Advanced microwave miniature and lossy tunable filters for wireless communication applications
Abstract
Microwave filters play very important roles in many RF/microwave applications,
which are employed to separate or combine different frequencies. Emerging applications,
such as wireless communications, challenge the design of microwave filters with more
functionalities and higher performance, such as reconfigurable or tunable, compact size,
light weight and lower cost.
In order to meet the increasing challenge requirements, the objective of this
dissertation is to develop new multilayer miniaturized filters, compact lossy microstrip
filters, and reconfigurable lossy filters. To achieve this, this dissertation is divided into
three main parts.
The first part focuses on the design of novel miniature bandpass filter with improved
performance. In this aspect, a novel microstrip bandpass filter using slow-wave open-loop
resonators is presented, which concentrates on the stopband rejection performance to
suppress the harmonic standing wave rather than the passband performance by using
multilayer LCP technology. The multilayer open-loop slow-wave resonator has not only
very compact size, but also exhibits an excellent wider upper stopband resulting from the
dispersion property. Based on this type of resonator, a five-pole bandpass filter has been
proposed, which has good stopband rejection and high selectivity as well as compact size
and light weight.
The second part is devoted to the design of compact lossy filters with improved
performance characteristics. To achieve this, lossy synthesis and extracted-pole
technology are combined together to design microstrip filters with flat passband and high
selectivity. Two six-pole filters has been analysed from the theoretical circuit model to
EM simulations, fabricated to demonstrate the response performance in narrowband and
wideband respectively.
The third part concentrates on the designs of novel varactor-tuned microstrip lossy
bandpass filters. Firstly, state-of-the-art literature review is given to have a general view
of reconfigurable bandpass filter with different tuning centre frequency and bandwidth characteristics. Then, three types of tunable microstrip bandpass filters with resistor
loading under symmetric tuning method are presented to introduce additional loss into
the passband to make it flat over the entire tuning range. The first filter is designed to
control the bandwidth and selectivity. The second one is designed to control the
bandwidth at fixed centre frequency, while the third filter is extended from the first one
to combine resistor loading and cross coupling. Finally, microstrip tunable bandpass lossy
filters with extracted-pole technology are proposed. Three six-pole filters of this type have
been analysed and fabricated. Due to the asymmetric tuning method, the number of tuning
components and dc bias schemes are increased, which is a kind of tradeoff with
performance.
For all the presented filters, theoretical analyses, implementations and
measurements have been given. All of the results achieved in this thesis make the
proposed filters attractive for their applications in modern wireless communication
systems.