Question
Wed January 30, 2013 By: Shilpa Dey
 

give a brief discription about modulation and its type .... please give a full note for this

Expert Reply
Thu January 31, 2013

Modulation is the process of varying one or more properties of a high frequency carrier signal, with respect to the message signal. Different types of modulation are: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).

The need for modulation can be summarized as follows:

(1)  The antenna needed for transmitting signals should have size at least ?/4, where, ? is the wavelength. The information signal, also known as baseband signal is of low frequency (and therefore the wavelength is high). If we need to transmit such a signal directly, the size of the antenna will be very large and impossible to build. Hence direct transmission is not practical.

(2)   The radiated power by an antenna is inversely proportional to the square of the wavelength. So, if we use high frequency signals, the power radiated will be increased.

(3)  If we transmit the baseband signals directly, the signals from different transmitters will get mixed up and the information will be lost.

Because of these reasons, we use the technology of modulation, for transmitting message signals effectively for long distances.

Different types of modulation are: amplitude modulation (AM), frequency modulation (FM) and phase modulation (PM).

Amplitude modulation: 

Amplitude modulation is the process of varying the amplitude of the carrier signal in accordance with theamplitude of the message signal. One important parameter associated with AM is the modulation index(or modulation depth). Modulation index is the ratio of amplitude of message signal to that of the carrier signal.

That is,

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frequency modulation: 

An important concept in the understanding of  FM is that of frequency deviation. The amount of frequency deviation a signal experiences is a measure of the change in transmitter output frequency from the rest frequency of the transmitter. The rest frequency of a transmitter is defined as the output frequency with no modulating signal applied. Frequency modulation, FM is widely used for a variety of radio communications applications. FM broadcasts on the VHF bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two way radio communications, and it is especially useful for mobile radio communications, being used in taxis, and many other forms of vehicle In just the same way that it is useful to know the modulation index of an amplitude modulated signal the same is true for a frequency modulated signal. The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index will vary according to the frequency that is modulating the transmitted carrier and the amount of deviation.

Pulse modulation is a technique in which the carrier wave used is a series of pulses, instead of a continuous wave. In the pulse modulation technique, the parameters of the carrier pulse, like, the pulse amplitude, pulse width or pulse position is varied in accordance with the message signal. These are known as PAM, PWM and PPM respectively.
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