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.