University of Jordan

Electrical Engineering Department

EE 429
Communications Lab

EXPERIMENT 4
Frequency Modulation and PLL

Lab Supervisor: Dr. Mohammed Hawa

Dr. Ahmad Mustafa
Lab Engineer: Eng. Reem Al-Debs

Prepared By: Dr. Mohammed Hawa

 EXPERIMENT 4
FREQUENCY MODULATION AND PLL

OBJECTIVE

When you have completed this exercise, you will be able to describe FM modulation and
demodulation along with their respective circuits including the VCO, quadrature detector and
phase-locked loop (PLL).

DISCUSSION

The three frequency modulation (FM) concepts you need to remember are:
1. The carrier frequency deviates in proportion with the message signal amplitude.
2. The message signal's frequency does not affect the carrier frequency but does affect the rate
of frequency deviation.
3. Amplitude variations of the FM carrier contain no message signal intelligence; only
frequency deviations contain the intelligence.

As illustrated in Figure 6-5, the carrier frequency in the FM signal is at its greatest or smallest
when the message signal's amplitude is at its maximum or minimum values. 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 peak amplitude of the message signal is constant but its frequency increases (for
example, from 2 kHz to 4 kHz), the maximum frequency deviation of the carrier signal does
not change. However, the same frequency deviation will occur 4000 times per second (4 kHz)
instead of 2000 times per second (2 kHz).

Because amplitude variations of the FM signal do not contain any message signal intelligence,
the FM carrier's amplitude can be limited within desired values (see Figure 6-6). Consequently,
noise amplitude spikes can be reduced by limiter circuits. Efficient class C amplifiers, which
may affect amplitude but not frequency, can also be used in FM equipment.

2-1
The bandwidth required by an FM signal depends upon two factors: the peak frequency
deviation of the carrier (f) and the frequency of the message signal itself (fm). Figure 6-8
shows the FM spectrum of a 450 kHz carrier signal with a 5 kHz modulating sinusoidal
message signal.

In the FM spectrum, two sidebands that are spaced equally above and below the carrier's
center frequency are called a sideband pair. The energy contained in each sideband pair
decreases as the sideband pair get further 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.

The FM modulation index () is the ratio of carrier peak frequency deviation (f) to the
maximum message signal frequency (fm):

 = f / fm

For example, if a 5 kHz message signal (fm) causing a carrier frequency deviation of ± 10 kHz
(i.e., f = 10 kHz), the modulation index would be:

 = 10/5 = 2

The maximum number of significant sideband pairs (SSP) for FM signals is given by Carson’s
rule which states that SSP =  + 1. The FM signal with a modulation index of 2, for example,
would have 3 significant sideband pairs. If a 450 kHz FM carrier signal were modulated by a
5 kHz message signal, it would have the sidebands spaced 5 kHz apart for 15 kHz on each side
of the 450 kHz center frequency. Even thought the frequency deviation is f = 10 kHz, the FM
bandwidth would be 30 kHz (435 kHz to 465 kHz). In other words the bandwidth of an FM
signal is BW = 2 (fm) ( + 1) = 2 (fm) (SSP + 1).

2-2
Advantages of FM modulation include good signal-to-noise ratio (SNR) and the ability to use
more efficient class C amplifiers because amplitude distortion does not affect the signal
quality. The FM modulation disadvantage is the requirement of wider bandwidth than AM
modulation.

One way to generate an FM signal is using a Voltage-Controlled Oscillator (VCO) circuit. In a

previous experiment, 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 used. In this PROCEDURE
section, you will use the VCO-LO block to generate an FM signal.

Remember that the potentiometer knob on the VCO-LO circuit block adjusts the output
amplitude. To adjust the VCO-LO output frequency, you can adjust the NEGATIVE SUPPLY
knob on the top left side of the base unit.

