exercise 2: demodulation (quadrature detector) · analog communications angle modulation and...

Student Manual

FACET by Lab-Volt 337

Analog Communications Angle Modulation and Demodulation

Exercise 2: Demodulation (Quadrature Detector)

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to explain demodulation of an FM signal

and describe the operation of a quadrature detector. You will use an oscilloscope to make signal

measurements.

DISCUSSION

FM demodulators are referred to as discriminators or frequency detectors.

A quadrature detector is one of several circuits that can demodulate FM signals.

Other FM demodulator circuits include the Foster-Seeley discriminator, the ratio detector, the

pulsecounting detector, and the phase-locked loop detector.

All of these circuits convert FM frequency variations into the amplitude and frequency of the message

signal.

The frequency deviations caused by 1 kHz and 2 kHz message signals with equal amplitudes are shown.

The X-axis is the FM frequency deviation, and the Y-axis is time. The intersection of the X and Y axes is

the center frequency of the FM carrier, or 0 frequency deviation.

The time span on the Y-axis is 1 ms.

Student Manual

338 FACET by Lab-Volt

Angle Modulation and Demodulation Analog Communications

The left plot shows the frequency deviations caused by a 1 kHz sine wave message signal.

The right plot shows the frequency deviations caused by a 2 kHz sine wave message signal.

Because the 1 kHz and 2 kHz message signals have equal amplitudes, the magnitudes of the frequency

deviations are equal.

A 1 kHz message signal causes two maximum frequency deviations (one positive and one negative) per

millisecond.

A 2 kHz message signal causes how many maximum frequency deviations per millisecond?

a. 1

b. 2

c. 3

d. 4

An FM discriminator converts the magnitude and rate of the frequency deviations into the amplitude and

frequency of the message signal.

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The FM discriminator on the circuit board is a quadrature detector.

T

DETECTOR, and a FILTER (see the image below).

At the quadrature detector’s input, the FM signal takes two paths.

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T

balanced modulator.

The phase detector outputs a signal with a frequency that is twice the FM frequency and a dc voltage that

varies with the phase difference between the two inputs.

Because the phase difference between the phase detector’s input signals varies with the FM signal’s

frequency deviations, the baseband output voltage changes with the amplitude and frequency of what

signal?

a. FM carrier signal

b. message signal

dc output voltage as the recovered message signal.

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PROCEDURE

The following PROCEDURE on the quadrature detector is divided into two sections:

• Phase Shifter and Limiter

• Phase Detector and Filter

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

will measure and calculate.

Phase Shifter and Limiter

In this PROCEDURE section, you will observe how a phase shifter changes the phase of an FM carrier

signal and how a limiter reduces the amplitude of the phase-shifted signal.

circuit.

The FM signal takes two paths at the quadrature detector’s input.

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Because the resonant frequency (fr) of the LC network equals the FM center frequency, it is a purely

resistive impedance to the FM center frequency.

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However, frequencies greater or less than froriginal FM signal.

r of the

if the frequency is lower than the tuned circuit.

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In reference to the original FM signal, frequency deviations on each side of the FM center frequency will be

a. in phase.

b.

c.

The phase-shifted FM signal is input to the limiter.

The limiter has two diodes connected from the output to ground with their polarities reversed: anodes

connect to cathodes.

The reversed-polarity diodes limit the output amplitude and minimize any AM modulation that the phase

shifter may cause.

On the VCO-LO circuit block, insert the two-post connector in the 452 kHz terminals.

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

circuit block.

Connect the oscilloscope channel 1 probe to FM on the QUADRATURE DETECTOR circuit

block.

With the potentiometer knob on VCO-LO, adjust the unmodulated FM carrier signal at FM for

300 mVpk-pk.

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Pull out the oscilloscope TIME VARIABLE knob, and adjust the channel 1 signal so that one

Adjust the FM frequency by turning the NEGATIVE SUPPLY knob on the left side of the base

unit until the waveform on channel 2 has a maximum amplitude.

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frequency is equal to what frequency?

a. the LC network resonant frequency (fr)

b. half the frequency of the original FM signal

What is the phase difference between the carrier signal on channel 1 and the PHASE

Phase difference = degrees (Recall Value 1)

between the input and output signals? (The signals are shown in the image below.)

a.

b. capacitor

c. LC network

d. limiter

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Adjust the NEGATIVE SUPPLY knob on the base unit CW and then CCW to vary the FM

frequency.

Why did the phase difference between the input and output signals increase and decrease, while

the amplitude always decreased?

a. The LC network causes a phase shift when the FM frequency varies.

b.

LIMITER output change phase with the frequency deviations of the modulated FM signal?

a. yes

b. no

and valleys?

a. The limiter restricts the amplitude of the output signal.

b. The output signal is out of phase with the input signal.

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amplitude of the input signal (channel 1) to about 100 mVpk-pk and then to 300 mVpk-pk by

turning the potentiometer on the VCO-LO circuit block CCW, and then CW.

When the input signal was reduced to 100 mVpk-pk

a.

b. sine wave

Phase Detector and Filter

signal.

