GENERAL CONCEPTS Communication is the process of conveying

or transferring messages from one point to another.

Generally it can be classified into two types (i) Communication within line of sight (ii) Communication beyond the line of

sight between point to point.

BLOCK DIA. Of communication system

1) Information source :

messages can be in the form of words, groups of words, code, symbols, sound signal etc.

out of these messages, only the desired message is selected and conveyed or communicated

the function of information source is to produce the required message, which has to be transmitted.

2) Transmitter

A transmitter comprises of electrical and electronic components that convert the message signal into a suitable form for propagating over the communication medium.

This is often achieved by modulating the message signal with a carrier signal (high frequency signal) to carry the modulating or message signal.

3) The channel and the noise:channel means the medium through which

the message travels from the transmitter to the receiver.

The channel can be of many forms like coaxial cable, microwave links radio wave links, or an optical fiber.

During the process of transmission and reception the signal gets distorted due to noise introduced in the system.

Noise is an unwanted signal which tends to interfere with the required signal.

Noise signal is always random in nature.

Noise may interfere with signal at any point in a communication system.

4) Receiver:The main function of the receiver is to

reproduce the message signal.This reproduction of the original signal is

accomplished by a process known as the demodulation or detection.

Demodulation is the reverse process of modulation carried out in transmitter.

The demodulated signal is in electrical form.

5) Destination:Destination is the final stage.The demodulated electrical signal is

converted into the original message.

Types of communication system1) Analog communication systemIt is designed to transmit analog

information using analog modulation schemes such as amplitude modulation and angle modulation.

2) Digital communication systemIt is designed to transmit digital

information using digital modulation schemes such as PCM, DM, DPCM etc.

3) Hybrid communication systemIt is designed to use digital modulation

schemes for transmitting sampled and quantized value of analog signals.

Definition of Modulation:Modulation is the process by which some

characteristics of a carrier wave is varied in accordance with the instantaneous value of a modulating signal or message signal.

The carrier wave is changed in accordance with the intensity (i.e amplitude) of the message signal.

The resultant wave is called modulated wave or radio wave.

Need for modulation:(i) Practical Antenna LengthTheory shows that in order to transmit a wave

effectively the length of the transmitting antenna should be approximately equal to the wave length of the wave

As the audio frequency ranges from 20 HZ to 20 kHZ, if they are transmitted directly into space, the length of the transmitting antenna required would be extremely large.

For instance, to radiate a frequency of 20 kHZ directly into space, we would need an antenna length of

This is too long for an antenna to be constructed practically.

For this reason, it is impracticable to radiate audio signal directly into space; on the other hand if a carrier wave, say of 1000 KHz is used to carry the signal, we need an antenna length of 300 meters only.

So an increase in the carrier frequency range, decreases the antenna length.

(ii) Operating rangeThe energy of a wave depends upon its

frequency. The greater the frequency of the wave, the

greater the energy possessed by it. As the audio signal frequencies are small

they cannot be transmitted over large distances if radiated directly into space.

The only practical solution is to modulate a high frequency carrier wave with audio signal and permit the transmission to occur at this high frequency (i.e Carrier frequency)

(iii) Wire CommunicationOne desirable feature of radio transmission is

that it should be carried with out wires i.e radiated into space.

At audio frequencies, radiation is not practicable because the efficiency of radiation is poor.

However, efficient radiation of electrical energy is possible at high frequencies (> 20 kHz).

(iv) MultiplexingIf more than one signal uses a single channel

then modulation may be used to translate different signals to different spectral location, thus enabling the receiver to select the desired signal.

Application of multiplexing includes data telemetry, broadcasting and long distance telephones.

(v) To Overcome Equipment LimitationsOccasionally in signal processing applications the frequency of the signal to be processed and frequency range of processing apparatus do not match.

If the equipment is elaborate and complex. It is operated in some fixed frequency range, thus translating the frequency range of the processing signal corresponding to this fixed range of equipment. Modulation can be used to accomplish this frequency translation.

(vi) Narrowbanding:without modulation, the ratio of highest to

lowest message (audio) signal would be 20 KHz / 20 hz = 1000.

