Student Manual 12 FACET by Lab-Volt Introduction to Analog Communications Analog Communications Exercise 1: Analog Communication Concepts EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe the basic principles of analog radio communications and recognize a message signal, a carrier signal, and a modulated signal. You will use an oscilloscope to make signal measurements. DISCUSSION $QDORJ VLJQDOV DUH FRQWLQXRXV DQG YDU\ LQ DPSOLWXGH IUHTXHQF\ DQGRU SKDVH Digital signals are discontinuous waves and have a high or low state. An analog communication system transmits and receives analog signals. How are the many simultaneous radio communications that radiate through space prevented from interfering with one another? a. The radio communications are multiplexed or combined. b. Each radio communication uses a different carrier frequency. c. Each radio transmitter uses a different modulation method.
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Student Manual
12 FACET by Lab-Volt
Introduction to Analog Communications Analog Communications
Exercise 1: Analog Communication Concepts
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to describe the basic principles of analog radio
communications and recognize a message signal, a carrier signal, and a modulated signal. You will use
an oscilloscope to make signal measurements.
DISCUSSION
Digital signals are discontinuous waves and have a high or low state.
An analog communication system transmits and receives analog signals.
How are the many simultaneous radio communications that
radiate through space prevented from interfering with one
another?
a. The radio communications are multiplexed or
combined.
b. Each radio communication uses a different carrier
frequency.
c. Each radio transmitter uses a different modulation
method.
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Two types of signals compose a radio signal:
• a low frequency message signal that contains the communication intelligence,
• and a high frequency carrier signal that is commonly called the RF signal.
It is normally a low-frequency AF signal in the 20 Hz to 20 kHz range.
What is the frequency range of carrier signals?
a. 9 kHz to 300 GHz
b. 30 kHz to 100 kHz
c. 100 kHz to 300 kHz
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Because message signals from different sources are in the same frequency range, each message signal
is transferred to a unique carrier frequency to prevent interference during transmission.
Modulation is the process of combining the message signal with a carrier frequency in the transmitter by
changing a characteristic of the carrier signal with the message signal.
The high frequencies for carrier signals also permit practical antenna sizes; low frequency signals require
an extremely long antenna.
What characteristic of the carrier signal is varied by modulation?
a. amplitude
b. frequency
c. phase
d. Any of the above
In amplitude modulation, the message signal causes the amplitude of the carrier signal to vary; the
amplitude variations of the carrier signal’s envelopes contain the message signal.
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FM and PM are types of angle modulation.
In FM, the message signal amplitude causes the FM carrier frequency to deviate; the frequency
deviations contain the message signal.
In PM, the message signal amplitude and frequency cause the FM carrier phase and frequency to
deviate; the phase and frequency deviations contain the message signal.
When a low frequency message signal modulates a high frequency carrier signal, frequencies on each
side of the carrier frequency are formed.
The frequencies on each side of the carrier signal are sidebands.
The bandwidth of an AM signal is the difference between the highest and the lowest frequencies on each
side of the carrier frequency.
What frequency contains the message signal?
a. the carrier signal frequency
b. the sideband frequencies
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carrier frequency with an adequate bandwidth.
The FM frequencies assigned to different users in the U.S. are shown.
Noise is any random signal with a frequency in the radio frequency spectrum that disturbs the
communications.
Radio transmitters are designed to minimize noise creation, and receivers are designed to reduce noise
picked up during transmission.
Radio signals are transmitted by what method?
a. radiated through space
b. conducted through a wire
c. All of the above
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Analog Communications Introduction to Analog Communications
When radio signals are sent through space, the transmitting antenna radiates the signal, which is picked
up by the antenna at the receiver.
from among the many other radiated frequencies.
What is the process of recovering the message signal from the carrier signal?
a. modulation
b.
c. demodulation
demodulated.
The receiver’s detector section demodulates the carrier signal to recover the low frequency message
signal.
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Introduction to Analog Communications Analog Communications
PROCEDURE
circuit board, and connect the SIGNAL GENERATOR to M on the MODULATOR, as shown.
Set switches S1, S2, and S3 to OFF.
channel 1. Set channel 1 to ac.
Connect the oscilloscope channel 1 probe to M (message signal input) of the MODULATOR.
NOTE: When making oscilloscope measurements or observations, be sure that you connect the probe’s
ground clip to a ground terminal on the ciruit board.
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Analog Communications Introduction to Analog Communications
While observing the signal on channel 1 of the oscilloscope, adjust the SIGNAL
GENERATOR for a 200 mVpk-pk, 2 kHz sine wave signal at M.