A simplified schematic of the VCO-LO circuit block is shown in Figure 6-12. The oscillator
block consists of two transistors that are connected in a cross-coupled oscillator configuration.
The oscillator's frequency is determined by the tuning of the LC network shown. You can tune
the LC network by changing the value of the voltage applied at the anode of the varactor diode
CR2. The voltage is applied by the NEGATIVE SUPPLY voltage source. The value of the
NEGATIVE SUPPLY voltage affects the CR2 capacitance which, in turn, affects the tuning of
the LC network. 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. The VCO-LO feeds its output through a buffer to the VCO-
LO potentiometer, which acts as a simple voltage divider. You adjust the potentiometer to set
the VCO-LO output amplitude.
Test Point
(T) LC Tuning
Network

At test point T, you can measure the DC voltage at CR2's anode. To use the VCO-LO as an FM
modulator, the message signal is fed at terminal (M), and it causes the voltage at CR2 to vary.
You can observe the FM signal at the (FM) OUT terminal.

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 pulse counting detector,
and the phase-locked loop detector. All of these circuits convert FM frequency variations into
the amplitude of the message signal.

2-3
The QUADRATURE DETECTOR circuit block on your kit includes a PHASE SHIFTER, a
LIMITER, a PHASE DETECTOR (MIXER), and a FILTER. A simplified schematic of the
quadrature detector is shown in Figure 6-24.

At the quadrature detector's input, the FM signal takes two paths. In one path, the FM signal is
input to a phase shifter (which is simply a resonant LC circuit). The phase shifter converts
frequency deviations into phase deviations (slightly above and slightly below 90°). The phase
shifter signal is then fed to a LIMITER.

The phase shifter circuit is composed of an amplifier, a capacitor and an LC network (see
Figure 6-24). The FM signal goes first through an amplifier, which is a non inverting op amp
with a gain of about 2. The capacitor shifts the FM signal by 90°. 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. Consequently, the 90° phase shift of the center
frequency is not affected by the LC network.

However, frequencies greater or less than fr are shifted less or more than 90°, respectively,
from the original FM signal. In reference to the original FM signal, frequency deviations on
each side of the FM center frequency will be greater than or less than 90° out of phase because
the frequency deviations produce the phase deviations.

The original FM signal and the phase-shifted/limited FM signal are then fed to a phase
detector, which is a balanced modulator. The balanced modulator combines the input
frequencies to produce sum and difference frequencies. Because the FM inputs have equal
frequencies, the sum frequency is twice the FM frequency. However, the difference frequency
component becomes a DC voltage that varies with the phase difference between the two inputs
(around 90°). Because FM frequency deviations are converted to phase differences at the
output of the PHASE SHIFTER/LIMITER, 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.

A low pass filter (RC network) at the phase detector's output removes the high-frequency
signal and passes the varying dc output voltage as the recovered message signal.

The phase-shifted FM signal is input to a LIMITER (shown in Figure 6-25). The limiter has two
Zener 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 amplitude changes that the phase shifter may cause.

2-4
The following components (see Figure 7-3) make
up the PHASE-LOCKED LOOP circuit block on
your kit:
 PHASE DETECTOR
 FILTER
 AMPLIFIER
 VCO

The PHASE DETECTOR, which is detailed in Figure 7-4, is an MC1496 balanced modulator
whose function is similar to that of the phase detector on the QUADRATURE DETECTOR
circuit block. It performs a full-wave multiplication of the RF and VCO input signals. When the
RF input frequency (fi) equals the VCO output frequency (fvco), the phase detector's output
includes the sum frequency of the inputs (fi + fvco) and a difference component, which is a dc
voltage.

The low-pass RC filter removes the sum frequency and passes the dc voltage of the PLL's
output signal, which is also the feedback signal. In order for the PLL to operate in a closed
loop, you must insert a two-post connector on your circuit board to connect the filter and
amplifier, as shown in Figure 7-4. The amplifier increases the feedback signal's voltage to the
VCO. The DC voltage feedback signal controls fvco to match fi, by changing the capacitance of a
varactor diode in the VCO circuit.

