exercise 1: frequency and phase modulation - lab-volt 1: frequency and phase modulation ... you will...

Student Manual

306 FACET by Lab-Volt

Angle Modulation and Demodulation Analog Communications

Exercise 1: Frequency and Phase Modulation

EXERCISE OBJECTIVE

When you have completed this exercise, you will be able to describe frequency modulation and an FM

circuit. You will also be able to describe phase modulation and a PM circuit. You will use an oscilloscope

and a multimeter to make measurements.

DISCUSSION

Following are the three frequency modulation (FM) concepts you need to remember.

1. The carrier signal frequency deviates only with the message signal amplitude.

2. The message signal’s frequency does not deviate the carrier signal’s frequency but does affect the

rate of deviation.

3. There are no desired amplitude variations of the FM carrier; only frequency deviations contain the

message signal intelligence.

The maximum carrier frequency deviation (plus or minus) occurs when the message signal’s amplitude is

at what value?

a. maximum or minimum peak

b. zero

When the message signal is at its zero reference, the carrier frequency deviation is zero because the

carrier is at its center frequency.

If the message signal’s amplitude is constant but the frequency increases (for example, from 2 kHz to 4

kHz), the amount of the carrier signal’s frequency deviation does not change.

However, the same frequency deviation will occur 4000 times per second (4 kHz) instead of 2000 times

per second (2 kHz).

Student Manual

FACET by Lab-Volt 307

Analog Communications Angle Modulation and Demodulation

Because FM amplitude variations do not contain any message signal intelligence, the FM carrier’s

amplitude can be limited within desired values.

may affect amplitude but not frequency, can be used in FM equipment.

The percentage of modulation describes the extent of the carrier frequency deviations.

The carrier signal’s center frequency is its unmodulated frequency: the frequency with no message signal

(or zero amplitude).

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modulation.

What percentage of modulation (% Mod.) is a deviation of ±37.5 kHz?

% Mod. = % (Recall Value 1)

Like AM, the FM carrier signal sidebands contain the message signal intelligence. If the frequency

deviation is held to a minimum, only two FM sidebands are produced.

the carrier center frequency are produced.

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The bandwidth required for an FM signal depends upon two factors: the message signal amplitude and

the frequency.

A sideband pair are two sidebands that are spaced equally above and below the carrier’s center

frequency. The energy contained in each sideband pair decreases as the sideband pairs are removed

from the center frequency.

A point is reached at which a sideband pair contains so little energy that they can be disregarded.

The point is determined by the modulation index.

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The FM modulation index (MI) is the ratio of the amount of carrier frequency deviation (fcd) to the

message signal frequency (fm).

MI = fcd m

For example, if a 5 kHz message signal (fm) caused a carrier frequency deviation of ±10 kHz (fcd), the MI

would be 2.

sidebands spaced 5 kHz apart for 20 kHz on each side of the FM center frequency.

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If the sideband frequency range on each side of the center frequency is 20 kHz, what is the FM

bandwidth?

FM bandwidth = kHz (Recall Value 2)

In PM, carrier signal modulation occurs when the carrier signal changes its phase (and frequency) with

the changes in the message signal amplitude and frequency.

The amount of phase shift is proportional to the message signal amplitude.

When the carrier’s phase changes, frequency deviations also occur.

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The amount of frequency deviation is directly proportional to the rate and the total amount of phase shift.

The rate of phase shift is directly proportional to the message signal’s frequency.

Therefore, the amount of frequency deviation in PM is directly proportional to the message signal’s

amplitude and frequency.

In FM, do changes in the message signal’s amplitude and frequency cause frequency deviations in the

carrier signal?

a. yes

b. no

Advantage and Disadvantages of FM and PM

The PM and broadcast FM disadvantages are wide bandwidths and the necessity of line-of-sight signal

propagation for FM frequencies.

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PROCEDURE

The following PROCEDURE is divided into two sections.

• Frequency Modulation (FM)

• Phase Modulation (PM)

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

measure and calculate.

Frequency Modulation (FM)

In this PROCEDURE section, you will frequency modulate a carrier signal, measure its parameters, and

observe its characteristics.

An FM signal is generated with the VCO-LO circuit block.

In previous units, you used the VCO-LO circuit block to generate a 452 kHz or a 1000 kHz sine wave,

depending on the position of the two-post connector.

In this PROCEDURE section, you will place the two-post connector in the 452 kHz position. (Do not make

this placement yet.)

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The potentiometer knob on the VCO-LO circuit block adjusts the output amplitude.

To adjust the VCO-LO output frequency, you will adjust the NEGATIVE SUPPLY knob on the left side of

the base unit.

T

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The oscillator’s frequency is determined by the tuning of the LC network.

You can tune the LC network by changing the value of the NEGATIVE SUPPLY voltage at the anode of

varactor diode CR2.

The value of the negative voltage affects the CR2 capacitance, which, in turn, affects the tuning of the LC

network.

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As the NEGATIVE SUPPLY voltage becomes more negative, VCO-LO’s output frequency increases.

