UnitNo: 1

AM Modulation & Demodulation

Introduction to Communication Systems
Electromagnetic spectrum, Elements of electronic communication systems, Modulation -Types, need for
Modulation., Amplitude Modulation principles, AM envelope, frequency spectrum & BW, Modulation index, %
modulation, AM transmitters: Block diagram of low level DSBFC, High level DSBFC. Block diagram of SSB,
Vestigial sideband (VSB).
Receiver parameters: Sensitivity, Selectivity, dynamic range, fidelity, Types of AM receiver: TRF and Super
heterodyne (block diagram), AM detection using diode detector, distortion in diode detector. Negative peak
clipping & diagonal clipping

Analog Signals : It is continuous function of time. It has amplitude

for all values of time.
Time period(T):The time taken by a periodic waveform to complete
one cycle is called time period of waveform. It is measured in seconds.
Wavelength(λ):The distance between successive troughs or crests. It is
measured in meter/s.
Frequency(f):The number of cycles completed by the waveform in one
second is called frequency. It is measured in Hertz(Hz)
1kHz=1000Hz=1X103 Hz
1MHz=1X106Hz
1GHz=1X109 Hz
1THz=1X1012Hz
Relations:
f=1/T
λ=Velocity X Time
λ=Velocity X 1/f
Examples:
1) Calculate the wavelength of signal having 1khz frequency

λ=Velocity X 1/f
Velocity:3x108m/s
λ =(3x108) /(1X103)
λ=300000m or 300km
Electronic Communication Systems
1) The word communicate refers to pass on and the act of communicating is
termed as communication. In everyday life, we are interested in communicating
some information which may include some thought, news, feeling and so on.

2) Thus, in a broad sense, the term communication refers to the

transmission of information from one place to the other.

3) The information transmission between humans sitting very close (example,

across a table) may take place via one or more of the following means: speech,
facial expressions and gestures.

ELEMENTS OF A COMMUNICATION SYSTEM

1) Figure shows the generic block diagram of a communication system

.

2) Any communication system have five blocks, the information

source, destination blocks, transmitter, channel and receiver.
3)
4)

5)
1) Information Source
1) The information comes from the information source, which
originates it.

2) We use the words information and message interchangeably, it is better to understand

the basic difference between the two .

3) For example, It is raining today at my place is the information and the speech
corresponding to it is the message signal.

2) Transmitter
1) The objective of the transmitter block is to collect the incoming message signal and modify
it in a suitable fashion (if needed), such that, it can be transmitted via the chosen channel to
the receiving point.

2) The functionality of the transmitter block is mainly decided by the type or nature of the
channel chosen for communication.

3) This transmitter block involves several operations like amplification, generation of high-
frequency carrier signal, modulation and then radiation of the modulated signal.

4) The amplification process essentially involves amplifying the signal amplitude values and
also adding required power levels.

5) The high-frequency signal is essential for carrying out an

important operation called modulation.

6) There are three types of modulations : amplitude modulation, phase modulation and
frequency modulation.

7) The modulated signal from the modulator is transmitted or radiated into the atmosphere
using an antenna as the transducer which converts the signal energy to free space
electromagnetic waves and vice versa.

3)Channel

1) Channel is the physical medium which connects the transmitter with that of the receiver.

2) The physical medium includes copper wire, coaxial cable, fibre optic cable, wave guide
and free space or atmosphere.

3) The choice of a particular channel depends on the feasibility and also the purpose of
communication system.

4) The nature of modification of message signal in the transmitter block is based on the
choice of the communication channel.

5) This is because the message signal should smoothly travel through the channel with least
opposition so that maximum information can be delivered to the receiver.
.

4) Receiver
1) The receiver block receives the incoming modified version of the message signal from the
channel and processes it to recreate the original (non-electrical) form of the message signal.

2) The super heterodyne receiver includes processing steps like reception. amplification,
mixing, demodulation and recreation of message signal.
3) Demodulation is the most important one which converts the message signal available in the
modified form to the original electrical version of the message.

4) Thus demodulation is essentially an inverse operation of modulation.

5) The output of a receiver may be fed to a loud speaker, video display unit, teletypewriter,
various radar displays, television picture tube, pen recorder or computer.

5) Destination

1) The destination is the final block in the communication system which receives the message
signal and processes it

NEED FOR MODULATION

1) The term modulation means regulate. The process of regulating is modulation. Thus, for
regulation we need one physical quantity which is to be regulated and another physical
quantity provides regulation. In electrical communication, the signal to be regulated is termed
as carrier. The signal which provides regulation is termed as modulating signal. Message acts
as modulating signal. The modulation
process is the most important operation in the modem communication systems.

2) The distance that can be travelled by a signal in an open atmosphere is directly

(inversely) proportional to its frequency (wavelength). Most of the message signals like
speech and music are in the audio frequency range (20 Hz-20 kHz) and hence they can hardly
travel for few meters on their own.

3) Further the for efficient radiation and reception, the transmitting and receiving
antennas would have to have length comparable to a quarter-wavelength of the frequency
used.

I) For example a message at 1 MHz, its wavelength is 300 m (3 X 108/ I X 106) and hence
antenna length should be about 75 m.

II) For a signal at 15 kHz. the antenna length will be about 5000 m. A vertical antenna of
this size is impracticable.

6) There is one more important reason against transmitting signal frequencies directly; all
message is concentrated within the same range (20 Hz-20 kHz for speech and music, ), so that
all signals from the different sources would be hopelessly and inseparably mixed up.
ELECTROMAGNETIC SPECTRUM AND TYPICAL APPLICATIONS
1) As the name indicates, an electromagnetic (EM) wave is a signal made of oscillating
electric and magnetic fields. The Table shows the entire range of EM spectrum.

