Communications I
Laboratory #5
1
Reception of Frequency Modulated Signals
FM Demodulation
OBJECTIVES
The purpose of this experiment is to show how the frequency-modulated signals
are demodulated via a Phase lock loop and quadrature detector. Also, in this experiment
you will be able to see the signal in the oscilloscope and its power spectrum in different
points in the receiver.
EQUIPMENT LIST
1.
2.
3.
4.
5.
6.
Oscilloscope and probes
ANACOM 2 Trainer
Power supply
Cables
N to BNC connector adapter
HP-VEE
DISCUSSION
The basic FM receiver uses the superheterodyne principle. See figure 1. In the
block diagram shows that it has many similarities to the AM receiver. The only apparent
difference are the use of a discriminator in place of detector, the addition of a deemphasis
network and the fact that AGC may or may not be used. The mixer, local oscillator and
IF amplifiers are basically similar to those discussed for AM receivers.
The discriminator extracts the intelligence from the high-frequency carrier. A
discriminator is a device in which amplitude variation is derived in response to frequency
or phase variations.
The deemphasis network is required to bring the high-frequency intelligence
back to the proper amplitude relationship with the lower frequencies. Remember high
frequencies were preemphasized at the transmitter to provide them with greater noise
immunity.
Prepared by: Prof. Rubén Flores Flores
Communications I
Laboratory #5
2
The AGC is optional in FM receivers. The use of limiters in FM receivers
essentially provides and AGC function. The limiter is a circuit whose output is a constant
amplitude for all inputs above a critical value. Its function in an FM receiver is to
remove any residual or unwanted amplitude modulation and amplitude variations due to
noise. In addition, the limiting function also provides AGC action, since signals from the
critical minimum value up to some maximum value provide a constant input level to the
detector.
Discriminators (Detector)
The FM discriminator extracts the intelligence that has been modulated onto the
carrier via frequency variations. In this laboratory we are going to see two types of
detector: the PLL and Quadrature detector.
The phase-lock loop (PLL) is an electronic feedback control system as
represented by the block diagram in figure #2. The comparator compares the input signal
and the output of the VCO and develops an error signal proportionate to the difference
between the two. This error signal drives the VCO to change frequency so that the error
is reduced to zero. If the VCO frequency equals the input frequency, the PLL has
achieved "lock" and he control voltage will be constant for as long as the PLL input
frequency remains constant. If the PLL input is an FM signal, the low-pass filter output
is the demodulated signal. The modulated carrier changes frequency according to the
modulating signal. The function of the PLL is to hold the VCO frequency in step with
this changing carrier. If the carrier frequency increases the error signal developed by the
phase comparator rises to make VCO frequency rise. On the other hand if the carrier
frequency decreases the comparator output drops to decrease the VCO frequency. Thus
we see that the error signal matches the modulating signal back at the transmitter; the
error signal is the demodulated output. The VCO input control signal causes the VCO
output to match the FM signal applied to the PLL. If the FM carrier drifts, the PLL
readjust itself. The PLL FM discriminator requires no tuned, it adjust itself to any carrier
frequency drift.
Prepared by: Prof. Rubén Flores Flores
Communications I
Laboratory #5
3
Figure 2: PLL block diagram
The Quadrature detectors derive their name from use of the FM signal in phase
and 90° out of phase. The two signals are said to be in quadrature at a 90° angle. The
circuit in figure 3 shows an FM quadrature detector using an exclusive-OR gate. The
limited IF output is applied directly to one input and the phase-shifted signal to the other.
Notice that this circuit uses the limited signal that has not been changed back to a sine
wave. The L, C and R values used at the circuit’s input are chosen to provide a 90° phase
shift at the carrier frequency to the signal 2 input. The signal 2 input is a sine wave due
to the LC circuits effects. The upward and downward frequency deviation of the FM
signal results in a corresponding higher or lower phase shift. With one input to the gate
shifted, the gate output will be a series of pulses with a width proportional to the phase
difference. The low-pass RC filter at the gate output “sums” the output, giving an
average value that is the intelligence signal.
