Continuous Frequency and Bandwidth Tunable Combline Cavity Bandpass Filters with Internally Mounted Motors

Novel design methods for implementing continuous centre-frequency and bandwidth tunability are reported. Coupling bandwidths are tuned through the mechanical rotation of a metallic plate suspended between resonators. Centre-frequency tuning is achieved via the vertical actuation of a moving part mounted within each resonator. In addition, each resonator and transformer is designed so that motors can be housed within them — adding tunability without compromising the overall filter volume. A second-order filter, based on Combline cavity structures, capable of bandwidth tunability of 49MHz to 67MHz for all centre-frequencies in the range of 1:751GHz to 1:998GHz is presented. Passband

insertion loss is kept below 1:2 dB for all tuning states with return loss above 10 dB. A five-pole filter is also fabricated with centre-frequency tunability of 14% from 1:764GHz to 2:015GHz and 15 dB bandwidth tunability of 107% from 41MHz to 85MHz. Midband insertion loss is kept below 1:4 dB in all tuning states. The five-pole filter is compared to other tunable filters in the UHF range and is shown to reduce midband insertion loss by 36% from the next best filter (2:2 dB to 1:4 dB).