Implementation of band-limited signal sampling and coherent detection in airborne pulsed Doppler radar

This papers studies a method of developing in-phase and quadrature samples of band-limited IF waveforms. The method employs only one analog to digital (A/D) convertor to sample and digitize the IF signal directly, eliminating the need for IF to be mixed to zero IF in form of two parallel baseband channels. An imaginary frequency rejecting filter and a secondary resampling circuit for frequency shifting are used to provide in-phase and quadrature components I and Q from the digital samples. Thus the problem of matching gain and phase response difference between the two channels is avoided. The digital frequency shifting circuits were simplified by properly selecting the IF carrier and the sampling interval. The experimental results show that the imaginary frequency level less then -50 dB is obtained by using 8 order filter.