Figure 2: Quadrature Detector
Prepared by: Prof. Rubén Flores Flores
Communications I
Laboratory #5
4
PROCEDURE
1.
Connect the power supply to the trainer with all equipment tuned off. Follow the
following diagram.
+12 V
Power Supply
+12 V
0V
0V
-12 V
-12 V
ANACOM 2
2.
Select the following conditions in the trainer
a. Mixer/Amplifier Amplitude dial :
Maximum clockwise direction
b. VCO switch (PLL detector block):
ON position
c. Modulation switch
:
Reactance
3.
Turn on the power supply. Connect the oscilloscope to test point 1. Turn Audio
Oscillator amplitude dial to the maximum position. Using the oscilloscope adjust
the Audio Oscillator frequency to 2 kHz. This signal is the modulating signal.
4.
Using a cable jumper, connect the Audio Oscillator output to the Modulator audio
input. Now connect the FM output with the AMPLITUDE LIMITER input;
connect the AMPLITUDE LIMITER output with the PLL detector input. Finally,
connect the PLL output with the LPF/AMP input.
5.
Turn the REACTANCE MODULATOR CARRIER FREQUENCY dial to the
middle position.
6.
Turn the LOW PASS FILTER/AMPLIFIER gain dial to the maximum position.
7.
Connect the oscilloscope channel one to test point 14 and channel two to test point
73. Measure frequency, peak to peak voltage of both channels. Are both signals
similar? Find the phase difference between the transmitted and received signals.
8.
Remove the Amplitude Limiter block from the receiver; connect the FM output to
the PLL Detector input. Compare the transmitted and received signals. Are they
similar? Measure frequency, peak to peak voltage of both channels.
9.
Now connect the FM output with the AMPLITUDE LIMITER input; connect the
AMPLITUDE LIMITER output with the Quadrature detector input. Finally,
connect the Quadrature output with the LPF/AMP input.
10.
Turn off the PLL Detector’s VCO.
Prepared by: Prof. Rubén Flores Flores
Communications I
Laboratory #5
5
11.
Connect the oscilloscope channel one to test point 14 and channel two to test point
73. Measure frequency, peak to peak voltage of both channels. Are both signals
similar? Find the phase difference between the transmitted and received signals.
12.
Remove the Amplitude Limiter block from the receiver; connect the FM output to
the Quadrature Detector input. Compare the transmitted and received signals. Are
they similar? Measure frequency, peak to peak voltage of both channels.
13.
Connect probe of channel one to test point 71 and probe of channel two to test
point 73. Determine the amplifier’s Gain in dB.
Prepared by: Prof. Rubén Flores Flores
Communications I
Laboratory #5
6
RESULTS
Table 1: PLL Receiver with Limiter
PLL Detector with Limiter
TP 14 Tx
TP 73 Rx
fs
Vp-p
Vavg

Table 2: PLL Receiver without Limiter
PLL Detector without Limiter
TP 14 Tx
TP 73 Rx
fs
Vp-p
Vavg
Table 3: Quadrature Receiver with Limiter
Quadrature Detector with Limiter
TP 14 Tx
TP 73 Rx
fs
Vp-p
Vavg

Table 4: Quadrature Receiver without Limiter
Quadrature Detector without
Limiter
TP 14 Tx
TP 73 Rx
fs
Vp-p
Vavg
Table 5: Amplifier's Gain at the Receiver
Test Point
71
73
Prepared by: Prof. Rubén Flores Flores
Amplifier’s Gain
Vp-p
Gain (dB)
Communications I
Laboratory #5
QUESTIONS
1. Why is important an Amplitude Limiter in a receiver?
2. Did you find that the received and transmitted signal are the same? What’s the
difference and why?
3. Explain the difference between the Quadrature and PLL detectors. Which one is
superior?
4. Mention a few other FM detector circuits.
Prepared by: Prof. Rubén Flores Flores
7