Application of the Parks-McClellan Algorithm to the Design of Quadrature Demodulation Filters

摘要

New results are presented concerning the use of the Parks-McClellan algorithm to design filters for 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 to reduce computational cost complicates this problem. Since the in-phase (I) and quadrature (Q) filters become odd and even length filters, respectively, the matching of the passband gains becomes an important error source. Consequently, the problem is to find the best design for a pair of filters rather than the best design for a single filter. One issue is whether to design the I and Q filters separately, or derive them from a prototype filter. Another concerns techniques for designing fractional-band filters if these are desired. The performance data presented in this paper shows that the quadrature demodulator accuracy has a complex dependence on the approach and specifications used to design the filters. Since good matching of the filter gains in the passband occurred only under certain conditions, significant performance losses can occur unless some care is taken in designing the filters.