NOTE: Set the TIME VARIABLE to the CAL position.

detector.

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The phase detector is a balanced modulator.

A balanced modulator combines FM input frequencies to produce sum and difference frequencies.

Because the FM inputs have equal frequencies, the sum frequency is two times the FM frequency.

If the FM center frequency is 440 kHz, what is the sum frequency?

Sum frequency = kHz (Recall Value 1)

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However, when the input frequencies to a balanced modulator are equal, the difference frequency

Because the message signal caused the FM frequency deviations, which are converted to phase

differences, the phase detector’s difference dc voltage component varies directly with the message signal.

Therefore, the output of the phase detector contains the sum frequency and the message signal.

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and passes the message signal.

Connect the circuit and channel 1 and 2 oscilloscope probes as shown. Set both channels to

oscilloscope signals, as shown in the graph below.

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Connect the channel 2 probe to the PHASE DETECTOR output.

What is the PHASE DETECTOR’s output signal on channel 2?

a. sum frequency

b. difference frequency

What is the PHASE DETECTOR output difference component?

a. the FM center frequency

b. a dc voltage

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What varies the PHASE DETECTOR’s dc output voltage (difference component)?

a. the frequency variations

b. the phase variations

Connect the channel 2 probe to the input of the PHASE DETECTOR.

If necessary, adjust the FM frequency with the NEGATIVE SUPPLY voltage so that the phase

Set the voltmeter to measure dc volts. Connect the voltmeter lead to the PHASE

DETECTOR’s output, and connect the common lead to ground.

DETECTOR’s output.

V90 = Vdc (Recall Value 2)

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below) by adjusting the NEGATIVE SUPPLY voltage knob CCW.



below) by adjusting the NEGATIVE SUPPLY voltage knob CW.



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The table shows your measured dc voltages at the PHASE DETECTOR’s output.

Phase difference

of input signalsDC output voltage

Vdc (Step 8, Recall Value 3)



Calculate the voltage change from a 90

V135 voltage change:

Vdc (Step 8, Recall Value 3) – Vdc (Step 7, Recall Value 2) =

V45 voltage change:

Vdc (Step 9, Recall Value 4) – Vdc (Step 7, Recall Value 2) =

a. yes

b. no

graph below) by adjusting the FM frequency with the NEGATIVE SUPPLY knob.

You will modulate the FM carrier with a 300 mVpk-pk, 3 kHz message signal.

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

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Connect the channel 1 probe to T on VCO-LO.

Adjust the SIGNAL GENERATOR for a 300 mVpk-pk, 3 kHz sine wave at T on VCO-LO

(channel 1).

Connect the channel 1 probe to the QUADRATURE DETECTOR’s input at FM.

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Set the coupling to AC on channel 2.

Set the vertical mode to ALT, and trigger on channel 1.

These settings will allow you to view the modulated signal at a viewable amplitude, minus the

DC offset.

Compare the signals at the two inputs to the PHASE DETECTOR.

Is the phase of the FM signal on channel 2 varying with respect to the signal on CH 1?

a. yes

b. no

Connect the channel 2 probe to the PHASE DETECTOR’s output to observe the sum

frequency signal.

vertical input and set it to the zero reference on the bottom graticule.

On channel 2, is the dc level, which is the zero reference (mid-point) of the sum frequency

signal, changing?

a. yes

b. no

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Connect the channel 1 probe to M at the VCO-LO circuit block to observe the message signal.

Trigger on channel 2.

Observe the dc variations of the PHASE DETECTOR output signal on channel 2.

Compare the message signal on channel 1 with the dc variations of the PHASE DETECTOR

output on channel 2.

Do the dc variations of the PHASE DETECTOR output have the same frequency as the

message signal?

a. yes

b. no

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Does the recovered message signal on channel 2 vary with the message signal amplitude

and frequency on channel 1?

a. yes

b. no

What frequency does the FILTER remove?

a. the sum frequency

b. the message signal frequency

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CONCLUSION

• A quadrature detector is an FM demodulator or discriminator that converts the FM frequency

variations into the message signal.

•

•

•

• The phase detector is a balanced modulator that outputs a dc voltage that varies with the phase

difference of the input signals.

•

signal.

REVIEW QUESTIONS

1. What function does a discriminator or frequency detector perform?

a. It converts FM frequency deviations into phase deviations.

b. It limits the amplitude of an FM signal.

c. It converts FM frequency deviations into the message signal.

d. It removes the FM sum frequency from the recovered message signal.

2. What does the term quadrature mean?

a. a frequency difference of 45 kHz

b.

c. shifting the phase of a signal

d.

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3.

a.

b.

c. It converts phase deviations to frequency deviations.

d. It converts phase deviations to the message signal.

4. What is the quadrature detector’s phase detector?

a.

b. limiter

c. balanced modulator

d. LC network

5. What are the output signals of the quadrature detector’s phase detector?

a. signals equal to half and three times the FM center frequency

b. the sum frequency of the inputs and the FM center frequency

c.

d. the sum frequency of the inputs and a dc voltage that varies with the message signal

exercise 2: demodulation (quadrature detector) · analog communications angle modulation and...

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