modulation & demodulation

Modulation & Demodulation

Satish Chandra

www.satish0402.weebly.com

Modulation

• The low energy message signal is mixed with the high energy or high frequency carrier signal to produce a new high energy signal which carries information to a larger distance.

• The Message signal contains information whereas the carrier signal contains no information. Carrier signal is used just to transmit the information to a long distance.

Modulation

• The question arises how the message signal should be added to the carrier signal.

• The solution lies in changing some characteristics (amplitude, frequency or phase) of a carrier signal in accordance with the amplitude of the message signal. This process is called modulation.

• Modulation means to “change”.

Modulation

• Modulation is the process of mixing a low energy message signal with the high energy carrier signal to produce a new high energy signal which carries information to a long distance.

or • Modulation is the process of changing the

characteristics (amplitude, frequency or phase) of the carrier signal, in accordance with the amplitude of the message signal.

• A device that performs modulation is called modulator.

Modulation

• In modulation process, three types of signals are used to transmit information from source to destination. They are:

–Message signal

–Carrier signal

–Modulated signal

Need for Modulation

• Because of the following reasons:

–Avoids mixing of signals

– Increase the range of communication

–Wireless communication

–Reduces the effect of noise

–Reduces height of antenna

Types of Modulation

Analog modulation

• In analog modulation, the analog signal (sinusoidal signal) is used as a carrier signal that modulates the analog message signal.

• In analog modulation, the characteristics (amplitude, frequency or phase) of the carrier signal is varied in accordance with the amplitude of the message signal.

• The sinusoidal carrier wave can be given by the equation.

Analog modulation

• Since the three variables are the amplitude, frequency, and phase angle, the modulation can be done by varying any one of them.

• Thus there are three modulation types namely:

–Amplitude Modulation (AM)

– Frequency Modulation (FM)

–Phase Modulation (PM)

AM

• Amplitude Modulation

FM

• Frequency Modulation

PM

• Phase Modulation

Amplitude Modulation

• Amplitude modulation is a type of modulation where

– the amplitude (signal strength) of the carrier signal is varied in accordance with the amplitude (signal strength) of the message signal.

– the information (message signal) is transmitted over a carrier wave by varying its amplitude in accordance with the amplitude of the message signal.

– the height of the carrier signal is changed in accordance with the height of the message signal.

AM

In amplitude modulation, only the amplitude of the carrier wave is changed while the frequency and phase of the carrier wave remain constant.

Analysis of an AM Wave

Consider a sinusoidal modulating signal or message signal (am) of angular frequency (ωm) and amplitude (Am) given by:

and carrier wave (ac) of angular frequency (ωc) and amplitude (Ac) given by:

The amplitude of the modulated wave (A) is given as:


The instantaneous value of the amplitude modulated wave (a) can be given as:

This is an equation of amplitude modulated (AM) wave.

Modulation index/factor

• Modulation index/factor or modulation depth describes

– how the amplitude, frequency or phase of the carrier signal and message signal affects the amplitude, frequency or phase of the modulated signal.

– how the amplitude of the carrier signal and message signal affects the amplitude of the amplitude modulated (AM) signal.


• Amplitude modulation index is defined as the ratio of the maximum amplitude of message signal to the maximum amplitude of carrier signal, i.e.,

where,

• Am is the maximum amplitude of the message signal.

• Ac is the maximum amplitude of the carrier signal.

• Generally expressed in percentage.


• There are three types of modulation:

1. Perfect-Modulation

2. Under-Modulation

3. Over-Modulation

Perfect-Modulation

Perfect-modulation occurs when the maximum amplitude of the message signal or modulating signal is exactly equal to the maximum amplitude of the carrier signal (Am = Ac).

Under-Modulation

Under-modulation occurs when the maximum amplitude of the message signal or modulating signal is less than the maximum amplitude of the carrier signal (Am < Ac).

Over-Modulation

Over-modulation occurs when the maximum amplitude of the message signal or modulating signal is greater than the maximum amplitude of the carrier signal (Am > Ac).


Consider the expression of AM wave given by


In the last equation, the first term represents unmodulated carrier, the second term represents lower sideband and the last term represents upper sideband.

This contains the full carrier and both the sidebands. Hence, it is also called Double Sideband Full Carrier (DSBFC) system.

Frequency Spectrum

• The carrier is an un-modulated sinewave which has a single value of frequency (eg: 3 MHz) and carries no useful information.

• When such a carrier is modulated with a message signal, other frequencies can be detected in it.

• These new frequencies that are caused by modulation are called sidebands.

• These sidebands are created above and below the carrier frequency.

Frequency Spectrum

• The sidebands that are created above the carrier frequency are called upper sidebands and the sidebands that are created below the carrier frequency are called lower sidebands.

i.e. fUSB = fc + fm and fLSB = fc – fm

where, fc is the carrier frequency

fm is the message signal frequency

fLSB is lower sideband frequency

fUSB is upper sideband frequency

Frequency Spectrum

½mAc

½mAc

Bandwidth of Amplitude Modulation

• The bandwidth of the signal can be obtained by taking the difference between the highest and lowest frequencies of the signal. From the above figure, we can obtain the bandwidth of AM wave as,

BW = fUSB – fLSB

= (fc + fm) – (fc – fm)

BW = 2 fm

Demodulation

• As the name indicates the demodulation process is the opposite of modulation.

• In the demodulation process the audio or other message signal carried by amplitude variations on the carrier is extracted from the overall message signal to appear at the output.

Demodulation

Diode rectifier envelope detector

modulation & demodulation

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