Novel cross-coupling topologies for planar microwave bandpass filters with asymmetric frequency response

Abstract

In this paper the problem of designing simple low-cost planar microwave bandpass filters with strongly asymmetric characteristics, filters that can efficiently attenuate an adjacent channel, is investigated. The usual quadruplet structure of a fourth-order filter offers two transmission zeros, located one on each side of the passband. For one of the two adjacent channels, a considerably higher attenuation can be obtained by designing filters with both transmission zeros in the same (upper or lower) stopband. The new topology investigated in this paper offers the possibility of designing such quadruplets with two transmission zeroes located both on the same side of the passband, in prescribed and controlled positions. Filters with this novel structure can provide very high attenuation not only at a single frequency, but in a larger adjacent frequency band.