At 0 Vdc, the output frequency is about 310 kHz.

At 10 Vdc, the output frequency is about 510 kHz.

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A buffer is between the oscillator and the VCO-LO potentiometer, which you adjust to set the VCO-LO

output amplitude.

At test point T, you can measure the dc voltage at CR2’s anode.

The sine wave message signal input is at (M), and it causes the voltage at CR2 to vary.

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You can observe the FM signal at (FM) OUT.

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

Set the VCO-LO amplitude potentiometer fully clockwise (CW).

Connect the oscilloscope channel 2 probe to (FM) OUT on VCO-LO.

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Set the voltmeter to volts dc. Connect the dc voltmeter to T on the VCO-LO circuit block.

Adjust the NEGATIVE SUPPLY for 4.0 Vdc at T.

Connect the oscilloscope channel 1 probe to T.

Adjust the channel 1 and 2 oscilloscope traces so they appear as shown.

The channel 1 dc trace, which indicates 4.0 Vdc at T, should be on the second division line

from the top of the oscilloscope display.

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Accurately measure the period (T) between the peaks of the unmodulated FM carrier signal

on channel 2.

T = s (Recall Value 1)

From the period (T), calculate the center frequency (f) of the unmodulated FM carrier signal.

T = s (Step 7, Recall Value 1)

= kHz (Recall Value 2)

You will vary the voltage to simulate the voltage change caused by the message signal,

which will be a sine wave.

While observing your oscilloscope screen, adjust the NEGATIVE SUPPLY knob CW and then

CCW so that the dc voltage on channel 1 varies by about ±1 Vdc.

On channel 2, does the FM carrier frequency change as the dc voltage on channel 1 varies?

a. yes

b. no

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You will determine the frequency deviation of the FM carrier when the message signal

amplitude changes by 1 Vdc.

Adjust the NEGATIVE SUPPLY knob CW to change the voltage at T on the VCO-LO circuit block

to 5.0 Vdc.

Accurately measure the period (T) between the peaks of the modulated FM carrier signal on

channel 2.

T = s (Recall Value 3)

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From the period (T), calculate the frequency (f) of the modulated FM carrier signal.

T = s (Step 11, Recall Value 3)


Calculate FM frequency deviation when the message signal’s amplitude changes by 1 Vdc.

The FM center frequency is kHz (Step 8, Recall Value 2) and the frequency with

a 1 Vdc message signal is kHz (Step 12, Recall Value 4).

Frequency deviation = kHz (Recall Value 5)

To return the carrier frequency to the kHz (Step 8, Recall Value 2) center

frequency, adjust the NEGATIVE SUPPLY knob CCW to change the voltage at T on the

VCO-LO circuit block from 5.0 Vdc to 4.0 Vdc.

You will now observe the effect of a 2 Vpk-pk, 5 kHz message signal on the FM carrier

frequency.

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

Adjust the SIGNAL GENERATOR for a 2.0 Vpk-pk, 5 kHz sine wave at T. This adjustment is

equivalent to varying the voltage at T ±1 V.

Set the sweep to 0.5

FM signal.

When the message signal amplitude was 1 V, the voltage at T decreased to 5 V and the

frequency increased to kHz (Step 12, Recall Value 4).

When the message signal amplitude is 1 V, the frequency will decrease to:

(Step 8, Recall Value 2) (Step 13, Recall Value 5)

= kHz

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Calculate the modulation index (MI) for an FM signal that has a frequency deviation (fdev) of

± (Step 13, Recall Value 5) kHz with a 5 kHz message signal (fm).

MI = fdev m

= (Step 13, Recall Value 5

= (Recall Value 6)

With an MI of (Step 18, Recall Value 6

pairs (SSP) from the table.

If the MI is not a whole number, use the next highest MI to select the number of SSPs.

On the next screen you will calculate the bandwidth of a message signal given a center

frequency and an SSP.

As shown, the sideband pairs are spaced evenly on each side of the center frequency and

are spaced 5 kHz apart.

BW = SSP x 5 kHz x 2


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Phase Modulation (PM)

In this PROCEDURE section, you will phase modulate a carrier signal, measure the phase change, and

observe its characteristics.

You use the PHASE MODULATOR circuit block to produce a PM signal.

The inputs to the phase modulator are a 452 kHz carrier signal and a 5 kHz message signal.

Test point T permits you to measure the 0 to 10 Vdc POSITIVE SUPPLY voltage.

A terminal between the MODULATOR and the LIMITER permits you to observe the PM signal before it is

limited.

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The tuning of the LC network determines the phase shift of the carrier signal.

You tune the LC network by changing the value of the POSITIVE SUPPLY voltage at the cathode of

varactor diode CR5.

The value of the POSITIVE SUPPLY voltage determines the CR5 capacitance, which affects the tuning of

the LC network.

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You will adjust the POSITIVE SUPPLY voltage so that the MODULATOR’s output (input to the LIMITER)

is in phase with the carrier signal.

As the POSITIVE SUPPLY voltage becomes more positive, the MODULATOR’s output leads the input.

