Name________________________
Exercise 25
Measuring Nonsinusoidal Waveforms on an Oscilloscope
Purpose
The purpose of this exercise is to demonstrate voltage and time measurements of
nonsinusoidal waveforms.
Objectives
After completing this exercise, you should be able to:
1.
Examine a nonsinusoidal waveform and observe its properties.
2.
Measure voltage and time values of a nonsinusoidal waveform.
3.
Operate a function generator and observe its output waveforms on the
oscilloscope.
4.
Calculate and measure the duty cycle and average value of a nonsinusoidal
waveform.
Lab Preparation
Review Sections 14.5 and 15.2 of Fundamentals of Electronics: DC/AC Circuits.
Materials
1 dual-trace oscilloscope and probe (1)
1 function generator with leads
1 DMM with leads
Discussion
To generate nonsinusoidal waveforms, most labs are equipped with a function
generator. The function generator typically can provide three distinctive waveforms. They are
the sine wave (which we have already observed), the triangular wave, and the square wave.
Most function generators have a DC offset control that can insert a DC voltage reference (either
negative or positive) into the AC waveform. In this exercise, you will measure various voltage
and time values of a square wave.
Procedure
1.
Connect the CH 1 probe (1) across the function generator terminals. Select the squarewave function, an amplitude of 4 V pp, 0 V offset, and a frequency of about 1 kHz. (Be
sure that you have set up a ground reference on the center of the graticule.)
2.
The display should be a symmetrical signal, going equally positive and negative with
respect to the ground reference at the centerline.
3.
Measure and record the baseline, peak-to-peak amplitude, and period in Table 25.1.
Then, using the known period (T), calculate the PRF (pulse repetition frequency) and
confirm that it matches the setting on the function generator.
4.
Measure and record the pulse width (PW) in Table 25.1.
5.
From the period and the pulse width, calculate and record the duty cycle (DC) and
then the average value in Table 25.1.
6.
Confirm the average value by measuring the generator output with a DMM in the
DCV position.
7.
Momentarily switch the input coupling from DC to AC and observe the display.
What did the waveform do?
8.
If the DMM did not measure 0 V, check to make sure that the DC offset was
established at 0 V.
9.
Turn the DC offset control so that the waveform rises by 2 V. The baseline should
now be 0 V with the peak at 4 V (scope with DC coupling).
10.
Repeat all measurements in Steps 3–6, recording the data in Table 25.1.
11.
Momentarily switch the input coupling from DC to AC and observe the display.
What did the waveform do?
12.
Now insert a –2-V DC offset so that the square wave is negative going from –4 V to
0 V.
13.
Repeat all measurements in Steps 3–6, recording the data in Table 25.1.
14.
Momentarily switch the input coupling from DC to AC and observe the display.
What did the waveform do?
Table 25.1
DC
Offset
Baseline
(V)
Amplitude
(Pk-Pk)
Period
(s)
PRF
(Hz)
Pulse
Width
(s)
Duty
Cycle
Average Value, V
Calculated
0V
+2 V
-2 V
Questions
1.
A square wave has a baseline of –2 V and a peak-to-peak voltage of 10 V. What is
the most positive voltage in this waveform?
Measured
2.
What is the average value of the waveform in Question 1?
3.
Sketch two square waves, one with an average value of 0 V and one with an average
value of 3 V. Label all pertinent amplitude and time quantities.
4.
Sketch two waveforms with the same duty cycle but different frequencies. Label all
pertinent time and amplitude values.