Exercise 2: Circuit Board Familiarizationlucian/ro/SistemeDeComunicatii/labvolt/1...Student Manual...

Student Manual

FACET by Lab-Volt 27

Analog Communications Introduction to Analog Communications

Exercise 2: Circuit Board Familiarization

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to describe analog radio communications

circuits and locate them on the ANALOG COMMUNICATIONS circuit board. You will use an oscilloscope

to make signal measurements.

DISCUSSION

The following circuits are found in radio transmitters and receivers:

• oscillators

•

• modulators

• mixers

•

• LC networks

• limiters

• detectors

The ANALOG COMMUNICATIONS circuit board includes all of these circuits.

The following seven circuit blocks make up the ANALOG COMMUNICATIONS circuit board:

• VCO-LO

• VCO-HI

•

• PHASE-LOCKED LOOP

•

• PHASE MODULATOR

• QUADRATURE DETECTOR

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28 FACET by Lab-Volt

Introduction to Analog Communications Analog Communications

The two voltage controlled oscillators (VCOs) on the circuit board are the VCO-LO circuit block (452 kHz

or 1000 kHz) and the VCO-HI circuit block (1455 kHz).

An input voltage controls the VCO output frequency.

What is the function of the oscillators on the circuit board?

a. They provide the high frequencies that are necessary for

transmitting and receiving the low frequency message signal.

b. They provide the low frequency message signal.

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long distances.

signal can be demodulated and understood.

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The ANALOG COMMUNICATIONS circuit board contains an amplitude modulator, a frequency modulator,

and a phase modulator.

The VCO-LO circuit block functions as a frequency modulator as well as an oscillator; a message signal

applied to the oscillator input frequency modulates the oscillator’s carrier signal.

What is the function of a modulator?

a. It recovers the low frequency message signal from the high frequency carrier signal.

b. It combines the message signal with the carrier signal so that the message can be transmitted at

the higher carrier frequency.

Mixers increase or decrease the frequency of a signal.

Because a balanced modulator multiplies two input signals and outputs the sum and difference

frequencies, the balanced modulator can function as a mixer.

Filters remove undesired frequencies and pass desired frequencies.

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the undesired high frequencies.

What is one of the components that is adjusted when you tune in to your favorite radio station?

a.

b. a detector that demodulates the carrier signal

Circuits composed of inductors (L), capacitors (C), varactors (CR), and resistors (R), which are called

networks, can perform several functions in radio receivers and transmitters.

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LC networks are used in

• oscillators to set the output frequency,

•

• and circuits to match output and input impedances of circuits.

What is the impedance of an LC network at its resonant frequency (fr)?

a. The impedance is capacitive and the signal’s phase is shifted.

b. The impedance is resistive and the network’s output signal is maximum.

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A limiter is a circuit that restricts a signal’s peak-to-peak amplitude to a maximum value in order to

eliminate any high amplitude noise.

The limiters on the circuit board have diodes with their anodes and cathodes connected; this arrangement

limits a signal’s voltage within the forward drop of the diodes.

What is the function of a detector?

a. It recovers the message signal.

b. It detects the transmitted radio frequency.

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The ANALOG COMMUNICATIONS circuit board includes an envelope detector for an AM signal, a

product detector for an SSB signal, a quadrature detector for an FM signal, and a phase-locked loop that

is used as a detector for FM and PM signals.

When it is necessary to generate a signal in the PROCEDURE, these switches make the transmitter

setup and adjustment easier to perform.

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When S1 is set to ON, the transmitter’s MODULATOR is automatically balanced to output a DSB signal.

When S2 is set to ON, the transmitter’s MIXER is automatically balanced to output a DSB signal.

When S3 is set to ON, the ANTENNA MATCHING NETWORK is automatically set for a 330

impedance.

When S1, S2, or S3 is OFF, manual adjustments must be made.

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PROCEDURE

The following PROCEDURE is divided into three sections:

• Oscillator Circuit Blocks

•

• Angle Modulation and Demodulation Circuit Blocks

Each section starts with an explanation of the circuit blocks that you will examine and get to know by

connecting communication circuits.

Oscillator Circuit Blocks

In this PROCEDURE section, you will examine and use the VCO-LO and VCO-HI circuit blocks.

Information about the functions of the S1, S2, and S3 setup switches can be found in the Resources

section of this manual. Circuit setup guides and the switch function guide are also located in the

Resources.

The VCO-LO circuit block is an oscillator circuit that performs three functions: it can output a 1000 kHz

signal, a 452 kHz signal, or an FM signal.

The VCO-HI circuit block outputs a signal in the 1455 kHz range.

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Locate the VCO-LO circuit block on the ANALOG COMMUNICATIONS circuit board.

Insert a two-post connector in the 1000 kHz terminals.

On the VCO-LO circuit block, set the potentiometer knob completely counterclockwise

(CCW).

What is the function of the VCO-LO potentiometer knob?

a. It adjusts the VCO-LO frequency.

b. It adjusts the VCO-LO amplitude.

