A DIGITAL QUADRATURE DEMODULATION SYSTEM

Existing digital quadrature demodulation techniques sample the input a t either 2B Hz or 4B Hz, select the even samples as the in-phase (I), and interpolate the odd samples to give the quadrature (a), output. Th e signal bandwidth is B. We propose a demodulation system to produce I and Q samples at arbitrary sampling rate greater than 2B Hz. The syst em eliminates the IF downconversion step with a special sampling schem e. The even samples correspond to the I component, white the a compone nts are the filtered output. The filter can be a lowpass or least squa res (LS) filter. The lowpass filter design is based on trade-offs betw een the filter length and the degree of oversampling. It produces simi lar results as previous work when the sampling rate is 2B Hz or 4B Hz. Unlike existing methods which assume the input is white, a LS filter, on the other hand, can make use of input signal characteristics to ac hieve a better result. The higher the correlation in the input, the la rger the improvement. The cost for LS filtering is a coefficient updat e step if the input is time varying. A scheme to cancel de offset from analog to digital (A/D) converters is also given.