A Tunable Microstrip Bandpass Filter With Two Independently Adjustable Transmission Zeros

In this letter, a tunable microstrip bandpass filter (BPF) with two independently adjustable transmission zeros (TZs) is reported. By introducing two microwave varactors to tune the TZs, the center frequency and the bandwidth of the BPF can be separately adjusted within a broad tuning range by varying the dc bias voltages applied to the varactors. Theoretical analysis and design approach are described. The experimental result of a demonstrative BPF shows that a less than 4 dB insertion loss, a relative bandwidth of 9% and a center frequency tunable from 1.4 to 2 GHz are obtained. The measured performance of the fabricated filter shows good agreement with the proposed theory.