Tunable microstrip bandpass filter based on cascade quadruplet topology

This paper presents an approach for designing 4-pole tunable microstrip bandpass filter with constant fractional bandwidth (CFBW). To achieve sharp roll-off responses, a cross-coupling scheme is adopted in the filter design. Cascade quadruplet (CQ) topology is used in the design of 4-pole tunable filters. Four transmission zeros are generated in proximity of the passband, resulting a sharp roll-off response. For the propose of demonstration, the 4-pole tunable bandpass filter with CFBW is designed, fabricated, and measured. The filter can be tuned over a wide fractional range up to 50%. The fabricated 4-pole filter exhibits a tuning range of 860 MHz-1290 MHz, with a FBW of 15.9 ± 2.8% and an insertion loss of 2.6-3.6 dB. Measured results of the filter present sharp roll-offs and agree well with the simulated counterparts.