Family Name: Date Performed:
First Name: Date Submitted:
Group Number: Instructor:
Communications I Laboratory
Experiment No. ECE 412B-12
The Varactor Modulator
.OBJECTIVES
At the end of the experiment, students must be able to:
1. describe the characteristics of an FM transmission system
2. outline the operation of a Varactor Modulator
II. PRE-LAB REQUIRED READING
The students are expected to have read the following topics/resources:
Pages 283-286 & 288-290, Electronic Communication Systems: Fundamentals through
Advanced, 5 Edition by Wayne Tomasi
th
Pages 85-87, Communications Electronics, 2 Edition by Louis E. Frenzel
nd
Pages 69-71 & 200-207, Electronic Communication Systems, 2 Edition by Roy Blake
nd
III. EQUIPMENT
The students must borrow the following materials from the laboratory assistant:
1 oscilloscope with XY capability
1 voltmeter or multi-meter
1 ANACOM 2 board
1 DC power supply
IV. PROCEDURE
Read and understand carefully before performing the following procedures.
1. Connect the ANACOM 2 board to the power supply as shown in Figure 12-1.
2. Set the following initial conditions on ANACOM 2.
a. All the switched faults to OFF
a. VCO switch in the Phase Locked Loop Detector block set to OFF.
a. AMPLITUDE and FREQUENCY presets in the AUDIO Oscillator block set to
maximum (fully clockwise).
0. Switch on the power supply.
0. Use your oscilloscope to measure the frequency and the peak-to-peak amplitude of the
output signal of the Audio Oscillator at tp1 and record it in Table 12-1.
0. Connect the output of the Audio Oscillator to the Audio Input socket of the Modulator
Circuits block.
0. Set the Reactance/Varactor switch to the Varactor position.
0. Reduce the Amplitude preset in the Audio Oscillator block to minimum (fully
counter-clockwise).
0. Set the Carrier Frequency preset in the Varactor Modulator to minimum (fully
counter-clockwise).
0. Set the Amplitude preset in the Mixer/Amplifier to maximum (fully clockwise).
0. Use your oscilloscope to measure the frequency and the peak-to-peak amplitude of the
output signal at tp34 and record it in Table 12-1.
0. Continue to monitor the output waveform at tp34. Adjust the time base frequency of your
oscilloscope to display approximately 2 complete waveforms per division. This figure is not
critical. Note that the carrier is not being frequency modulated at the moment.
0. Adjust the focus control on your oscilloscope and closely observe the cycles
towards the right-hand side of the screen as you increase the Audio Oscillator
Amplitude preset control to maximum (fully clockwise).
0. Ensure that the Carrier Frequency preset is set to minimum (fully
counter-clockwise).
0. Measure and record in Table 12-2 the oscillator output frequency at tp34 and
the DC input voltage measured at tp21 when the Carrier Frequency preset is set to
minimum.
0. Turn the Carrier Frequency preset is set to maximum (fully clockwise).
0. Measure and record in Table 12-2 the oscillator output frequency at tp34 and the DC
input voltage measured at tp21 when the Carrier Frequency preset is set to maximum.
0. Switch off the power supply.
Figure 12-1
V. DATA
Write your calculated and measured data on the tables provided. Table 12-1
Waveform type Frequenc Peak-to-Peak Voltage
y
Audio Oscillator signal at tp1
Mixer/Amplifier signal at
tp34 Table 12-2
Waveform type Signal Frequency at DC Voltage at tp21
tp34
Carrier Frequency minimum
Carrier Frequency
maximum
VI. GRAPH
Illustrate the graphs or waveforms in the experiment.
VII. QUESTIONS
Answer the following questions by writing the letter of your choice inside the
parenthesis. If it is not a multiple question write your answer in the space provided
after the question.
1. Frequency modulation occurs when ( )
a. the amplitude of the carrier wave is controlled by the frequency of the information signal
a. the Frequency of the carrier wave is controlled by the amplitude of the information signal
a. the Amplitude of the carrier wave is controlled by the amplitude of the information
signal.
a. The frequency of the carrier wave is affected by the phase of the information signal
0. FM transmissions are less susceptible to noise because: ( )
a. the frequency of FM noise is not within the audio range
a. noise affects the amplitude of the signal and, generally, FM receivers do not respond
to amplitude variations.
a. it is easily filtered out by a low pass filter.
a. they use noise free transmission media.
0. In an FM modulator, increasing the modulating frequency: ( )
a. Has no effects on the FM waveform
a. Increases the number of frequency variations that occur in each second
a. Increases the amplitude of the FM waveform
a. Increases the number of amplitude changes that occur in each second
0. If the modulating signal applied to an FM modulator increased from 2 volts to 4 volts and
this resulted in the carrier frequency changing from 2.5 MHz to 3 MHz, the frequency deviation
is: ( )
a. 2.5 MHz
a. 1.25 MHz
a. 125 MHz
a. 250 MHz
0. For a given modulating signal amplitude, there will be: ( )
a. more distortion in the demodulated audio signal when using a varactor modulator that
with a reactance modulator
a. more changes per second in carrier frequency when a reactance modulator is used
a. no difference in the levels of distortion in the demodulated audio signal caused by
the varactor modulator and the reactance modulator
a. more distortion of the demodulated audio signal when using a reactance modulator than
with a varactor modulator.
VIII. OBSERVATIONS/DISCUSSION
Discuss your observations on the experiment performed. Write your answers on the space
provided.
