The design of digital quadrature demodulators using the Parks-McClellan algorithm

New results are presented concerning the design of digital quadrature demodulators based on quadrature mixing and lowpass filtering concepts. The use of a 4:1 ratio between the sampling rate and intermediate frequency reduces the computational cost, but also causes the inphase and quadrature filters to become odd and even length filters, respectively. Consequently, the mismatch between the passband gains of the filters becomes an important source of error. The performance data presented in this paper shows that the Parks-McClellan (1973) algorithm can be used to design pairs of I and Q filters whose passband frequency responses are closely matched relative to the size of the passband ripple. However, this favourable result is dependent on the choice of the design specification and the number of filter coefficients