Study Of Cross Coupled Filter

High performance narrowband microwave filter is essential unit in advanced microwave communication systems. In some special occasion such as microwave space and terrestrial communication system, in which the sharp cutoff at the edge of the passband and high selectivity, low insertion loss and linear in-band group delay are needed. Generally, advanced microwave filter possesses two types of configuration. One is direct coupled filter, which has only one signal path. The other is cross coupled filter, which has at least two signal paths. Generalized Chebyshev filtering function can be used to synthesize both kinds of filter.This paper is mainly devoted to general coupling matrix synthesis methods for generalized Chebyshev filtering functions. Microstrip and waveguide structure is used to realize direct coupled filter and cross coupled filter. The cross coupled filter is synthesized at the beginning of prescribed filter specifications (center frequency, band width, in-band return loss and normalized port impedance). At first, recursive expression of the transfer and reflection function of generalized Chebyshev filter is obtained based on expression of theoretical response. Coupling matrix in terms of every resonator and loaded quality factor of source and load is extracted. Finally, by using theory of coupled resonator, the realizable coupling coefficient of coupling matrix and loaded quality factor is realized in practical microwave circuit. The feasibility and practicability of these kinds of design method are verified by final simulated results.