When fi changes, there is an initial phase change between the PHASE DETECTOR'S RF and
VCO input signals. The phase change causes the PHASE DETECTOR'S dc output voltage
(difference component) to change. The amplified change in the dc output voltage that is fed
back to the VCO causes fvco to match the change in fi.

2-5
Any PLL has three characteristics that you need to know about (see Figure 7-2):
 Free-running frequency fo (about 456 kHz on your kit)
 Capture range (between 454 kHz and 458 kHz approximately on your kit)
 Lock range (between 400 kHz and 480 kHz approximately on your kit)

Usually the lock range of a PLL is

greater than the capture range, and
the free-running frequency is in the
middle of both ranges. The PLL
operates as follows: Once fi goes
within the capture range, fvco can
track fi for all values of fi so long as it
is within the lock range. If fi leaves
the lock range fvco reverts back to the
free-running frequency.

The dynamic range of the PHASE DETECTOR'S difference component dc voltage determines
the lock range. This is because when the dc voltage reaches its limit and does not change with
an input phase change, it can no longer cause fvco to match the change in fi; fvco then reverts to
the free-running frequency fo. And since the phase detector's difference component is a dc
voltage, it is not affected by the cutoff frequency of the low-pass filter.

Notice, however, that if fi doesn’t go inside the capture range of the PLL in the first place,
which is about  2 kHz around the VCO's free-running frequency (fo), then fvco will remain at
fo, and the PLL will not follow the input frequency.

The filter cut-off frequency determines the capture range. This is because when fi and fvco are not
equal, the sum and difference frequencies are output from the phase detector. To capture fvco
the difference frequency (fi – fvco) has to be within the cutoff frequency of the low-pass filter. If
the difference frequency is greater than the filter's cutoff frequency, the filter will remove the
difference frequency and prevent a feedback signal to the VCO.

When the filter's output is not connected to the amplifier (open loop), there is no feedback
signal to the VCO. With an open loop, fvco is set by the dc bias voltage (about -4.8 Vdc) to the
free-running frequency (fo) of the VCO.

When a PLL is locked, the phase detector's input frequencies (fi and fvco) are equal but 90° out
of phase. When fi changes, the 90° phase difference between fi and fvco changes (see
Figure 7-15). The initial phase change between fi and fvco causes the phase detector's dc voltage
difference component to change. Every variation in fi causes a phase change with fvco, which
then causes the dc voltage difference component to change. The dc voltage, or the feedback to
the VCO, causes fvco to change so that it equals fi .

After capture, if the bandwidth of the FM signal stays within the PLL's lock range, the PLL
recovers the message signal. However, if the bandwidth of the FM signal becomes greater than
the PLL's lock range, fvco returns to its free-running frequency (fo).

2-6
PROCEDURE A - FREQUENCY MODULATION (FM)

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

1. Locate the VCO-LO circuit block. Insert the two-post connector in the 452 kHz terminals
(Figure 6-13). Set the VCO-LO amplitude potentiometer fully CW.

2. Connect the oscilloscope channel 2 probe to (FM) OUT on VCO-LO. Set channel 2 for
20 mV/DIV and the sweep to 0.5 μs/DIV, and trigger on channel 2.

NOTE: Whenever you make oscilloscope measurements, be sure that you connect the probe's
ground clip to a ground terminal on the circuit board. Also remember that the probe is x10.

3. Connect the oscilloscope channel 1 probe to test point T. Set channel 1 for 100 mV/DIV and
its coupling to DC. Adjust the NEGATIVE SUPPLY knob for (-4.0) Vdc at T.

4. After you have measured the 4 Vdc, adjust the vertical position of channel 1 and 2
oscilloscope traces so they are not overlapping. You might need to adjust the VCO-LO
potentiometer knob slightly to get the appropriate signal on channel 2.

