In order to meet the increasing demands of advance wireless communications and
radar systems, several novel types of bandpass filters and bandstop filters have been
developed in this thesis.
A new type of varactor-tuned dual-mode bandpass filters have been presented to
achieve a nearly constant absolute bandwidth over a wide tuning range by using a single
DC bias circuit. Since the two operating modes (i.e., the odd and even modes) in a dualmode
microstrip open-loop resonator do not couple to each other, tuning the passband
frequency is accomplished by merely changing the two modal frequencies
proportionally. Design equations and procedures are derived, and two two-pole tunable
bandpass filters and a four-pole tunable bandpass filter of this type are demonstrated
experimentally.
Miniature microstrip doublet dual-mode filters that exhibit quasi-elliptic function
response without using any cross coupling have been developed. It shows that a single
two-pole filter or the doublet can produce two transmission zeros resulting from a
double behaviour of the dual-mode resonator of this type. Electromagnetic (EM)
simulation and experiment results of the proposed filters are described.
Parallel feed configuration of a microstrip quasi-elliptic function bandpass filter
has been built with a pair of open-loop dual-mode resonators. By employing this new
coupling scheme, a novel filter topology with three-pole quasi-elliptic function
frequency response can be obtained, leading to good passband performance, such as low
insertion loss and good matching at the mid-band of passband. A designed three-pole
bandpass filter of this type is demonstrated experimentally.
A new class of dual-band filters based on non-degenerate dual-mode microstrip
slow-wave open-loop resonators, which support two non-degenerate modes that do not
couple, have been introduced. Different feed schemes that affect the filtering
characteristics are investigated. Examples of dual-band filters of this type are described
with simulation and experiment results.
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In order to achieve a wide spurious-free upper passband, a novel design of
bandstop filter with cancellation of first spurious mode by using coupled three-section
step impedance resonators (SIRs) has been developed. This cancellation occurs when
two transmission poles coincide with the first spurious mode (transmission zero) by
properly choosing the step impedance ratio and the gap between the SIR and the main
transmission line. A stripline bandstop filter and a microstrip bandstop filter of this type
are designed, fabricated and tested. As a preliminary investigation, the microstrip filter
is tuned electronically using ferroelectric thin film varactors
