Experiment 1

American International University- Bangladesh

Department of Electrical and Electronic Engineering
EEE4106: Telecommunications Engineering Laboratory

Title: Study of AM and FM Transmitters and Receivers

Introduction:

Analog modulation is important for analog communication system. Analog modulation

techniques like AM (Amplitude Modulation) and FM (Frequency Modulation) still find
application in various application such as radio and TV broadcasting system. AM and FM
also provides basics for understanding and design of all other types modulation
technologies.

The objectives of this experiment are:

1) To understand basic principles of Amplitude Modulation (AM) and Frequency

Modulation (FM)
2) To design and implement modulator and demodulator of AM and FM
3) To observe modulation process of AM and FM practically

Theory and Methodology:

Amplitude Modulation (AM)

Modulation means that some aspects (Amplitude, frequency, and phase) of one signal
(the carrier signal which is a high frequency signal) varies according to an aspect of a
second signal (the modulating signal or the information signal to be transmitted).
Amplitude modulation is the process in which the amplitude of the carrier wave varies in
accordance with a modulating wave. Wave shapes of AM are shown in Fig. 1.

Amplitude modulation generates a pair of sidebands: Upper Sideband (USB) and Lower
Sideband (LSB) for every sinusoidal component in the carrier. USB appears as sum of
carrier frequency and modulating signal frequency and LSB as subtraction of carrier
frequency from modulating signal frequency. The amplitude of the two sidebands
increases in proportion to the amount of modulation (modulation index), but never
exceeds half of the carrier amplitude level. As an example of amplitude modulation and
its sideband production, consider a modulating signal (information signal) of 1,000 Hz
sine wave and the carrier signal with a 10,000 Hz sine wave. After modulation,
modulated signal contains 3 components: the carrier signal of 10 KHz plus two sideband
signals: USB 11 KHz and LSB 9 KHz.

© Dept. of EEE, Faculty of Engineering, American International University-Bangladesh (AIUB) 1

 Fig. 1: Wave shapes of Amplitude Modulation

Modulator is used in transmitter of AM communication system (Fig. 2) for modulating of

transmitting information signal. Function of demodulator is reverse of modulation that
recovers the information (modulating) signal from the modulated signal. Demodulator is
used in the receiver of AM communication system (Fig. 3).

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 Fig. 2: Block diagram of AM Transmitter

Fig. 3: Block diagram of AM Receiver

Frequency Modulation (FM)

Frequency modulation (FM) consists in varying the frequency of the carrier wave in
accordance with the instantaneous value of the modulating signal voltage (Fig. 4). Thus
the amplitude of the carrier signal does not change in FM. This is an advantage since any
incidental disturbance such as atmospheric disturbance or man-made static that primarily
appears in the forms of variations of amplitude of the carrier voltage may be eliminated
in an FM receiver which is made insensitive to amplitude variations. Evidently the
frequency variation called the frequency deviation is proportional to the instantaneous
value of the modulating signal voltage. The rate at which this frequency variation takes
place is obviously equal to the modulating signal frequency. In FM all the modulating
signals having the same amplitude will deviate the carrier frequency by the same amount,
say 50 KHz, no matter what their frequencies. Similarly all the modulating signals of the
same frequency say 1 KHz will deviate the carrier at the same rate of 1000 times per sec,
no matter what their individual amplitudes. The frequency deviation, i.e. the max
variation in frequency from the mean value is given by

fd = fmax- fc = fc- fmax

Where,

fc= carrier frequency

fmax= Max carrier frequency

fmin= Min carrier frequency

Modulation index mf is the ratio of frequency deviation to the modulating signal frequency
and is indicated by

mf = fd/fm .

© Dept. of EEE, Faculty of Engineering, American International University-Bangladesh (AIUB) 3

 Fig. 4: Wave shapes of Frequency Modulation

Transmitter and receiver of FM communication system are shown in Fig. 5 and 6

respectively. Message signal is processed to FM signal at the transmitter end by the
modulator for its convenient transmission and at the receiver end, message signal is
extracted from the FM signal by the demodulator.

