Engineering Tutorials

Very rarely base band transmission is used for long distance transmission. A technique called modulation is used for the purpose. In modulation there is a carrier signal. The carrier signal may be an analog sinusoidal signal of a fixed frequency or a train of pulses of certain frequency .

The information signal is placed on the carrier. The information signal introduces certain change in the parameter of the carrier. In the case of analog sinusoidal carrier, it is possible to change the amplitude or frequency or phase or two of them or all of them by the information signal.

The point to modulation is to take a message bearing signal and superimpose it upon a carrier signal for transmission. For ease of transmission carrier signals are generally high frequency for several reasons [Book03]:

1. For easy (low loss, low dispersion) propagation as electromagnetic waves.
2. So that they may be simultaneously transmitted without interference from other signals.
3. So as to enable the construction of small antennas (a fraction, usually a quarter of the wavelength).
4. So as to be able to multiplex that is to combine multiple signals for transmission at the same time.

Modulation is the process of putting an information signal on a carrier signal for some technical advantages.

Demodulation is the opposite of modulation. That is separation of the information signal from the received modulation signal. For data transmission demodulation is the process of recovering the data signal from the received asked sinusoidal signal.

Modulation is the process by which some characteristic of a carrier signal is varied in accordance with a modulating signal. Many ways exist to modulate a message signal m(t) to produce a modulated (transmitted) signal x(t). For amplitude, frequency, and phase modulation, modulated signals can be expressed in the same form as

s(t) = A (t) cos(2 ƒ_c t+ (t))

where A(t) is a real-valued amplitude function (a.k.a. the envelope), ¦_c is the carrier frequency, and θ(t) is the real-valued phase function.

Communication systems are often organized according to the following structure

Fig 1.1 Communication systems