ELECTRONICALLY TUNABLE COMBLINE FILTER USING GALLIUM ARSENIDE VARACTOR TUNING DIODES

Abstract

Electronically tunable microwave narrowband bandpass filters have wide applications in many military and commercial microwave systems. The filters are attractive because they exhibit fast tunability, have a reasonably wide tuning band, acceptable passband performance and are low in cost. These same requirements are admirably met by tunable combline bandpass filters where Gallium Arsenide (GaAs) varactors are utilized as the electronic tuning elements. This thesis presents the systematic design of a fast electronically tunable combline filter using GaAs varactor tuning diodes. These filters suffer from the effect of frequency dependent coupling between the filter resonators. A solution for this problem is presented in this thesis. The design method developed in this thesis also takes into account the effects of varactor parasitic elements. This design method consists of properly characterizing the tuning diodes, calculating the varactor parasitics from the measured results and designing new terminating matching circuits to yield improved and predictable filter performance over the tuning band.