Modulation

In analog transmission, the sending device produces a high frequency signal that acts as a basis for the information signal. This base signal is called the carrier signal or carrier frequency. The receiving device is tuned to the frequency of the carrier signal that it expects from the sender. Digital information is then modulated in the carrier on the carrier signal by modifying one or more of its characteristics. This kind of modification is called modulation and the information signal is called the modulating signal.
Modulation technique
  • Amplitude modulation is a form of modulation in which the amplitude of carrier wave varied in direct proportion to that of modulating signal.
  • Phase modulation is the encoding information into a carrier wave by variation of its phase in accordance with an input signal.
  • Frequency modulation is a form modulating which represents information as variations in the instantaneous frequency of a carrier wave.
Relation with data communication

The most important question associated with a communication channel is the maximum rate which it can transfer information. Information can only be transferred by a signal if the signal is permitted to change. Analogue signals passing through physical channels may not change arbitrarily fast. The rate at which a signal may change is determined by the bandwidth. It is governed by Nyquist-Shannon law, a signal of bandwidth B may change at a maximum rate 2B. If each change is used to signify a bit, the maximum information rate is 2B.
Therefore places a limit on the maximum rate at which we can transfer information. What really matters is the Signal-to Noise ratio(SNR).This defined by ratio of signal power S and to noise power N, and is often expressed in decibels



There is theoretical maximum to the rate at which information passes error free over channel. This maximum is called the channel capacity C. The famous Hartley-Shannon law states that the channel capacity
C is given by