OSCILLOSCOPES AND FUNCTION GENERATORS
This is an introductory lab intended to provide training on the use of basic electronic test equipment such as oscilloscopes and function generators. During the lab work, the function generators will be set to generate some waveforms with specific shapes and voltages. The signal parameters will then be measured and verified using the oscilloscope.
Function generator is a device that produces a desired signal of specified frequency, shape and amplitude. This implies that you would be able to adjust the frequency, set the shape or function (sinusoidal, triangle and square/rectangular) and change the amplitude level. There is also another adjustment possible and that is the offset. Offset is the DC level of the signal either positive or negative. Figure 1 shows an Agilent 33220A Function Generator.
Oscilloscope is an instrument that captures the shape of a signal and displays it on screen. An oscilloscope usually has two or four channels and one can see one to four signals simultaneously on its screen. Figure 2 shows an Agilent 54622A Oscilloscope we use in this lab. This one has two channels.
In order to use these instruments, we have to make sure they are reset to the factory settings. Here is the procedure to achieve this task:
Agilent 33220A Function Generator (FG):
- Turn the instrument on.
- Press "Store/Recall" key.
- Select the soft key corresponding to "Set to Defaults".
- Select "Yes" soft key.
Now Agilent 33220A is set to the factory default values. But we are not done yet. We need to make one more modification in the output of 33220A; change the output impedance from 50 Ω to "High Z". Here is the procedure:
- Press "Utility" button.
- Select "Output Setup" soft key.
- Press "Load" softkey to change the value from "50 Ω" to "High Z"
- Press "Done".
Agilent 54622A Oscilloscope (Oscope):
- Turn the instrument on.
- Press "Save/Recall".
- Select the soft key corresponding to "Default Setup".
The Agilent 54622A oscilloscope is now set to the factory default values.
- Use the function generator to generate a triangular waveform. Set the signal amplitude to 2V and the signal frequency to 10 kHz. Display this waveform on the oscilloscope. Measure the voltages at the positive and negative peaks (Vposp and Vnegp) of this signal. Also measure T1 and T2 as shown in Figure 3. How do you compare the oscilloscope readings with the signal amplitude and frequency indicated by the function generator? Explain your answer.
- Using the function generator, obtain a non-uniform rectangular waveform, with 4 V peak-to-peak amplitude and a minimum voltage at about +0.5 V. Set the frequency to 400 kHz. Try to set TS = 2TM. A typical signal is shown in Figure 4. Follow the instructions in "Collecting Data From Agilent 54622A Oscilloscopes"
to download the data.
Figure 4 - Rectangular Waveform
- Using the oscilloscope,
- Measure accurately the frequency of the signal.
- Measure accurately the two intervals TM and TS as shown in Figure 4. How do you correlate these two measurements with the results obtained in (a) above? Find the "Duty Cycle" of this signal. The duty cycle is defined as the time a rectangular wave is at high value with respect to the period of the rectangular wave. It is usually indicated in %, e.g., if the period of the rectangular wave is 10 millisecond and the rectangular is in high position for 2 millisecond, then the duty cycle is (2/10) x 100 % = 20%.
- Measure the high and low voltages of this signal.
- Measure the rise and fall times of the edges of the waveform: trise and tfall.
- Compare the measured data with the setting of the function generator. Comment on the differences.
- Sinusoidal waveform
- Set the frequency of the function generator to 10 KHz sine wave and amplitude to 1-Vpp (peak-to-peak) and offset to zero. Observe the signal on the oscilloscope.
- Next change the offset voltage with respect to ground and observe how the waveform looks like. First, set the the offset to +0.5 Volt, Next to -0.5 Volt.
For each part, collect the data from the oscilloscope using the above mentioned procedure and write down all the measurements you have made. The finished product should be turned in at the beginning of the following session.