5. Accurately measure the period (T) between the peaks of the unmodulated FM carrier signal
on channel 2. From the period (T), calculate the center frequency (f) in kHz of the unmodulated
FM carrier signal (f = 1/T).

....................................................................................

2-7
6. 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
test point T on the VCO-LO circuit block to (5.0) Vdc.

7. Accurately measure the period (T) between the peaks of the modulated FM carrier signal on
channel 2. From T, calculate the frequency of the modulated FM carrier signal. Record f in kHz.
....................................................................................

8. Calculate the VCO sensitivity (kf) which is the frequency deviation when the message
signal’s amplitude decreases by 1 Vdc. The FM center frequency is the value you calculated in
step 5, and the frequency with a (1) Vdc message signal is the value you just calculated in the
previous step. Record your answer in kHz/Volt.
....................................................................................

9. To return the carrier frequency to the center frequency, adjust the NEGATIVE SUPPLY knob
to (4.0) Vdc.

10. You will now observe the effect of a 2.0 Vpk-pk, 5 kHz message signal on the FM carrier
frequency. Connect the FUNCTION GENERATOR to (M) on the VCO-LO circuit block, as
shown in Figure 6-15.

11. Set the sweep to 50 µs/DIV, and trigger on Channel 1. Adjust the FUNCTION
GENERATOR for a 2.0 Vpk-pk, 5 kHz sine wave at T. This adjustment is equivalent to varying
the voltage at T by ± 1V. Now, set the sweep to 0.5 μs/DIV and trigger on channel 2. Observe
channel 2, which shows an FM signal like the one in Figure 6-16.

2-8
12. Explain why doesn’t the FM signal shown in Figure 6-16 look similar to the FM signal
shown earlier in Figure 6-5.

....................................................................................

13. Since the message signal is 2.0 Vpk-pk (i.e., its peak is 1 V), the frequency deviation (f) of the
FM signal is equal to kf × 1 V = kf. Calculate the modulation index () for the FM signal that has
a sensitivity (kf) of the amount you determined in step 8 and with a 5 kHz message signal (fm).

....................................................................................

14. Using the  you calculated above, find the number of significant sideband pairs (SSP). If 
is not a whole number, use the next highest  to find the number of SSP.

....................................................................................

15. Now calculate the bandwidth (BW) of your FM signal in kHz.

....................................................................................

PROCEDURE B – FM DETECTION USING QUADATURE DETECTOR

In this PROCEDURE section, you will study the FM quadrature detector by observing how a
phase shifter changes the phase of an FM carrier signal and how the phase detector and filter
recover the message signal.

1. On the VCO-LO circuit block, insert the two-post connector in the 452 kHz terminals, and
disconnect the FUNCTION GENERATOR from the message (M) input if it is already
connected (see Figure 6-26).

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

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

block, and trigger on channel 1.

2-9
4. Set channel 1 for 10 mV/DIV and the sweep to 0.5 μs/DIV. With the potentiometer knob on
VCO-LO, adjust the unmodulated FM carrier signal at FM for 300 mVpk-pk.

5. Connect the oscilloscope channel 2 probe to the output of the PHASE SHIFTER/LIMITER on
the QUADRATURE DETECTOR circuit block. Set channel 2 for 20 mV/DIV. 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.

6. Count how many horizontal divisions represent one cycle (360°) on channel 1 signal.
....................................................................................

7. When the output amplitude of the PHASE SHIFTER/LIMITER is maximum, the FM center
frequency is equal to what frequency: the LC network resonant frequency (fr) or the message
signal frequency?
....................................................................................

8. Measure the phase difference between the unmodulated FM carrier signal on channel 1 and
the PHASE SHIFTER/LIMITER output signal on channel 2? Note: You know how many
horizontal divisions represent 360° from step 6 above.
....................................................................................
9. In the PHASE SHIFTER/LIMITER circuit, what component causes a phase shift of 90°
between the input and output signals: the amplifier, the capacitor, the LC network, or the
limiter?
....................................................................................