2) For the classification purpose, the EM spectrum is divided into small segments and each
segment is given a nomenclature.

3) Apart from this detailed classification, the EM spectrum is also broadly classified into
two broad categories, namely, audio frequency (AF) for the frequency range 20 Hz - 20 kHz
and the radio frequency (RF) range for frequencies more than 20 kHz.
TERMINOLOGIES IN COMMUNICATION SYSTEMS
Time (t) : IT is a fundamental quantity with reference to which all communications happen. It is
typically measured in seconds (sec).

Frequency (f): is another fundamental quantity with reference to which all signals in a
communication system are more commonly distinguished. Frequency is defined as the number
of oscillations per second and is measured ill hertz (Hz).

Wavelength (λ): It is yet another fundamental quantity used as an alternative to frequency for
distinguishing communication signals. Wavelength is defined as the distance travelled by an EM
wave during the time of one cycle. EM waves travel at the speed of light in atmosphere or vacuum,
that is, 3 X I 08 m/s.

The wavelength of a signal can then be found by using the relation λ = c / f

For instance, if the frequency of a given signal is 30 MHz, then its wavelength is λ=10m.

Spectrum: The frequency domain representation of the given signal.

Bandwidth: It is the range of frequencies over which the information is present in the original
signal and hence it may also be termed as signal bandwidth.

Channel Bandwidth: The range of frequencies required for the transmission of modulated signal.

Modulation: In terms of signal and channel bandwidths, modulation is a process of transforming

signal from signal bandwidth to channel bandwidth .

Demodulation: On the similar lines, demodulation is the reverse process of modulation, that is,
transforming signal from channel bandwidth to signal bandwidth.

Baseband Signal: Message signal in its original frequency range.

Baseband Transmission: Transmission of message signal in its original frequency range.

Broadband Signal: Message signal in its modulated frequency range.

Broadband Transmission: Transmission of message signal in the modulated frequency range.

Amlitude Modulation:

1) In Amlpitude Modulation the amplitude of carrrier signal is varied by the

modulating signal or message signal
2) It can also be difined as the ampitude of the carrier is made proportional to the
instanteneous ampitude of the modulating signal
3) The amplitude is also called as DSBFC because in modulating signal there are
carrier frequency along with two side bands
AM Transmitters

1) Transmitters that transmit AM signals are known as AM transmitters

2) Thesetransmitters are used in medium wave (MW) and short wave (SW) frequency bands
for AM broadcast.

3) The MW band has frequencies between 550 KHz and 1650 KHz,

4) The SW band has frequencies ranging from 3 MHz to 30 MHz.

5) The two types of AM transmitters that are used based on their transmitting powers
are:

I) High Level transmitters

II) Low Level transmitters

6) High level transmitters use high level modulation, and low level transmitters use low
level modulation.

7) The choice between the two modulation schemes depends on the transmitting power
of the AM transmitter.

8) In broadcast transmitters, where the transmitting power may be of the order of kilowatts,
high level modulation is employed.

9) Low level modulation is used where only a few watts of transmitting power are required.

1) High-Level AM Transmitters

Figure (a): Block Diagram of High Level AM Transmitter

1) In high-level transmission, the powers of the carrier and modulating signals are amplified
before applying them to the modulator stage,
The various sections are:

 Carrier oscillator
 Buffer amplifier
 Frequency multiplier
 Power amplifier
 Audio chain
 Modulated class C power amplifier

I) Carrier Oscillator
1) The carrier oscillator generates the carrier signal, which lies in the RF range. The frequency
of the carrier is always very high.

2) It is very difficult to generate high frequencies with good frequency stability, the carrier
oscillator generates a sub multiple with the required carrier frequency.

3) This sub multiple frequency is multiplied by the frequency multiplier stage to get the
required carrier frequency.

II) Buffer Amplifier

1). It first matches the output impedance of the carrier oscillator with the input impedance of the
frequency multiplier.

2) It then isolates the carrier oscillator and frequency multiplier.

3) This is required so that the multiplier does not draw a large current from the carrier
oscillator. If this occurs, the frequency of the carrier oscillator will not remain stable.

III) Frequency Multiplier

1) The sub-multiple frequency of the carrier signal, generated by the carrier oscillator , is now
applied to the frequency multiplier through the buffer amplifier.

2) The frequency multiplier generates higher harmonics of carrier oscillator frequency. The
frequency multiplier is a tuned circuit that can be tuned to the requisite carrier frequency that is to
be transmitted.

IV) Power Amplifier

1) The power of the carrier signal is then amplified in the power amplifier stage. This is the basic
requirement of a high-level transmitter.

2) A class C power amplifier gives high power current pulses of the carrier signal at its output.
V) Audio Chain
1) The audio signal to be transmitted is obtained from the microphone.
2) The audio driver amplifier amplifies the voltage of this signal.

3) This amplification is necessary to drive the audio power amplifier.

4) A class A or a class B power amplifier amplifies are used to boost the power of the audio signal.

VI) Modulated Class C Amplifier

1) This is the output stage of the transmitter. The modulating audio signal (message signal)and the
carrier signal, after power amplification, are applied to this modulating stage.

2) The modulation takes place at this stage. The class C amplifier also amplifies the power of the
AM signal to the required transmitting power.

3) This signal is finally fed to the antenna., which radiates the signal into space of transmission.

1) Low-Level AM Transmitters

Figure (b): Block Diagram of Low Level AM Transmitter

1) The low-level AM transmitter shown in the figure (b) is similar to a high-level

transmitter, except that the powers of the carrier and audio
signals are not amplified. These two signals are directly applied to the modulated class C power
amplifier.
Modulation takes place at the stage, and the power of the modulated signal is amplified to the required
transmitting power level. The transmitting antenna then transmits the signal.