three BNC cables One lot of colored clip leads and/or jumper wires DMM (digital multimeter) Use the DC offset in signal generator for the DC power supply Signal generator
BACKGROUND
The oscilloscope is primarily a voltmeter for observing time varying signals. It has a fairly low input impedance of one megohm (1M ) so it cannot be used when a load impedance of this size would distort the signal being measured. It is an excellent tool for measuring transient phenomenon such as impact forces on a load cell. Modern oscilloscopes can operate in both a digital mode and analog mode. They also have built-in computers for doing signal analysis such as Fourier transforms on the incoming signal. This type of measurement and analysis would be very useful in measuring impact response of a suspension system.
It is important that you do not indiscriminately turn the controls especially if you have not been instructed in their use and function. This can prevent the oscilloscope from being able to properly display an incoming signal.
An ideal meter will not disturb the circuit when taking measurements. Multimeters and oscilloscopes are not ideal instruments. You can determine the root-mean-square (rms) value of a sine wave displayed 푉 2 on an oscilloscope by the following equation: 푉 = 푝−푝 √ = 0.3535푣 = 0.707푣 푟푚푠 2 2 푝−푝 푝
If you are using one of the new digital oscilloscopes, you can read waveform parameters on the lower menu which displays Vrms, Vp-p, and frequency. and phase The voltage from a household outlet is 120 1 R. Frank Smith, Cal Poly Pomona University, 2016
ECE 231 Laboratory Exercise 3 Oscilloscope/Function-Generator Operation VAC. This is the rms value. The peak value is 1.414 *120= 169.7 voltages. The heating value of 120 VAC rms is exactly equal to a 120 VDC voltage source such as a photovoltaic panel.
PROCEDURE Part 1
1. Connect channel 1 of the oscilloscope to the signal generator and to the digital multimeter (set to voltage). See Figure 1. Make sure that the ground on the oscilloscope and signal generator are connected together. Both are internally grounded to the building ground system. 2. Set the signal generator to 1 KHz, 5 V pk-to-pk for each of the following waveforms: sine wave, triangle wave, and square wave. Increase the frequency to 10 kHz, and then 100 kHz. Connect a BNC cable to both the signal generator and the oscilloscope channel 1 (two cables required). Connect the red clip leads together. Plug your banana cable into the multimeter then connect the red clip to the red clip leads going to channel 1 and the signal generator. See Figure 1. The instruments are internally connected to the black lead so you shouldn’t have to do anything with the black lead. You can connect them all together if you want. The black lead should be at earth ground potential. Make sure the trigger is set to channel 1. 3. Plot what you see on the oscilloscope screen. in Figure 2. You can copy the signal seen on the oscilloscope and paste it into your lab report so that you don’t have to draw it by hand. 4. Compare the readings on the multimeter with what you see on the oscilloscope. Place the results in Table 1. Add dc offset to your input signal and describe what happens on the oscilloscope. Try to read just the offset using the multimeters and the oscilloscope. Change the oscilloscope Vertical Mode from GND, to AC, and then to DC. On the dc setting you see both the dc and ac signal. In the ac setting you only see the ac waveform. Describe what happens to the waveform displayed on the oscilloscope with and without DC offset. The multimeter should not be able to read the voltage as accurately as the oscilloscope. Record your readings in Table 1. The oscilloscope will automatically display the signal’s voltage value and frequency automatically. Use the soft keys to select voltage and time measurements.
2 R. Frank Smith, Cal Poly Pomona University, 2016
ECE 231 Laboratory Exercise 3 Oscilloscope/Function-Generator Operation
Figure 1. Test Setup
Time (sec.,msec., sec.)
3 R. Frank Smith, Cal Poly Pomona University, 2016
ECE 231 Laboratory Exercise 3 Oscilloscope/Function-Generator Operation Figure 2. Oscilloscope Display
Table 1. Measured and calculated results
Waveform Oscilloscope Multimeter Frequency Calculated RMS reading Vp-p reading voltage Sine wave 1 k Hz Sine wave +5dc 1 k HZ Triangular 1 kHz Square 1 kHz
Sine wave 10 kHz Sine wave +5dc 10 kHz Triangular 10 kHz Square 10 kHz
Sine wave 100 kHz Sine wave +5dc 100 kHz Triangular 100 kHz Square 100 kHz
5. Now slowly increase the frequency of the function generator until the multimeter has an error of at least 10%. The voltmeter reading will be less than the oscilloscope reading. 6. What is the frequency limitation of the multimeter. ______Hertz. Sine wave 7. What is the frequency limitation of the multimeter. ______Hertz. Square wave 8. What is the frequency limitation of the multimeter. ______Hertz. Saw tooth wave
2 Notes: If the amount of heat (joules) generated by a DC source (푖푑푐푅푇) is equal to the heat generated 푇 2 by an ac source over the same period T ,(∫0 푅 ∗ 푖 푑푡). Equating the energies and solving results in 1 푇 퐼 = 푖 = √ ∫ 푖2 푡 푑푡 . The following are 푉푟푚푠 equations for common waveforms: 퐷퐶 푟푚푠 푇 0 푡
푉푝−푝 √2 푉푝−푝 Sine wave 푉푟푚푠 = ; square wave 푉 푟푚푠 = 푉푝; triangle wave 푉푟푚푠 = 2 2 2√3 4 R. Frank Smith, Cal Poly Pomona University, 2016
ECE 231 Laboratory Exercise 3 Oscilloscope/Function-Generator Operation 1 휔 푟푎푑푖푎푛푠/푠푒푐표푛푑 = 푓푟푒푞푢푒푛푐푦 표푓 푤푎푣푒푓표푟푚 (Hz) = 푝푒푟푖표푑 푇 표푓 표푛푒 푐푦푐푙푒 2휋
9. Describe how you measure the frequency of a waveform from the oscilloscope display if you didn’t have soft keys to measure it automatically
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10. Why does the multimeter reading decrease as the frequency increases?
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Hint: See Exercise 6. The input circuit topology to many analog voltmeters is usually a low pass filter.
Write a professional comprehensive lab report using a word processor. Show your results and include a comprehensive conclusion. There are lots of sample lab reports on the internet.
Conclusion
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5 R. Frank Smith, Cal Poly Pomona University, 2016