10. Adjust the NEGATIVE SUPPLY knob on the base unit CW and then CCW to vary the FM
frequency. Why do you think the phase difference between the input and output signals
increase and decrease:
(a) because the LC network causes a different phase shift when the FM frequency varies, or
(b) because the 90° phase shift caused by the capacitor is changing?
....................................................................................

11. Why does the PHASE SHIFTER/LIMITER output signal on channel 2 have flattened peaks
and valleys:
(a) because the limiter restricts the amplitude of the output signal, or
(b) because the output signal is out-of-phase with the input signal?
....................................................................................

12. While observing the PHASE SHIFTER/LIMITER output signal (channel 2), reduce the
amplitude of the input signal (channel 1) to about 100 mVpk-pk and then back to 300 mVpk-pk by
turning the potentiometer on the VCO-LO circuit block CCW and then CW. When you
reduced the input signal to 100 mVpk-pk, did the PHASE SHIFTER/LIMITER output signal
(channel 2) become a sine wave or more flattened?
....................................................................................

2-10
13. The circuit is now connected as shown in Figure 6-29.

14. Make sure that the channel 1 signal is adjusted for 300 mVpk-pk, and adjust the NEGATIVE
SUPPLY knob on the BASE UNIT so that the channel 2 waveform is 90° out-of- phase with the
channel 1 signal (see Figure 6-30).

15. What is the PHASE DETECTOR'S output signal on channel 2: (a) the sum frequency or (b)
the difference frequency or (c) both?

....................................................................................
16. What is the PHASE DETECTOR output difference component (which is also the output of
the FILTER): the FM signal or a dc voltage?

....................................................................................

17. Use a voltmeter to measure dc volts. Connect the voltmeter lead to the FILTER’S output,
and connect the common lead to ground. With a 90° phase difference between the input
signals, measure and record the dc voltage at the FILTER’S output (V90°)

....................................................................................

18. Set the phase difference between the signals on channels 1 and 2 to 135° by adjusting the
NEGATIVE SUPPLY voltage knob CCW. With a 135° phase difference between the input
signals, measure and record the dc voltage at the FILTER’S output (V135°).

....................................................................................

19. Set the phase difference between the signals on channels 1 and 2 to 45° by adjusting the
NEGATIVE SUPPLY voltage knob CW. With a 45° phase difference between the input signals,
measure and record the dc voltage at the FILTER’S output (V45°).

....................................................................................

20. When the phase difference was increased or decreased from 90°, did the dc output voltage
change?
....................................................................................

2-11
21. Set the phase difference between the signals on channels 1 and 2 back to 90° (Figure 6-30)
by adjusting the FM frequency with the NEGATIVE SUPPLY knob.

22. Now, you will modulate the FM carrier with a 300 mVpk-pk, 3 kHz message signal. Connect
the FUNCTION GENERATOR to (M) on the VCO-LO circuit block (Figure 6-32).

23. Connect the channel 1 probe to T on VCO-LO. Set channel 1 for 10 mV/DIV, set the sweep
to 0.1 ms/DIV, and trigger on channel 1.

24. Adjust the FUNCTION GENERATOR (message signal) for a 300 mVpk-pk, 3 kHz sine wave
at T on VCO-LO (channel 1).

25. Connect the channel 2 probe to the PHASE DETECTOR'S output (just before the FILTER) to
observe the sum and difference frequency signals. Set channel 2 for 20 mV/DIV. Set the
vertical mode to DUAL (or ALT depending on your oscilloscope), and keep trigger on
channel 1. On channel 2, is the dc level, which is the zero reference (midpoint) of the sum
frequency signal changing?

....................................................................................

26. 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?

....................................................................................

27. Connect the channel 2 probe to the output of the FILTER. Set channel 2 to 5 mV/DIV.
Observe the message signal on channel 1 and the QUADRATURE DETECTOR output on
channel 2. Vary the message signal frequency and amplitude. Does the recovered message
signal on channel 2 vary with the message signal amplitude and frequency on channel 1?