FM ensures higher noise immunity to the message signal than that of AM. But FM
requires larger bandwidth. FM is popularly find application in radio broadcasting (FM
radio system) and terrestrial TV broadcasting system for transmission of audio signal.
FM can also be used in analog satellite communication system.

© Dept. of EEE, Faculty of Engineering, American International University-Bangladesh (AIUB) 4

 Fig. 5: Block Diagram of FM Transmitter

Fig 6: Block Diagram of FM Receiver

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Pre-lab Homework:
 Gain in depth knowledge about the principle of AM and FM modulation.
 Gain basic knowledge about super-heterodyne radio receiver, quadrature detector
and varactor modulator.

Apparatus:
 Oscilloscope

 The ANACOM 1/1 AM Transmitter

 The ANACOM 1/2 AM Receiver

 The ANACOM 2 FM Transmitter and Receiver

 Microphone

 Power supplies (+12 V, 0 V, -12 V)

Precautions:
 Students are not allowed to work alone in the laboratory.

 Understand the operation of the trainer boards before using them. Permission
from the lab supervisor must be obtained if any changes to the
settings/configuration are to be made.

 Handle all the trainer boards with care.

 Do not take any things out of the laboratory without special permission.
Experiment must be completed within the given time.

 Report all damage to equipment, hazards and potential hazards to the lab
instructor and lab staff.

 Do not touch any exposed wires.

 Do not unplug the cable while the power is switched on.

Experimental Procedure:

AM modulation and demodulation:

1. On Anacom 1/1, check the frequency of the carrier and the frequency of the
message signal.

2. Draw the wave shape of the AM waveform.

© Dept. of EEE, Faculty of Engineering, American International University-Bangladesh (AIUB) 6
 3. Draw the output wave shape of the power amplifier and state the purpose of the
power amplifier.

4. Measure the length of the antenna provided on the ANACOM 1/1. Theoretically,
calculate the ideal length of antenna with the transmitted carrier of 1 MHz.

5. In ANACOM 1/2, draw the wave shape of the signal before RF amplifier and
after RF amplifier.

6. Draw wave shapes of the signal after IF amplifier stage.

7. Observe and draw the wave shape of the output of the AM demodulator

FM modulation and demodulation:

FM modulator and demodulator circuit in Anacom – 2 board with their connectivity is

shown in Fig. 7.

Fig 7 Anacom-2 module

1. Connect the Anacom-2 module to the appropriate power supply.

2. Ensure that the following initial conditions exists on the Anacom-2 module:
a) All switched faults OFF.
b) AMPLITUDE preset (in the MIXER/AMPLIFIER block) in fully clockwise
position.
c) VC0 switch (in PT, L block) in OFF position.

3. Turn on power to the ANACOM2 module.

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 4. Turn the AMPLITUDE PRESET (in the AUDIO OSCILATOR block) in fully
clockwise (Max.) position and observe its output (at t.p 1) on oscilloscope. This is
the modulating signal whose frequency (300 Hz to 3400 Hz) and amplitude can
be varied by FREQUENCY PRESET and AMPLITUDE PRESET on the block.
Leave the AMPLITUDE PRESET in fully counter clockwise (Mm.) position for
the time being.

5. Link the AUDIO OSCILLATOR output to the AUDIO INPUT of the

MODULATOR CIRCUIT as shown in Fig. 3 and put the
VARACTOR/VARACTOR switch in the VARACTOR position.

