Abstract
A model of tunable microwave narrowband lumped-element microstrip filters is developed, using varactor-loaded inductors to achieve frequency-dependent inductance. A bandwidth transformation technique is utilized, so that the bandwidth of the filters is determined by the inductance slope of frequency-dependent inductors dL/df. Large coupling capacitance with small element separations can still be applied to these tunable narrowband filters, hence maintaining their compactness. The centre frequency of the filters is tuned by varying the bias voltage of the varactors. The numerical results of a tunable 2-pole narrowband filter are presented. It has a bandwidth of 4 MHz with a tuning range of 90 MHz, from 0.92 GHz to 1.01 GHz, for a bias voltage from 0 V to 15 V, respectively. The varactor losses are also discussed and compared with experimental data.
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