So the antenna height has to vary up to a scale of 1000, which is impractical.

If modulation is used with a carrier of 1 MHz, then the ratio will be [106+ 20000] / 106 + 20] ≈ 1.

Definition Amplitude modulation may be defined as a

system in which the amplitude of the carrier wave changes in accordance with the instantaneous value of the modulating signal.

Let the carrier and modulating ( message ) signal be

Frequency spectrum of AM signal

Graphical representation of AM wave.

Phase representation of AM with carrier.It is the easy representation of AM, where Ac

is the carrier wave phasor (taken as reference phasor).

The two sidebands having a frequency of (ωc+ ωm) and (ωc - ωm) are represented by two phasors rotating in opposite directions with angular frequency of ωm.

The net or resultant phasor is s(t). It depends on the position of the sideband phasor and carrier wave phasor.

Phase representation of AM with carrier. ( contd…)

Modulation Index • The modulating signal is preserved in the

envelope of amplitude modulated signal only if Am < Ac .

• For this, ma < 1 is one of the conditions for proper AM transmission.

• There are three degrees ( depth) of modulation depending upon the amplitude of the message signal relative to carrier amplitude. They are

(i) Under modulation (ii) Over modulation (iii) Critical modulation

Under modulation ( ma <1 )In this case the modulation index ma <1 , ie.

Am <Ac.

It may be observed that the envelope is not reaching the zero amplitude axis of the AM wave form.

So the baseband signal may be fully recovered from the envelope of AM wave.

Under modulation ( ma <1 ) contd…

Over modulation (ma > 1) In this case, the amplitude of a baseband

signal exceeds maximum carrier amplitude Am > Ac.

Therefore certain portion of the envelope of the modulated signal crosses the zero axis.

So both positive and negative extensions of modulating signal are cancelled or clipped out

Over modulation (ma > 1) contd.. • Thus the envelopes of message signal are not

same. This is called envelope distortion.

• Due to this, envelope detector ( AM demodulator ) provides distorted message signal.

• Thus it is advisable to avoid over modulation otherwise original signal without distortion cannot be recovered.

• In this case, the modulation index ma is more than 1 .

Over modulation (ma > 1) contd..

Critical modulation (ma = 1 ) • In this case the modulation index ma = 1 i.e.,

Am = Ac.

• Here envelope of the modulated signal just reaches the zero amplitude axis.

• The message signal remains preserved. This is known as Critical modulation.

• In this case also the modulated signal can be recovered by using envelope detector without any distortion.

Critical modulation (ma = 1 )

Power relations in AM • The carrier component of the modulated wave

has the same amplitude as the un-modulated carrier.

• That is, the amplitude of the carrier is unchanged.

• The modulated wave contains extra energy in the two sideband components.

• Therefore, the modulated wave contains more power than the carrier had before modulation took place.

Power relations in AM -contd…

WKT, the amplitude of the sidebands depends on the modulation index Am/Ac.

Thus , the total power in the modulated wave will depend on the modulation index.

The total power in the modulated wave will be

Power relations in AM -contd…Where all three voltages are (rms) values (1 √2

peak ) and R is the resistance (e.g., antenna resistance), in which the power is dissipated.

The unmodulated carrier power component is,

Power relations in AM -contd…The power components due to side bands are,

Power relations in AM -contd…

Current Relation in AM

It = total or modulated currentIc = carrier current

Power Efficiency of AM wave It can be defined as the ratio of power in

sidebands to total power.Because sidebands only contain the useful

information ( modulating or message signal).

Power Efficiency of AM wave contd…

Power Content in Multiple-Tone AmplitudeModulationA multitone amplitude modulation is a type of

modulation in which the modulating signal consists of more than one frequency components.

Let us consider that a carrier signal Ac cos ωct is modulated by a modulating signal Vm(t).

Power Content in Multiple-Tone AmplitudeModulation contd…

Power Content in Multiple-Tone AmplitudeModulation contd…

Power Content in Multiple-Tone AmplitudeModulation contd…

Power Content in Multiple-Tone AmplitudeModulation contd…