The Strongly Coupled Resonator Quadruplet

A new structure, namely the strongly coupled resonator quadruplet (SCRQ), is presented in this article for the realization of compact pseudoelliptic evanescent mode filters. The SCRQ consists of a quadruplet of resonators, and it can be seen as the combination of a strongly coupled resonator triplet (SCRT) and a fourth resonator, the latter being exclusively coupled to the triplet by means of a regular coupling. Compared to the SCRT, which at the passband frequencies generates two poles and a transmission zero, the SCRQ generates an additional pole and an additional zero. The ability to locate the two zeros extremely close to both passband edges represents a unique feature of the SCRQ. The new structure is successfully validated by the experimental results of a manufactured 5th-order filter employing an SCRQ between two standard resonators.