A broadband tunable combline filter using active devices

Abstract

Electronically tunable microwave filters with fast tuning speed can be realized using varactor tuned combline microstrip structures. Even when GaAs varactors are employed, the resonator Q is limited by the varactor losses and the filter bandwidth has to be more than 8-10%. The authors present a solution to the problem. The varactor losses are compensated by coupling a negative resistor in the tuned circuit. This resistor is realized using an active device. Computer simulation results for a filter with 3% bandwidth are presented. Performance results with and without the negative resistance are presented to bring out improvement due to negative resistance.<>