....................................................................................

PROCEDURE C - PHASE-LOCKED LOOP

In this PROCEDURE section, you will study the characteristics of the phase-locked loop on
your Analog Communications Lab Kit.

2-12
1. You will first determine the VCO's free-running frequency. Do not insert a two-post
connector between the FILTER and AMP in the PHASE-LOCKED LOOP circuit block just yet.
Set oscilloscope channel 2 to 20 mV/DIV, and set the sweep to 0.5 μS/DIV. Connect the
channel 2 probe to the VCO output, and trigger on channel 2. Connect the voltmeter to the
VCO input, and connect the voltmeter common lead to a ground terminal on the circuit board
(See Figure 7-5.)

2. Accurately measure the period (T) between the peaks of the waveform. Each horizontal
division is 0.5 μs. Record your answer below.
....................................................................................

3. From the period (T) of the VCO output signal, calculate the free-running frequency
(fo = 1/T). Record your answer in kHz.

....................................................................................

4. Set the voltmeter to measure volts dc. Measure and record the VCO dc input voltage (Vi).

....................................................................................

5. Does Vi control the VCO output's amplitude or its frequency?

....................................................................................

6. On the VCO-LO circuit block, insert a two-post connector in the 452 kHz position. Connect
the (FM) OUT terminal on VCO-LO to the RF terminal at the PHASE DETECTOR input on the
PHASE-LOCKED LOOP circuit block, as illustrated in Figure 7-7.

7. Set oscilloscope channel 1 to 5 mV/DIV. Connect the channel 1 probe to the RF input of the
PLL. Adjust the potentiometer knob on the VCO-LO circuit block for a 150 mVpk-pk signal at
RF.

2-13
8. Set the oscilloscope vertical mode to DUAL, and trigger on ALT. The signals should appear,
as shown in Figure 7-8.

9. While observing the RF and VCO signals on the oscilloscope screen, slowly increase the RF
frequency (the period decreases) by turning the NEGATIVE SUPPLY knob completely CW.
Slowly decrease the RF frequency (the period increases) by turning the NEGATIVE SUPPLY
knob completely CCW.

10. Did the change in the RF frequency (fi) on channel 1 affect the VCO frequency (fvco) on
channel 2? Why or why not?

....................................................................................

11. Place a two-post connector in the terminals between the FILTER output and the AMP input
to close the feedback loop.

12. While observing the RF and VCO signals on the oscilloscope, slowly increase fi (channel 1)
by turning the NEGATIVE SUPPLY knob CW. When the VCO signal starts to track (follow)
the RF signal, stop turning the NEGATIVE SUPPLY knob CW. The signals should appear, as
shown in Figure 7-9.

13. What is the name of the frequency range in which the VCO signal starts to track the RF
input signal?

....................................................................................

14. What determines that range of the PLL?

....................................................................................

2-14
15. On the oscilloscope screen, compare fvco and fi by overlaying the signal traces. Are the
frequencies about equal?

....................................................................................

16. While observing the oscilloscope screen, turn the NEGATIVE SUPPLY knob slightly CCW,
and then slightly CW. Does fvco track fi?

....................................................................................

17. While observing the VCO's dc input voltage (Vi), vary fi by turning the NEGATIVE
SUPPLY knob slightly CCW, and then slightly CW. When fvco tracks fi, does Vi change?

....................................................................................

18. What is the name of the frequency range over which fvco tracks fi?

....................................................................................

19. While observing the RF and VCO inputs to the PHASE DETECTOR, slowly increase fi by
turning the NEGATIVE SUPPLY knob CW. When fvco stops tracking fi, stop turning the
NEGATIVE SUPPLY knob CW. This is the point at which the VCO signal snaps back to its
free-running frequency. You may have to repeat this step to obtain the exact frequency at
which tracking stops.

