Exercise 1: Balanced Modulator and LSB Filter - Lab-Volt · The balanced modulator’s outputs are the sum (455 kHz USB) and difference (449 kHz LSB) frequencies. You will suppress

Student Manual

FACET by Lab-Volt 193

Analog Communications SSB Transmission

Exercise 1: Balanced Modulator and LSB Filter

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to describe how a balanced modulator produces

power consumption and a narrow bandwidth. You will use an oscilloscope to make signal measurements.

DISCUSSION

The balanced modulator converts the message and carrier signals to a DSB signal.

The balanced modulator frequency-translates a message signal to two sidebands and suppresses the

carrier signal frequency to produce a DSB signal.

c) is relatively low (452 kHz). A low fc

provides a higher percentage of difference between upper and lower sideband frequencies than a high fc

would provide.

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194 FACET by Lab-Volt

SSB Transmission Analog Communications

removes the 449 kHz LSB signal.

SSB signal is the 455 kHz USB.

kHz USB frequency is present.

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The advantages of SSB transmission are that:

• only 16.5% of the total 100% modulated AM signal power is required because the carrier and one of

the sidebands are not present.

• the SSB bandwidth is less than half of the AM bandwidth.

These advantages are offset, however, because more components are required for an SSB transmitter

than for an AM transmitter.

If the message is 3 kHz and the carrier is 452 kHz (fc), what are the frequencies of the LSB and USB?

a. 446 kHz and 452 kHz

b. 449 kHz and 455 kHz

c. 452 kHz and 458 kHz

PROCEDURE

The following procedure is divided into two sections.

• Balanced Modulator: Convert the Carrier and Message Signals to a DSB Signal

• LSB Filter: Produce an SSB Signal by Filtering the LSB Signal

Each section starts with an explanation of the signals that you will observe and the parameters that you

will measure and calculate.

Balanced Modulator: Convert the Carrier and Message Signals to a DSB Signal

In this PROCEDURE section, you will produce a DSB signal in a balanced modulator by modulating a

452 kHz carrier signal with a 3 kHz message signal. You will observe the difference between a DSB signal

and a 100% modulated AM signal.

If you must restart this section at a later time, setup the circuit by completing the steps in the Adjust SSB

Transmitter for a DSB to the LSB Filter section in the Resources of this manual. Circuit setup guides and

the switch function guide are also located in the Resources.

The 3 kHz message signal and the 452 kHz carrier signal are inputs to the balanced modulator.

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The balanced modulator’s outputs are the sum (455 kHz USB) and difference (449 kHz LSB) frequencies.

You will suppress the carrier frequency (452 kHz) in the output by adjusting the modulator’s null

potentiometer.

When you adjust the modulator’s potentiometer, the output signal goes from less than 100% modulated

(a); to 100% modulated (b); to greater than 100% (c); to a DSB signal that has the carrier signal

suppressed (d).

Notice the difference between a 100% modulated AM signal and a DSB signal.

In a 100% modulated signal, the envelopes do not cross the zero reference line.

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The 100% modulated signal contains both the carrier and sidebands; the amplitudes of the carrier and

message signals are equal.

Because the carrier signal is suppressed in a DSB signal, the zero reference line bisects (cuts in half) the

envelopes.

A DSB signal does not contain the carrier signal; only the sidebands are present.

The DSB envelope has the message signal’s waveform, which is shown as the heavy sine wave.

Which signal is a DSB signal?

a. A

b. B

c. C

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COMMUNICATIONS circuit board, and connect the circuit shown. Be sure to place a two-

post connector in the 452 kHz position on the VCO-LO circuit block. VCO-LO connects to

the MODULATOR’s carrier signal input (C), and the SIGNAL GENERATOR connects to the

MODULATOR’s message signal input (M).

Connect the oscilloscope channel 1 probe to the message signal input (M). While observing

the signal on channel 1, adjust the signal generator for a 300 mVpk-pk, 3 kHz sine wave

(message signal) at M.

Connect the oscilloscope channel 2 probe to the carrier signal input (C). While observing the

signal on channel 2, adjust the amplitude knob on VCO-LO for a 100 mVpk-pk carrier signal at

C.

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Turn the NEGATIVE SUPPLY knob on the left side of the base unit completely

counterclockwise (CCW) to adjust the VCO-LO frequency to less than 452 kHz.

Set switches S1, S2, and S3 to OFF. Adjust the MODULATOR’s potentiometer knob fully

CCW.

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Connect the oscilloscope channel 2 probe to the MODULATOR’s output. Set the

oscilloscope vertical mode to channel 2, and trigger on channel 1 (the message signal). Set

Slowly turn the MODULATOR’s potentiometer knob clockwise (CW) until the AM signal

(channel 2) is less than (<) 100% modulated, as shown.

Continue to turn the knob slowly CW until the AM signal is greater than (>) 100% modulated, as

shown.

