COMMUNICATION SYSTEMS

The main communication systems we are using now
1)voice(300-3.5khz)
2)audio(20hz-20khz)
3)video(0-4.5Mhz)
4)Data communications(Eg:broadband)


Block diagram of communication system



Transmitter is the first block in the block diagram of communication system. Here the message which is to be sent is generated.In Encoders process in which the actual message is converted in to symbols for transmission. In this a sequence of characters are put in a specialized format for efficient transmission.Noisy channel is nothing but the medium through which the message is transmitted.Decoder is used to decode the encoded message and retrieve the actual message.Receiver can be said as the target to which the information need to be delivered.
Need for modulation in communication systems

1)To reduce the antenna height
2)Multiplexing of signals
In analog communications we use (Frequency Division Multiplexing)
In digital communications we use (Time Division Multiplexing)
3)To reduce the effect of noise

Bandwidth

Range of frequencies occupied by a signal is called bandwidth.
bandwidth of signal must be as low as possible.
bandwidth of channel is as high as possible.
To reduce bandwidth we have to reduce insignificant frequencies.

Domain conversions in signals

Fourier transforms
For studying the frequency characteristics of signals some times we have to convert time domain signals in to frequency domain for that use use a tool called Fourier transforms.
If the signal is narrow band in one domain it will be wide band in other domain.For example the Fourier transform of rectangle function is sinc.In sinc function low frequencies is significant so we use low pass filter. A signal cannot be band limited and time limited simultaneously.We can define a Fourier transform for stable signals only.



Frequency shifting property of Fourier transforms
or
Modulation property of Fourier transforms

Modulation is defined as the process of frequency translation.Spectrum is shifted from low frequency to high frequency region.



Frequencies in a dc signal is zero.In modulation we have to multiply base band signal and carrier signal so that the resulting signal is shifted to left side and right side.
In FM communication the signal range is from 88Mhz to 108 Mhz
In radar or satellite it is about in the range of Ghz
Analog Communications

Modulation is defined as the process in which a characteristic parameter of the carrier signal is varied according to message signal.The major modulation technique in analog modulation is amplitude modulation.Sinusoidal signal carries information in the form of amplitude.when doing modulation we have to consider a fact that bandwidth and power should be as minimum as possible.

The major modulation schemes used in analog communication systems are
1) Amplitude modulation
2)Double sideband modulation
3)Single sideband modulation
4)vestigial side band modulation
Frequency modulation
1)Narrow Band Frequency Modulation
2)Wide Band Frequency Modulation
3)Phase Modulation

AMPLITUDE MODULATION
AM is defined as the process in which the peak amplitude of the carrier is varied according to the message signal.


The bandwidth of signal in amplitude modulation is 2*highest frequency of message signal.
Frequency above carrier frequency is upper cut off frequency.Frequency below carrier frequency is lower cut off frequency.
amplitude modulation is of singletone and multitone modulation.Singletone consists of only one frequency.Multitone consist of more than one frequency.
The major disadvantage of AM signal is carrier takes more power than sidebands.
GENERATION OF AM SIGNALS
square law modulator