Microwave Lowpass Filters with a Constricted Equi-Ripple Passband

Lowpass filtering requirements in modern systems with RF bandwidth-limited signals, such as cellular, PCS and UMTS, usually demand lowpass-type selectivity. The standard DC-to-cutoff equi-ripple passband, however, is seldom required. Here, standard lowpass prototypes may be considered non-optimum or even wasteful in terms of an excessive passband width. Lowpass functions with a finite-interval equiripple passband produce higher selectivity and allow control of the impedance level within the lowpass network [1]. Levy [1, 2, 9] has shown that special Zolotarev functions are suitable as quasi-lowpass approximation functions. Horton [3] has described Zolotarev quasi-elliptic lowpass filters with finite transmission zeros. However, Zolotarev functions are relatively complicated for run-of-the-mill lowpass filter design work, and easy-to-use filter tables have only been published for limited cases. This paper presents a quasi-lowpass prototype network similar to a standard Chebyshev prototype, but with a constricted equi-ripple passband of variable width. A fast-converging iterative approximation method is introduced. Tables of normalized prototype elements (gparameters) for various network degrees, passband ripples and passband widths are given for direct use in lowpass filter design at RFand microwave frequencies. A detailed design example is given at the end.