20. On the oscilloscope screen, compare fvco to fi by overlaying the signal trace. Are the
frequencies equal?

....................................................................................

21. On channel 1, accurately measure the period (T) between peaks of the RF input signal
waveform. Each horizontal division is 0.5 μs.

....................................................................................

22. From T, calculate the frequency of the RF input signal (fi = 1/T).

....................................................................................

23. Turn the NEGATIVE SUPPLY knob completely CW. While observing the inputs to the
PHASE DETECTOR, slowly decrease fi (channel 1) by turning the NEGATIVE SUPPLY knob
CCW, When the VCO signal starts to track (follow) the RF signal, stop turning the NEGATIVE
SUPPLY knob CCW.

24. While observing the RF and VCO inputs to the PHASE DETECTOR, slowly decrease fi by
turning the NEGATIVE SUPPLY knob CCW. When fvco stops tracking fi stop turning the
NEGATIVE SUPPLY knob. This is the point at which the VCO signal snaps back to its free-
running frequency. You may have to repeat this step to obtain the exact frequency at which
tracking stops.

2-15
25. On channel 1, accurately measure the period (T) between peaks of the RF input signal
waveform. Each horizontal division is 0.5 μs.

....................................................................................

26. From T, calculate the frequency (KHz) of the RF input signal (fi = 1/T).

....................................................................................

27. You determined that when fi is between the frequencies you calculated in steps 22 and 26,
fvco tracks fi. What is the width of the lock range of the PLL?

....................................................................................

28. Place a two-post connector between the FILTER and AMP on the PHASE-LOCKED LOOP
circuit block.

29. On the PHASE-LOCKED LOOP circuit block, connect the channel 1 probe to RF at the
PHASE DETECTOR input, and connect the channel 2 probe to the VCO output. Set channel 1
to 50 mV/DIV, set channel 2 to 200 mV/DIV, and set the sweep to 0.5 μs/DIV. Trigger on
channel 1.

30. Adjust the NEGATIVE SUPPLY knob on the base unit completely CCW.

31. Slowly increase fi (channel 1) by turning the NEGATIVE SUPPLY knob CW. When the fvco
signal starts to track fi and Vi is about -4.0 Vdc, stop turning the NEGATIVE SUPPLY knob CW.
The signals should appear, as shown in Figure 7-23.

32. Connect the FUNCTION GENERATOR'S output to the (M) terminal on the VCO-LO circuit
block. Connect the channel 1 probe to (M), Adjust the FUNCTION GENERATOR for a
150 mVpk-pk, 3 kHz sine wave message signal at (M).

33. Connect the channel 2 probe to the PHASE DETECTOR output. Set channel 2 to 1.0 V/DIV
and the oscilloscope sweep to 0.2 ms/DIV. The oscilloscope signals should appear, as shown in
Figure 7-24.

2-16
34. What signals compose the PHASE DETECTOR output signal on channel2?

....................................................................................

35. Is the varying dc voltage: the FM carrier signal, or the recovered message signal?

....................................................................................

36. Connect the channel 2 probe to the FILTER output on the PHASE-LOCKED LOOP circuit
block. Set channel 2 to 500 mV/DIV.

37. Slightly vary the frequency and amplitude of the message signal from the FUNCTION
GENERATOR. Do the frequency and amplitude of the recovered message signal vary with the
message signal?

....................................................................................

38. Connect the channel 2 oscilloscope probe to the VCO input. Set channel 2 to 1 V/DIV. Does
the message signal feedback to VCO change fvco or not?

....................................................................................

39. Now connect the channel 2 oscilloscope probe back to the FILTER output. Set channel 2 to
500 mV/DIV and channel 1 to 100 mV/DIV. At the FUNCTION GENERATOR, increase the
message signal amplitude on channel 1 to 500 mVpk-pk. Is the signal on channel 2 the recovered
message signal or not? Explain?

....................................................................................

2-17