High-Selectivity Tunable Planar Combline Filter With Source/Load-Multiresonator Coupling

A tunable planar combline filter exhibiting multiple transmission zeros is reported in this letter. By introducing two new coupled lines, the source/load-multiresonator coupling phenomenon is obtained without using additional combline resonators. The transmission zero locations can be adjusted by means of the length of these coupled lines to improve the upper stopband rejection. Moreover, all of these transmission zeros can be tuned at the same time as the passband without distorting the filtering-response shape. Parasitic coupling effects of the proposed filter topology and guidelines for their compensation are also discussed. The described combline filter is experimentally validated with the design and manufacture of a three-stage electronically-tunable prototype with five transmission zeros.