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Set the NEGATIVE SUPPLY knob on the left side of the base unit completely CCW.

What is the function of the NEGATIVE SUPPLY knob?

a. It adjusts the VCO-LO frequency.

b. It adjusts the VCO-LO phase.

Connect the channel 1 probe to OUT on the VCO-LO circuit block, and connect the probe

ground clip to a ground terminal on the circuit board.

NOTE: When making oscilloscope measurements or observations, be sure that you connect the probe’s

ground clip to a ground terminal on the circuit board.

equal a 300 mVpk-pk signal?

a. 1

b. 2

c. 3

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Slowly turn the VCO-LO potentiometer knob clockwise (CW) until the output signal on channel 1

is 300 mVpk-pk.

While observing the signal on channel 1, slowly turn the NEGATIVE SUPPLY knob about a

and use the bottom portion for coarse adjustments.

Did the VCO-LO frequency increase or decrease?

a. increase

b. decrease

With a sweep setting of 0.5

equal one cycle of a 1000 kHz signal?

a. 1

b. 2

c. 3

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Adjust the NEGATIVE SUPPLY knob so that the VCO-LO output frequency is 1000 kHz, as

shown above.

Remove the two-post connector from the 1000 kHz terminals.

Insert the two-post connector in the 452 kHz terminals.

Adjust the VCO-LO potentiometer knob to set the output signal on channel 1 to 200 mVpk-pk.


equal 1 cycle of a 452 kHz signal?

-6))

Divisions = (Recall Value 1)

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Adjust the NEGATIVE SUPPLY knob so that 1 cycle of the channel 1 signal equals

(Step 10, Recall Value 1) divisions, which sets the VCO-LO frequency at

452 kHz.

Locate the VCO-HI circuit block on the ANALOG COMMUNICATIONS circuit board.

On the VCO-HI circuit block, set the potentiometer knob, which adjusts the amplitude,

completely CCW.

Set the POSITIVE SUPPLY KNOB on the right side of the base unit completely CCW.

What is the function of the POSITIVE SUPPLY knob?

a. It adjusts the VCO-HI frequency.

b. It adjusts the VCO-HI phase.

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Connect the channel 1 probe to OUT on the VCO-HI circuit block, and connect the probe ground

clip to a ground terminal on the circuit board.

Slowly turn the VCO-HI potentiometer knob CW until the output signal on channel 1 is

300 mVpk-pk.

While observing the signal on channel 1, slowly turn the POSITIVE SUPPLY knob about a

the bottom portion for coarse adjustments.

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Did the VCO-HI frequency increase or decrease?

a. increase

b. decrease

Set the oscilloscope sweep to 0.2


one cycle of a 1455 kHz signal?

-6))

Divisions = (Recall Value 2)

Adjust the POSITIVE SUPPLY knob so that one cycle of the channel 1 signal covers

(Step 18, Recall Value 2) divisions. This adjustment sets the VCO-HI

frequency at 1455 kHz.

AM/SSB Transmitter and Receiver Circuit Blocks

RECEIVER circuit blocks.

Information about the functions of the S1, S2, and S3 setup switches can be found in the Resources


Resources.

• transmit an AM signal

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• or transmit an SSB signal.

You can adjust the ANTENNA MATCHING CIRCUIT impedance to 330 by connecting the 330

CALIBRATION CIRCUIT.

The 51 resistor R5 simulates the antenna impedance.

connector.

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Resistor R8 (1 M ) simulates transmission losses.

The AM receiver consists of the RF FILTER, RF AMPLIFIER, MIXER, IF FILTER, and ENVELOPE

DETECTOR.

The SSB receiver consists of the RF FILTER, RF AMPLIFIER, MIXER, IF FILTER, PRODUCT

DETECTOR, and AUDIO FILTER.

An AUTOMATIC GAIN CONTROL can be connected in the SSB receiver circuit.

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circuit board. Connect the SSB transmitter circuit, as shown.

While you are becoming familiar with the circuit board, you will not set the amplitudes and

frequencies of the message or carrier signals.

How many two-post connectors did you use to connect the SSB transmitter circuit?

a. 2

b. 3

c. 4

Does your SIGNAL GENERATOR connect to M and your VCO-LO connect to C on the

MODULATOR, respectively?

a. yes

b. no

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Does your MODULATOR connect to the LSB FILTER?

a. yes

b. no

Does the VCO-HI circuit block connect to C on the MIXER?

a. yes

b. no

board. Connect the SSB receiver circuit, as shown.

Connect the VCO-HI circuit block to C on the MIXER, and connect the VCO-LO circuit block to C

on the PRODUCT DETECTOR.

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Did you connect the TRANSMITTER, R8, RF FILTER, RF AMPLIFIER, MIXER, IF FILTER,

PRODUCT DETECTOR, and AUDIO FILTER?

a. yes

b. no

Connect the AUTOMATIC GAIN CONTROL into the SSB receiver circuit.

