Oscilloscope/Function-Generator Operation

ECE 231 Laboratory Exercise 3

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.

EQUIPMENTREQUIRED

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 forobserving 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 on an oscilloscope by the following equation:

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 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

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.
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.
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.
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.

Figure 1. Test Setup

Time (sec.,msec., sec.)
Figure 2. Oscilloscope Display

Table 1. Measured and calculated results

Waveform / Oscilloscope reading Vp-p / Multimeter reading / Frequency / Calculated RMS 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

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.
What is the frequency limitation of the multimeter. ______Hertz. Sine wave
What is the frequency limitation of the multimeter. ______Hertz. Square wave
What is the frequency limitation of the multimeter. ______Hertz. Saw tooth wave

Notes: If the amount of heat (joules) generated by a DC source () is equal to the heat generated by an ac source over the same period T ,(. Equating the energies and solving results in . The following are equations for common waveforms:

Sine wave; square wave; triangle wave

(Hz) =

Describe how you measure the frequency of a waveform from the oscilloscope display if you didn’t have soft keys to measure it automatically

______

Why does the multimeter reading decrease as the frequency increases?

______

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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R. Frank Smith, Cal Poly Pomona University, 2016