6. As like AUDIO block the amplitude of the FM carrier can be adjusted by the
AMPLITUDE preset on the MIXER/AMPLIFIER block and tile frequency (4~l
kHz to 458 kHz) can be varied by the CARRIER FREQUENCY preset on the
VARACTOR MODULATOR block. In the VARACTOR MODULATOR block
put the CARRIER FREQUENCY preset in its midway position and
AMPLITUDE preset in fully clockwise position. Monitor the signal at t p.34. It is
the un-modulated carriers as the amplitude of the modulating signal is zero.
7. Turn the CARRIER FREQUENCY preset to its fully counter clockwise position-
this corresponds to minimum base bias voltage. Monitor signal at t.p 34
(Oscillator output) and at t.p.21 (base bias voltage). Now slowly turn the
CARRIER FREQUENCY preset clockwise and record the oscillator frequency
(with a frequency meter at t.p.34) for each 0. 1 Volts intervals of the base voltage.
Plot the oscillator frequency Vs basebias voltage as shown in Fig. I.5.
8. If it is possible to change the base bias voltage with sinusoidal modulating signal
a sinusoidal change in oscillator frequency can be obtained. Thus frequency
modulation is performed with a VARACTOR modulator.
9. Now keeping the CARRIER FREQUENCY preset in fully CCW position observe
the FM output at t.p.34. Now turn AMPLITUDE preset (in AUDIO
OSCILLATOR block) to its fully clockwise position and note what happens to the
FM output. Decrease amplitude of the modulating signal by turning
AMPLITUDE preset (in AUDIO OSCILLATOR block) slowly CCW and
observe the frequency deviation in the FM output
10. Return the AMPLITUDE preset (in AUDIO OSCILLATOR block) to its fully
CW position. Vary the frequency of the modulating signal by adjusting the
FREQUENCY preset (in AUDIO OSCILLATOR block) and observe whether the
FM output pattern changes or not. The change in AUDIO OSCILLATOR
frequency does not effect the amount of frequency deviation-it actually
determines how many times per second the carrier deviates from its center
position. But oscilloscope can not show the rate of change of frequency deviation
and for this reason it appears that the AUDIO OSCILLATOR frequency have no
effect. Now turn the CARRIER FREQUENCY preset slowly CW and observes
the frequency deviation.

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Demodulation Technique using Quadrature detector:

Apply theFM signal to the input of the Quadrature detector block as shown in Fig.

1. Now monitor the output of Quadrature detector block (at t.p. 52) along with input
audio signal (at t.p. 14) at dual mode and compare the two signals. The signal
should contain two components: a) A sine wave at the same frequency as the
audio signal at t.p. 14; b) A high frequency ripple component of small amplitude.
2. Toremove the high frequency ripple, apply the signal at t.p.52 to the LOW PASS
FILTER/AMPLIFIER block. Now observe the signal at the output of the LOW
PASS FILTER at t.p.73.
3. To reduce the amplitude variation connect the AMPLITUDE LIMITER as shown
in Fig 1.6 and observe the signal at AMPLITUDE LIMITER output (at t.p.68)
and at LOW PASS Fig: Connection diagram of an FM transmitter and receiver
FILTER output (at t.p.73).Compare the final output with and without
AMPLITUDE LIMITER.

Questions for report writing:

1. Draw all the wave shapes and explain those waveshapes. Also give reasoning
if you find any unexpected result.

2. Draw the frequency spectrum of both AM and FM waves. Also give

comments on your observations.

3. Write down the basic operation of the Varactor diode.

4. What is the function of the amplitude limiter?

5. Explain the basic operation of the Quadrature detector.

6. Write down advantages and disadvantages of FM transmission over AM

transmission

7. Explain terms DSB, DSB-SC, SSB. Write down advantages and

disadvantages of all the three AM transmission.

Discussion and Conclusion:

Analyze the findings of the experiment and explain about deviation of the outputs. Make
discussion about the sources of errors and possible ways of improving of the
experimental findings.

References:
© Dept. of EEE, Faculty of Engineering, American International University-Bangladesh (AIUB) 9
 1. Simon Haykin, “Communication systems”
2. B. Forouzan, “Data Communication and Networking”
3. Telecommunication Lab Manual of AIUB

© Dept. of EEE, Faculty of Engineering, American International University-Bangladesh (AIUB) 10