A Compact Reconfigurable Low-Pass Filter With Wide-Stopband Rejection Bandwidth

In this letter, a new varactor-tuned low-pass filter (LPF) with sharp roll-off factor at different tunable frequencies and wide stopband is presented. By loading the low-impedance stubs of stepped-impedance resonators with the varactor diodes in shunt and a tunable coupling network in series, the cutoff frequency is tuned. To increase the stopband, two semicircular slots are etched in the ground plane, and the patches are connected to feed lines at both ends. A prototype of the proposed tunable LPF is designed, fabricated, and tested. Simulation and measured results demonstrate a good agreement. Measured results show the tuning range 75% (1-2.2 GHz), with a roll-off factor of 50 and 70.6 dB/GHz at lower and upper cutoff frequencies, respectively. The stopband is extended up to 20 GHz for lower cutoff, maintaining a rejection level >10 dB.