What is the channel 1 signal at M on the MODULATOR?
a. carrier signal
b. message signal
Increase the frequency of the message signal to 5 kHz and then decrease the frequency to
2 kHz by varying the AF FREQUENCY knob on the SIGNAL GENERATOR.
When you increased the frequency to 5 kHz, did the signal period (the time between one
cycle) decrease?
a. yes
b. no
Increase the amplitude of the message signal to 400 mVpk-pk and then decrease the
amplitude to 200 mVpk-pk by varying the AF LEVEL knob on the SIGNAL GENERATOR.
When you increased the amplitude of the message signal to 400 mVpk-pk, did the period
increase, decrease, or stay constant?
a. increase
b. decrease
c. stay constant
Locate the VCO-LO circuit block on the ANALOG COMMUNICATIONS circuit board. Place a
two-post connector in the 1000 kHz position on the VCO-LO circuit block.
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Introduction to Analog Communications Analog Communications
Connect OUT on the VCO-LO circuit block to C (carrier input) at the MODULATOR.
set the sweep to 0.5
Connect the channel 2 probe to C.
While observing the signal on channel 2, set the amplitude of the VCO-LO output to
200 mVpk-pk by adjusting the potentiometer knob on the VCO-LO circuit block.
Set the frequency of the VCO-LO output to 1000 kHz by adjusting the NEGATIVE SUPPLY
knob on the base unit.
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Analog Communications Introduction to Analog Communications
With the sweep set to 0.5
oscilloscope graticule divisions.
What is the signal on channel 2?
a. carrier signal
b. message signal
Adjust the MODULATOR’s potentiometer knob completely counterclockwise (CCW). Trigger
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Connect the channel 2 probe to the output of the MODULATOR.
Slowly turn the MODULATOR’s potentiometer knob so that the AM waveform on
oscilloscope channel 2 appears, as shown.
What type of modulation are you observing on channel 2?
a. amplitude modulation
b. frequency modulation
c. phase modulation
Connect the MODULATOR to the POWER AMPLIFIER by placing two-post connectors in
Connect the output of the RF POWER AMPLIFIER to the input of the ENVELOPE DETECTOR
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Analog Communications Introduction to Analog Communications
Connect the channel 2 probe to the output of the ENVELOPE DETECTOR.
Compare the message signal input to the MODULATOR on channel 1 to the output of the
ENVELOPE DETECTOR on channel 2.
What is the signal on channel 2?
a. the carrier signal
b. the recovered message signal
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On the SIGNAL GENERATOR, vary the amplitude and frequency of the message signal.
Does the amplitude and frequency of the recovered message signal on channel 2 change
with the channel 1 message signal?
a. yes
b. no
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Analog Communications Introduction to Analog Communications
CONCLUSION
• Analog signals are continuous, as compared to digital signals, which are either high (logic 1) or low
(logic 0).
• Message signals (information) have low frequencies from 20 Hz to 20 kHz.
• Carrier signals have frequencies from 9 kHz to 300 GHz.
• Modulation is the process of the message signal changing a characteristic (amplitude, frequency, or
phase) of the carrier signal so that the message can be transmitted at the higher carrier frequency.
• Analog radio signals are transmitted by conduction through a wire, or radiated through space.
• In the receiver, demodulation is the process of recovering the message signal from the modulated
carrier signal.
• Bandwidth of an AM signal is the difference between the highest and lowest frequencies.
REVIEW QUESTIONS
1. Carrier signals may have a frequency from 9 kHz to 300 GHz. In the PROCEDURE section, you used
a 1000 kHz carrier signal.
What is the frequency range of message signals?
a. 1 Hz to 10 Hz
b. 30 Hz to 50 kHz
c. 20 Hz to 20 kHz
d. 100 kHz to 200 kHz
2. What is the process of combining a message signal with a carrier signal for communication over long
distances?
a.
b.
c. demodulation
d. modulation
3. What type of modulation is shown?
a. frequency modulation
b. amplitude modulation
c. phase modulation
d. angle modulation
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4. In the PROCEDURE section, you modulated a 1000 kHz carrier signal with a 2 kHz message signal.
The frequency spectrum of the AM signal that is produced is shown.
What is the bandwidth of the AM signal?
a. 2 kHz
b. 4 kHz
c. 1002 kHz
d. 998 kHz
5. In the PROCEDURE section, what function did the ENVELOPE DETECTOR perform?