In previous post we started about basic digital modulation techniques,ASK,PSK and FSK.Let’s look at another digital modulation technique called as QAM [Quadrature Amplitude Modulation],which is actually a combination of ASK and PSK .
QAM is the combination of Amplitude modulation and Phase modulation. More technically, QAM is a system of modulation in which data or signal is transferred by modulating the amplitude of two separate carrier waves, mostly sinusoidal, which are out of phase by 90 degrees (sine and cosine). The modulated carriers are then summed, and the resulting signal is a combination of both phase shift keying (PSK) and amplitude shift keying (ASK).
Keep in mind that,the term Quadrature refers to their phase difference of 90 degrees. As Sine and Cosine are 90 degree out of phase.
[Any two periodic waveforms whose phase difference is 1/4 th of their output period are said to have a Quadrature phase relationship].As Sine n Cosine have time period of 360 degrees so, 1/4 th of 360 degree is 90 degree! :)
In next post we will talk about different orders of ASK,PSK n QAM modulation