Expansion method of bandstop for compline bandpass filters

Abstract

The article proposes a new method for expanding the stopband of combline bandpass filters (BPF) containing quarter-wave resonators. The filter contains resonators from segments of transmission lines of a quarter-wave length, one end of which is short-circuited. The quarter-wave (λ/4) resonators are oriented in one direction and are parallel to each other without mutual displacement. The disadvantage of such BPF is the many spurious passbands and not wide enough stopband located between the main passband and the first spurious passband. This method is based on the features of the input functions of λ/4 resonators, which appear at different coordinates of connection to these resonators. To obtain a wide stopband, spurious passband associated with higher (spurious) resonant frequencies 3f0, 5f0, 7f0, etc. are eliminated. For this, the resonators are connected to each other at special points located at a certain distance from the short-ends of λ/4 resonators. Using the proposed method, it is possible to suppress a significant number of spurious passbands in combline BPFs with λ/4 resonators, and make the stopband wide enough. In the article, a combline filter with inductive couplings between resonators and between end resonators and load is constructed. However, the proposed method allows us to use the other couplings between resonators. The results of frequency characteristics simulation are presented. The ratio R of the resonant frequency of the first non-suppressed (spurious) passband to the resonant frequency of the main passband increases with an increase in the BPF's order n. For n = 3, 4, 5 the values of R = 11, 13, 15 were obtained, respectively.