Rf and if digitization in Radio Receivers: Theory, Concepts, and Examples

In quadrature sampling the signal to be digitized is split into two signals. One of these signals is 

multiplied by a sinusoid to downconvert it to a zero center frequency and form the in-phase 

component of the original signal. The other signal is multiplied by a 90° phase-shifted sinusoid 

to downconvert it to a zero center frequency and form the quadrature-phase component of the 

original signal. Each of these components occupies only one-half of the bandwidth of the 

original signal and can be sampled at one-half the sampling rate required for the original signal. 

Therefore, quadrature sampling reduces the required sampling rate by a factor of two at the 

expense of using two ADCs instead of one.