Continue to turn the knob slowly CW until the amplitude modulated signal appears, as

shown. What type of amplitude modulated signal appears on channel 2?

a. SSB

b. DSB

c. 100% modulated

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The carrier signal frequency (fc) is 452 kHz, and the message signal frequency (fm) is 3 kHz.

What frequencies are present in the frequency spectrum of the DSB signal?



c. 449 kHz, 452 kHz, and 455 kHz

LSB Filter: Produce an SSB Signal by Filtering the LSB Signal

kHz SSB signal. You will observe the properties of an SSB signal.

If you are not proceeding directly from the Balanced Modulator portion of this procedure or if you must

restart this section at a later time, setup the circuit by completing the steps in the Adjust SSB Transmitter

for a DSB to the LSB Filter section in the Resources of this manual. Circuit setup guides and the switch

function guide are also located in the Resources.

When the base unit’s NEGATIVE SUPPLY knob is fully CCW, the carrier frequency is less than 450 kHz.

shown.

while turning the NEGATIVE SUPPLY potentiometer knob slowly CW.

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455 kHz USB, which is the SSB signal, is output.

USB to two sidebands.

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changes from two sidebands to a 455 kHz SSB signal.

The 455 kHz SSB signal is the LSB because the carrier signal has been increased to 458 kHz.

a. It becomes almost completely attenuated.

b. Another DSB signal appears.

c. A greater-than-100%-modulated signal appears.

Connect the LSB FILTER to the MODULATOR with a two-post connector, as shown.

Set setup switch S1 to ON.

Set setup switches S2 and S3 to OFF.

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In the previous PROCEDURE section, you connected and adjusted the VCO-LO signal for 100 mVpk-pk;

you connected and adjusted the SIGNAL GENERATOR for a 300 mVpk-pk, 3 kHz signal.

Connect the oscilloscope channel 2 probe to the output of the LSB FILTER. Set the channel 2 attenuation

on the base unit should be fully CCW.

A trace like the one in display A appears at the LSB FILTER output on channel 2.

Increase the VCO-LO’s frequency to the MODULATOR by slowly turning the NEGATIVE SUPPLY knob

CW until the LSB FILTER’s output appears, as shown in display B.

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What is the LSB FILTER’s output signal that is shown on channel 2?

a. a greater-than-100%-modulated AM signal

b. a DSB signal

c. an SSB signal

Trigger the oscilloscope on channel 2, and set the oscilloscope sweep to 1

the channel 2 signal appears, as shown.

While observing the 455 kHz signal at the LSB FILTER’s output on channel 2, vary the

amplitude of the 3 kHz message signal to the MODULATOR by varying the AF LEVEL knob

on the SIGNAL GENERATOR. Does the amplitude of the 455 kHz signal vary with the

amplitude of the 3 kHz message signal?

a. yes

b. no

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How much of the message signal does the SSB signal contain?

a. none

b. some

c. all

present LSB and USB frequencies being input to the LSB FILTER?

a. A

b. B

Continue to increase the oscillator’s frequency by slowly turning the NEGATIVE SUPPLY

knob CW until the channel 2 signal appears, as shown. What type of signal is the LSB

FILTER’s output?

a. DSB

b. SSB



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Continue to slowly increase the oscillator’s frequency by turning the NEGATIVE SUPPLY

knob CW until channel 2 appears, as shown. What type of signal is the LSB FILTER’s

output?

a. DSB

b. SSB

Continue to increase the oscillator’s frequency by slowly turning the NEGATIVE SUPPLY

knob CW until the channel 2 signal is a thick line.

characteristic curve?

a. A

b. B

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CONCLUSION

• When the carrier signal is suppressed in an AM signal, the AM signal becomes a DSB signal

containing only the USB and LSB.

• You suppress the carrier signal by adjusting a balanced modulator’s null potentiometer to produce a

DSB signal from the message and carrier signals.

• Each sideband contains the complete intelligence of the message signal.

•

signal.

REVIEW QUESTIONS

1. Which waveform is a DSB signal?

a. A

b. B

c. C

d. D

2. What SSB transmitter circuit combines the message signal with a carrier to produce a signal with two

sidebands and a suppressed carrier?

a.

b.

c. matching network

d. balanced modulator

3. What is the function of the LSB FILTER?

a. It produces a DSB signal.

b.

c.

d.

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4. For the LSB FILTER on the circuit board to output a 455 kHz USB signal, why does the carrier signal

have to be exactly 452 kHz if the message signal is 3 kHz?

a.

b. because the balanced modulator’s sum frequency output has to be exactly 455 kHz

c. because there is only 6 kHz difference between the USB and the LSB

d. All of the above

5. What frequency portion of an AM signal contains the message signal intelligence?

a. carrier and USB

b. each sideband (LSB and USB)

c. LSB only

d. USB only

Documents

Exercise 1: Balanced Modulator and LSB Filter - Lab-Volt · The balanced modulator’s outputs are the sum (455 kHz USB) and difference (449 kHz LSB) frequencies. You will suppress