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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.

Student Manual

FACET by Lab-Volt 13

Analog Communications Introduction to Analog Communications

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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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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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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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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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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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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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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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?

a. mixing

b.

c. demodulation

d. modulation

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