The AUTOMATIC GAIN CONTROL connects from the PRODUCT DETECTOR output to what

SSB receiver component?

a. RF FILTER

b. RF AMPLIFIER

Angle Modulation and Demodulation Circuit Blocks

In this PROCEDURE section, you will examine and use the PHASE MODULATOR, QUADRATURE

DETECTOR, and PHASE-LOCKED LOOP circuit blocks, and the VCO-LO circuit block as an FM

modulator.

Information about the Functions of the S1, S2, and S3 setup switches can be found in the Resources


Resources.

The VCO-LO circuit block can also function as a frequency modulator.

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The 452 kHz output signal is the FM carrier frequency.

A message signal from the SIGNAL GENERATOR is input at (M) on the circuit block to frequency

modulate the 452 kHz carrier signal.

The PHASE MODULATOR circuit block consists of a phase MODULATOR and a LIMITER.

The VCO-LO circuit block provides a 452 kHz FM carrier signal at C that is phase modulated by a 5 kHz

message signal from a SIGNAL GENERATOR that is connected at M.

The LIMITER reduces the amplitude of the PM signal.

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DETECTOR, and the FILTER.

An FM signal is input from the VCO-LO circuit block.

The FM signal takes two paths.

amplitude.

The original and phase-shifted FM signals are input to the PHASE DETECTOR, where the message

signal is recovered.

The FILTER removes the high frequency from the PHASE DETECTOR output and passes the message

signal.

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The PHASE-LOCKED LOOP circuit block consists of the PHASE DETECTOR, FILTER, AMP, and VCO.

The RF signal is an FM signal from the VCO-LO circuit block.

A two-post connector connects the FILTER to the AMP to provide the feedback to the PHASE

DETECTOR.

The output of the FILTER is the recovered message signal.

The recovered message signal is also fed back through the AMP to the VCO.

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The VCO frequency tracks the FM (RF) signal frequency.

Connect the VCO-LO circuit block as a frequency modulator, as shown. The SIGNAL

GENERATOR connects to (M).

Connect the (FM) OUT on the VCO-LO circuit block to FM on the QUADRATURE

DETECTOR circuit block.

NOTE: You will only make connections. No signals will be input, nor will measurements be made.

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What is the function of the QUADRATURE DETECTOR?

a. modulation

b. demodulation

c. mixing

Connect the SIGNAL GENERATOR and VCO-LO circuit block to the PHASE MODULATOR

circuit block, as shown.

In the PHASE MODULATOR, what characteristic of the carrier signal does the message

signal change?

a. amplitude

b. frequency

c. phase

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Connect the SIGNAL GENERATOR to (M) on the VCO-LO circuit block.

Connect the VCO-LO circuit block to the PHASE-LOCKED LOOP circuit block, as shown.

On the PHASE-LOCKED LOOP circuit block, insert a two-post connector between the FILTER

and the AMP.

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In the PHASE-LOCKED LOOP circuit, the recovered message signal is output at what

component?

a. FILTER

b. VCO

c. PHASE DETECTOR

CONCLUSION

• The ANALOG COMMUNICATIONS circuit board contains seven circuit blocks.

•

detectors.

•

signals.

• The VCO-LO circuit block provides oscillator frequencies in the range of 452 kHz and 1000 kHz that

are controlled by the NEGATIVE SUPPLY voltage on the base unit.

• The VCO-HI circuit block provides an oscillator frequency in the range of 1455 kHz that is controlled

by the POSITIVE SUPPLY voltage on the base unit.

REVIEW QUESTIONS

1. How do you adjust the frequency of the VCO-HI oscillator signal?

a. by turning the NEGATIVE SUPPLY knob on the base unit

b. by inserting a two-post connector in the 1000 kHz terminals

c. by turning the POSITIVE SUPPLY knob on the base unit

d. by turning the potentiometer knob on the VCO-HI circuit block.

2. How do you adjust the amplitude of the VCO-LO oscillator signal?

a. by turning the NEGATIVE SUPPLY knob on the base unit

b. by turning the potentiometer knob on the MODULATOR

c. by turning the POSITIVE SUPPLY knob on the base unit

d. by turning the potentiometer knob on the VCO-LO circuit block

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3.

of the carrier by the message signal?

a. by turning the potentiometer knob on the MODULATOR

b. by adjusting the frequency of the carrier signal

c. by adjusting the frequency of the message signal

d. by turning the NEGATIVE SUPPLY knob on the base unit

4. The VCO-LO, PHASE MODULATOR, QUADRATURE DETECTOR, and PHASE-LOCKED LOOP

circuit blocks are associated with what type of radio signals?

a. amplitude modulated (AM) signals

b. single sideband (SSB) signals

c. angle modulated (FM and PM) signals

d. double sideband (DSB) signals

5. On the ANALOG COMMUNICATIONS circuit board, how do you connect the AM transmitter to the AM

receiver?

a. A connection is not required because the signal is radiated by an antenna.

b. Insert a two-post connector between R10 and the RF AMPLIFIER.

c. Place switch S3 in the ON position.

d. Insert a two-post connector between FROM TRANSMITTER and R8

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