As the POSITIVE SUPPLY voltage becomes more negative, the MODULATOR’s output lags the input.

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When a sine wave message signal is applied to M, the varying amplitude causes the phase of the

MODULATOR’s output to vary.

Because the FM or PM signal’s amplitude does not contain any message intelligence, signal spikes

caused by noise can be cut off to improve the signal-to-noise ratio of an FM or PM signal.

A LIMITER circuit keeps the PM signal’s amplitude within a desired range.

In the PHASE MODULATOR circuit block, the LIMITER circuit is an op amp with a gain of 1.0 that has two

Schottky diodes connected from the output to the input.

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The diodes’ polarities are reversed: anodes connect to cathodes.

The reversed diodes limit a signal’s positive and negative peaks to the value of the diode’s forward

voltage.

When the PM signal peak reaches about 0.2 V, the diode with its anode connected to the output conducts

and maintains the PM positive peak voltage at a value of about 0.2 V.

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In the same way, the other diode maintains the PM negative peak voltage at 0.2 V.

Undesired voltages above and below ±0.2 V are cut off by the LIMITER circuit.

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

Connect (FM) OUT on the VCO-LO circuit block to C at the MODULATOR on the PHASE

MODULATOR circuit block.

In this PROCEDURE section the oscilloscope probes must be set to X10 to prevent

loading down of the signals. Connect the oscilloscope channel 1 probe to C on the PHASE

MODULATOR circuit block.

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With the potentiometer knob on the VCO-LO circuit block, adjust the signal at C for a 600

mVpk-pk amplitude.

Set the voltmeter to volts dc. Connect the dc voltmeter to T on the VCO-LO circuit block.

Adjust the NEGATIVE SUPPLY knob on the left side of the base unit for 4.5 Vdc at T, which

sets the signal at C to about 475 kHz.

Connect the oscilloscope channel 2 probe to the terminal between the MODULATOR and

LIMITER.

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Adjust the POSITIVE SUPPLY knob on the right side of the base unit so that the

MODULATOR output signal (channel 2) is in phase with the VCO-LO signal (channel 1).

While observing the oscilloscope screen, slowly turn the POSITIVE SUPPLY knob on the

base unit CW and then CCW.

Varying the POSITIVE SUPPLY voltage simulates a message signal amplitude change.

As the POSITIVE SUPPLY voltage changes, does the MODULATOR output signal on

channel 2 change phase in relationship to the channel 1 input signal?

a. yes

b. no

Adjust the POSITIVE SUPPLY voltage so that the channel 1 and channel 2 signals are in

phase.

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You will observe the effect of a 3 Vpk-pk, 5 kHz message signal on the PM carrier frequency.

Connect the SIGNAL GENERATOR to M on the MODULATOR.

Connect the oscilloscope channel 1 probe to M.

Adjust the SIGNAL GENERATOR for a 3 Vpk-pk, 5 kHz sine wave at M on the MODULATOR.

Connect the oscilloscope channel 1 probe to C, and connect the oscilloscope channel 2

probe between the MODULATOR and the LIMITER. The probes must be set at X10.

set the vertical mode to ALT.

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Channel 1 shows the unmodulated carrier, and channel 2 shows the phase modulated

signal.

On channel 2, do you observe out-of-phase signals with different amplitudes?

a. yes

b. no

Connect the oscilloscope channel 1 probe to the LIMITER output. Compare the LIMITER’s

input and output signals.

Did the LIMITER reduce the amplitude of the PM signal?

a. yes

b. no

CONCLUSION

• In FM, only changes in the message signal amplitude cause carrier frequency deviations.

• In PM, changes in the message signal amplitude and frequency cause carrier frequency deviations.

• In FM and PM, any variations in the carrier amplitude do not contain message intelligence; the

amplitude can be limited to reduce noise.

• The modulation index is the ratio of frequency deviation and message signal frequency.

•

•

sideband pairs times two (2).

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REVIEW QUESTIONS

1. In FM, the carrier frequency deviates with changes in what parameter(s) of the message signal?

a. phase angle and amplitude

b. frequency only

c. amplitude only

d. frequency and amplitude

2. In PM, the carrier frequency deviates with changes in what parameter(s) of the message signal?

a. modulation index

b. frequency only

c. amplitude only

d. frequency and amplitude

3. Why can a limiter circuit be used to cut off the peaks and valleys of an FM or PM signal?

a. The message signal is contained in the frequency and phase deviations of the carrier, not in the

amplitude variations.

b. The message signal causes only small changes in the amplitude of the carrier.

c. FM or PM signals do not require much transmission power.

d. All of the above

4. A 4 kHz message signal causes an FM carrier signal to have a frequency deviation of ±20 kHz.

MI = fdev m

What is the MI?

a. 80

b. 5

c. 0.2

d. 24

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5. An FM carrier signal with an MI of 4 contains a 3 kHz message signal. Use the table to determine the

a. 12 kHz

b. 6 kHz

c. 42 kHz

d. 21 kHz

exercise 1: frequency and phase modulation - lab-volt 1: frequency and phase modulation ... you will...

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