The Design and Implementation of Continuous Frequency and Bandwidth Tunable Combline Cavity Filters

Design methods for the implementation of continuous centre frequency and bandwidth tunability in bandpass filtering structures are presented in this thesis. Initially, the use of discrete electronic components as providers of tunability are explored for use in planar filters. PIN and varactor diodes are used to control capacitance at key nodes in the circuit through use of voltage or current biasing. The limitations of these devices are explored in terms of linearity. Intermodulation distortion is measured using two-tone characterisation tests. Configurations of three-dimensional combline filters are analysed using HFSS. Filter structures are treated as a combination of resonant frequencies, external couplings and inter-resonator couplings which control centre frequency, return loss ripple level and bandwidth and respectively. A variety of bespoke components are introduced and compared with each other through simulation for their effectiveness of tunability with actuation. Novel, and independent techniques for tuning both resonant frequency and coupling bandwidths are proposed and implemented in physical TEM combline filters simultaneously. The method introduced for resonance tuning allows for the mounting of motorised actuators to be internal to the filter cavity - thereby adding the complexity of tunability to the structure without needing to increase the volume. Two filters of differing complexity incorporating the developed tuning methods (for centre frequency, return loss and bandwidth) are designed and fabricated to the same tuning specifications as each other - centre frequency from 1.8 GHz to 2.0 GHz and bandwidth from 40 MHz to 60 MHz. The first filter is a second-order combline cavity filter capable of bandwidth tunability from 49 MHz to 67 MHz for all centre-frequencies in the range of 1.751 GHz to 1.998 GHz. Passband insertion loss is kept below 1.2 dB for all tuning states with return loss above 10 dB. The second filter is a five-pole design. This filter achieves frequency tunability from 1.764 GHz to 2.015 GHz and 15 dB bandwidth tunability from 41MHz to 85MHz. Midband insertion loss is kept below 1.4 dB in all tuned states.