Tunable Constant-Bandwidth Substrate-Integrated Bandstop Filters

A new method for designing tunable bandstop filters with constant fractional and absolute bandwidths (ABWs) is presented. The constant bandwidth is achieved through a new coupling structure, which is passive and consists only of coupling elements and transmission lines. Its theory of operation is presented, and design principles and tradeoffs are examined in detail. Several prototype filters are fabricated and measured to demonstrate the performance and versatility of this method. An octave-tunable filter with less than 12% fractional bandwidth variation is demonstrated, along with an octave-tunable filter with 12.3% variation in ABW variation, a filter with a 50% tuning range and less than 4% variation in ABW, and a four-pole octave-tunable filter with less than 4% variation in its 10-dB ABW. These results represent state-of-the-art performance when compared to other constant-bandwidth bandstop filters at frequencies higher than those compatible with lumped-element filters.