Investigation On Analysis And Design Of Miniaturized Lossy Filters With Flat Passband

Microwave filter is one of the most important elements in various microwave systems for radar and communication applications.The filter design has to deal with particular electrical specifications and constraints that also concern its weight and cost.The conventional synthesis procedure is based on the assumption of low-loss or even lossless networks.This is approximately satisfied with high-Q resonators.However,the miniaturization and integration may cause significant degradation of unloaded Q-factors of microwave resonators,which bring some disadvantages,such as high insertion loss and unflatness in passband,and degraded frequency selectivity within transition bands.So high-performance miniaturized filters are always desired in filter development.In this paper,the coupling principle and synthesis method are first analyzed in detail.Then,the methods of extracting the external quality factors and coupling coefficients are introduced to pave the way for transforming coupling matrix to structure realization.Moreover,some methods to design lossy filter are compared.At last,two different types of lossy filters are designed based on the above theory.1)Four designs of lossy filters are compared,which are the synthesis of lossy coupling matrix,uniform Q method,resonator pairs with uniform Q's,and the sensitivity analysis to lossless filter synthesis.Under two typical conditions with or without transmission zeros together with the same design requirements,the in-band insertion loss and passband flatness,i.e.0.2-dB bandwidth,provided by the four methods are compared with each other.It can be concluded that the sensitivity analysis to lossless filter synthesis is the best method for Chebyshev lossy filters,considering the in-band insertion loss and flatness simultaneously.2)To achieve good flatness in passband,a lossy filter is designed with the method of lossy coupling matrix synthesis and the structure of substrate integrated waveguide(SIW)resonators.Based on the analysis of sensitivity,the in-line topology with resistive coupling is investigated.Then,the implementation of lossy filter with SIW resonators are carried out minutely.The resistive couplings,which can be determined by the sensitivity of filter response to the resistive coupling coefficients,are only added between some resonators.The realization of the filter will be simpler than the counterpart with the lossy coupling matrix synthesis method,also with smaller insertion loss.And the in-band flatness and the frequency selectivity are improved obviously.