High Q narrow-band tunable filters with controllable bandwidth

In the present work, the authors present a design technique to create widely-tunable high Q narrow-band filters with bandwidth control by utilizing low Q varactors, while maintaining the high Q of the original filter. Detailed design methodology and equations are presented to illustrate the concept and a widely tunable high Q evanescent-mode cavity bandpass filter with controllable bandwidth is demonstrated as an example of this design technique. The filter is designed and fabricated in low-loss Rogers TMM3 substrate. A thin copper membrane is laminated on top of the substrate to allow frequency tuning, achieved using commercially available 0.38 mm thick piezoelectric actuators. The feed-lines with the impedance matching and the varactor based bandwidth control section are incorporated in parallel with the coupling iris on the back-side of the filter. A constant absolute bandwidth filter is also presented, which is tuned from 0.89 GHz to 1.47 GHz with constant 25 MHz (plusmn 0.2 MHz) 3-dB bandwidth and less than 3 dB insertion loss over the entire tuning range. The extracted filter Q is in the range of 250-350, even though low Q varactors (Q < 30@1 GHz) with capacitance in the 0.5 pF to 4 pF range are utilized for the bandwidth control.