Tunable Narrow-band X-band Bandpass Filters

This dissertation proposes two narrow-band tunable X-band filters from different classes, one planar and one waveguide. The first is a suspended substrate varactor-tuned doubly end-loaded staircase filter that incorporates a geometrically decoupled biasing structure. The second design consists of a varactor-tuned evanescent mode waveguide filter that incorporates tuning screws and a symmetrical varactor mounting structure inside the guide. The filters are designed for miniaturization with competitive filter performance in terms of centre frequency tunability, insertion loss and bandwidth performance. A planar staircase filter with electrically decoupled biasing and centre frequency tunability is realised using a suspended substrate and surface mount varactor diodes. Each half-wave resonator is terminated at both ends with identical varactor diodes, controlled by a single biasing line running through the roof of the enclosure. For symmetrically loaded half-wave resonators such as used in the staircase filter, each resonant line is DC decoupled from the next and a zero-voltage point exists in the centre of each line. This point is stationary under changes in centre frequency, which is a significant advantage for tunable filters. The biasing structure is also non-resonant, which allows for good out-ofband performance. The proposed planar structure is illustrated by a fifth order staircase X-band filter. Using varactors at each open end of the resonators together with spatially decoupled bias feeds, a measured centre frequency tunability of 400 MHz is achieved around a nominal frequency of 11.9 GHz. The tunable evanescent mode waveguide filter is realised with in-waveguide mounted varactors and tuning screws. The mechanical tuning screws enable rough-tuning, while the varactors provide fine-tuning capabilities. The varactors are connected back-to-back on central metal plates which are grounded to the enclosure. The other end of each varactor is connected to a biasing wire which exits the enclosure through a small hole in the side of the waveguide. A centre frequency tunability of 1.3 GHz around a nominal frequency of 10.45 GHz is achieved by the manufactured filter prototype. Both filters are evaluated through theory and simulation in terms of the thermal resistance of the mounting structure and input power limitations due to the non-linearity and power dissipation of the varactor diodes. Across the full tuning range of the planar filter, the maximum input power limit is 8 dBm. For the waveguide filter, the simulation yielded a maximum input power limit for full range tunability of 2 dBm due to the nonlinearity of the hyperabrupt varactor diode, the applied voltage, and the power dissipation in the varactors. The proposed biasing structure is a novel method for biasing planar filters, and to the knowledge of the author, this is the first application of the specified in-waveguide mounted diode structure in tunable evanescent mode waveguide filters.