Measurement of Time Varying Sources

In this section, we will be using the function generator to provide a time-varying voltage which will then be measured using the oscilloscope.
Important: The function generator has an internal resistance of 50 . The voltage displayed on its front panel is the voltage that would appear at its terminals if it were connected to a 50 load. Under open circuit conditions, the voltage across the terminals of the function generator is twice as large as the value indicated on its front panel. This is an important concept and often misunderstood.

If you are located at a lab station that has no printer, complete items 15-17 after one of the lab stations containing a printer becomes vacant.

1. Set the function generator to produce a 10 kHz sine wave with 6 .
2. Connect the oscilloscope channel 1 probe to the leads on the function generator.
3. Locate the VERTICAL section on the front panel of the oscilloscope. It is the section which includes the connections of the scope probes to the scope.
4. Press the softkey labeled 1. A menu will appear on the bottom of the scope screen. The items that appear on this menu are: channel activation, coupling, bandwidth limiter, inversion, vernier, and probe calibration. Examine the setting for probe. It should be on 10. If it is not, set the probe value to 10. This tells the scope that the probe reduces the signal strength by a factor of 10. All of the scope probes in this lab are ``10x" probes. It is a good idea to get in the habit of checking the probe setting at the beginning of each lab.
5. Press Auto-Scale on the oscilloscope to view the signal produced by the function generator.
6. Using the automatic features of the oscilloscope, determine the peak-to-peak voltage. Observe that the measured peak-to-peak voltage is approximately twice the peak-to-peak voltage shown on the front panel of the function generator.
7. Using the automatic features of the oscilloscope, determine the frequency, and the period of the sinusoidal voltage.
8. Measure the peak-to-peak voltage by hand, i.e. count the vertical divisions from the minimum to the maximum and multiply by the number of volts per division. The volts per division setting is shown in the upper left corner of the scope screen.
9. Using the same screen, measure the period of the sine wave, i.e. count the number of horizontal divisions in one period of the sine wave. The time per division setting is shown at the top-right-center of the scope screen.
10. Using the formula, determine the frequency of the sine wave.
11. Connect the function generator across a 50 resistor. Leave the settings on the function generator unchanged.
12. Connect the oscilloscope channel 1 probe across the resistor (and function generator).
13. Press Auto-Scale on the oscilloscope to view the signal produced by the function generator.
14. Using the automatic features of the oscilloscope, determine the peak-to-peak voltage. Observe that the measured peak-to-peak voltage is approximately the same as the peak-to-peak voltage shown on the front panel of the function generator. Hence, the voltage at the terminals of the function generator is load dependent. Whenever you use the function generator to provide a voltage waveform and desire a specific amplitude, you must use either the multimeter or the scope to set the desired voltage because the value shown on the front panel of the function generator is valid only for a 50 load.
15. Press the Print/Utility softkey on the scope. It is located to the right of Autoscale. Then press the RS-232 Menu softkey. Make sure the resolution is set to low. Press the previous menu softkey.
16. Turn on the HP Deskjet printer. Wait a few moments and then press the print screen softkey on the scope. A printout of the waveform shown on the scope screen should result. Feel free to use the printer to obtain scope traces as needed to document your laboratory experiments.
17. Compare the values for the peak-to-peak voltage and frequency that were 1) specified by the function generator, 2) measured by the oscilloscope and 3) measured by hand. Do the measurements made by hand differ from those made by the oscilloscope? Which do you think are more accurate? Justify your answer.