Frequency Multiplexer With Improved Selectivity Using Asymmetric Response Directional Filters

In this letter, a novel approach to the design of frequency multiplexers consisting of cascaded directional filters (DFs) is proposed. It is shown that when channels are spaced closely enough, their selectivity can be improved without increasing the filters' order by taking advantage of two phenomena. Asymmetric frequency response of a constitutive DF allows to increase the attenuation slope on the one side of the multiplexer channel. In addition, the slope on the other side can be increased due to the creation of additional transmission zero resulting from the fact that bandstop response of the previous DFs within the cascade directly affects the response of the following channel. Theoretical analysis is provided together with the applicability condition and multiplexer design procedure. Moreover, an exemplary four-channel S-band multiplexer was manufactured and measured showing the selectivity improvement of as much as ~1.3 times.