NAME __________________________
Exercise 24A
Additional Waveform Measurements
Purpose
The purpose of this exercise is to demonstrate frequency and period measurements
of common electronic signals.
Objectives
After completing this exercise, you should be able to:
1.
2.
3.
To measure frequency and period using a frequency counter.
To measure frequency and period using an oscilloscope.
To use a function generator to create analog and digital waveforms of
various frequencies.
To distinguish the differences between analog and digital waveforms.
4.
Lab Preparation
Review Sections 14.3 and 15.2 of Fundamentals of Electronics: DC/AC Circuits.
Materials
1 trainer with hookup wires as needed
1
1
1
1
Function generator w/leads
Frequency counter with RF cable
Oscilloscope w/probe (1)
1-kΩ resistor
Discussion
Electronic signals can be divided into two broad categories--analog and digital. An
analog signal changes smoothly from one value to the next with respect to time (a
continuous change in amplitude occurs). Figure 24A.1 (a) shows a variety of signals found
in electronic circuits. All are considered analog in nature except for the square wave. A
digital signal (Figure 24A.1 (b)) abruptly changes from one level to the next with no
intermediate values. Thus, amplitude changes occur in steps.
Another characterization for electronic signals is something you have already been
studying, that is, electronic signals can be classified as AC (alternating current) or DC
(direct current) signals. You have already learned that DC is current flowing in one
direction only, while AC is bidirectional current flow. If we assume that the dashed lines in
Figure 24A.1 (a) are the zero reference points, then all signals represented in the figure
are AC. The waveform in 24A.1 (b) is a DC signal because that waveform does not cross
zero.
In this lab you will investigate several pieces of test equipment (oscilloscope,
function generator, and frequency counter) that are regularly used by technicians to repair
and test electronic circuits. You will use these pieces of equipment to measure both the
frequency and period of several types of electronic signals.
(a)
(b)
Figure 24A.1
Procedure
Complete each of the following steps in the order given.
A. Waveform Analysis:
Figure 24A.2
1.
Construct the circuit shown in Figure 24A.2 using the function generator as
the source. Use the breadboard of the analog trainer.
2.
Most function generators have at least three waveform choices--sine, square,
and triangle.
3.
Set the function generator to:
a. sine wave function
b. frequency of 1 kHz
c. output amplitude midway
d. DC offset set to 0V (some generators may not have one)
4.
There are three basic sets of controls for an oscilloscope.
a. vertical controls for each channel
b. horizontal time base settings
c. triggering controls
5.
Set the oscilloscope to:
a. Vertical: 1V/Div., DC coupled-Channel 1
b. Vertical Mode: CH 1
c. Sweep Time: 0.5mS/Div. (AS A STARTING POINT)
d. Triggering Mode: AUTO
e. Trigger Slope: Negative (-)
f. Trigger Source: CH 1
6.
Position the traces on the scope as follows:
a. Channel 1 - center horizontal line
7.
Connect the ground lead of the CH 1 scope probe to the
bottom of R1.
8.
Connect CH 1 probe (X1 position) of the scope to the top of R1.
9.
If a sine wave is not visible on the scope ask your instructor for assistance. If
you have a sine wave, adjust the amplitude control for a 2 Vpp signal.
10.
Is a sine wave analog or digital?
11.
Switch the output of the function generator to produce a square wave.
12.
Is a square wave analog or digital? __________________
13.
Switch the output of the function generator to produce a triangle wave.
14.
Is a triangle wave analog or digital? ____________________
15.
Describe the characteristics of the wave that made you choose your answers
for steps 10, 12, and 14.
________________
B. Frequency and Period Analysis:
16.
Switch the output of the function generator to produce a square wave again,
use this output for steps 17-24. (Frequency still at 1 kHz.)
17.
To measure the frequency of the waveform on the scope count the number
of centimeters (divisions) that the full wave covers. A full waveform (one
cycle) includes both one high and one low value. The number of divisions is:
____________ div
18.
Multiply the number of divisions by the setting of the horizontal time base
control.
TIME = # of div X time per div
TIME = ____________X_______________ = ______________
19.
Calculate the frequency of the waveform by dividing the time into 1 (f = 1/T).
Frequency = ___________________Hz
20.
Connect the frequency counter across R1.
21.
Set the counter to the period mode. Record the period of the waveform.
_______________sec
22.
Set the counter to the frequency mode. Record the frequency of the
waveform. ___________________Hz
23.
Are the values of steps 18 compared to 21 and 19 compared to 22 close to
each other?
YES/NO
24.
Set the function generator to the frequencies listed in the table and then
repeat steps 17 through 22 and complete the table.
Counter
Reading
(Period)
Counter
Reading
(Frequency)
Scope
Reading
(Period)
Scope
Calculation
(Frequency)
250 Hz
10 kHz
25 kHz
100 kHz
Questions
1.
Using complete sentences, describe the difference between an analog
and a digital signal.