Design of the Compact Tunable Filter With Modified Coupled Lines

A compact tunable bandpass filter (BPF) with a broad stopband is presented. The proposed filter is composed of two modified coupled lines and one varactor-loaded stub. In terms of measurement, the tunable BPF with the transmission zero within the higher passband skirt has a very wide tuning range, from 0.95 to 1.55 GHz (48%). Moreover, with the return loss > 10 dB, there is a controllable constant passband bandwidth 120 MHz. Furthermore, within the tuning range, the insertion loss is from 2.4 to 2.8 dB and the return loss is > 20 dB.