28 August 2011

Modulation

         Modulation is the process where the message information is embedded into the radio carrier. Message information can be transmitted in the amplitude, frequency, or phase of the carrier, or a combination of these, in either analog or digital form. Most first generation cellular systems such as AMPS use analog FM, because analog technology was well understood when these systems were developed. However, the pressing need for greater spectral efficiency lead to the use of digital modulation techniques in second generation digital cellular systems.

            It allows voice, pictures and other information’s to be transferred from one place to another. The modulation process is reversed at the receiver to recover the information.     
     
            The process of superimposing the audio signal wave on the high frequency carrier wave is known as modulation and the resulting wave is known as the modulated wave and it is transmitted as radio wave.

            A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator. A device that can do both operations is a modem (modulator–demodulator).  
          
            Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used. 
 
Different kinds of modulation :

            The three key parameters of a periodic waveform are its amplitude, its phase and its frequency.

1. Amplitude Modulation :

            If the amplitude of the carrier wave is varied in accordance with the signal voltage, the result is amplitude modulated wave. The process is known as the amplitude modulation (A.M).

2. Frequency Modulation :

            If the frequency of the carrier wave is varied in accordance with the signal voltage, the result is frequency modulated wave. The process is known as the frequency modulation (F.M).

3. Phase Modulation : 

            If the phase of the carrier wave is varied in accordance with the signal voltage, the result is phase modulated wave. The process is known as the phase modulation (P.M).

Need for Modulation :

* it improves the signal strength . It means, the low frequency 
signal can't transmit long distance when we modulate that signal 
it can be transmitted long distance.
 
* To transfer the message signal from one site to another site 
without any loss
 
* To increase the bandwidth of the signal
 
* Moreover, with high frequency electromagnetic wave, more 
power can be transmitted into space.
 
* To reduce the interference made when we transmit the 
signals with nearly same frequency in the audio frequency 
range (20-20k) Hz.
 
* The audio signals can be amplified and propagated through space
using antenna. For efficient radiation of em waves from the aerial,
the length of antenna should be at least one fourth of 
the wavelength of the signal. For example, if the frequency of
the signal is 1000 hertz, the corresponding wavelength is 300 km
and the length of the antenna must be about 75 km. This is 
practically impossible. However, if the em waves radiated by 
the antenna is of high frequency, the length of the 
antenna required will be conveniently small.
 
* To reduce the noise & distortions