Reconfigurable Low-Pass Filter With Sharp Roll-Off and Wide Tuning Range

A miniaturized reconfigurable low-pass filter (LPF) with a sharp roll-off, wide tuning range, and extended stopband is presented in this letter. The proposed LPF consists of two fixed low-pass (LP) cells and a tunable bandstop (BS) cell loaded with two pairs of varactor diodes. The BS cell with tunable stopband edges is employed to achieve the reconfigurable cutoff frequency and sharp roll-off characteristics in the whole LPF. The fixed LP cells are utilized to suppress the high passband of the BS cell, which obtain the tunable LP filtering response and wide stopband. For experimental demonstration purposes, a microstrip prototype of the proposed reconfigurable LPF with a wide fractional tuning range of 78.8% and roll-off rate over 103 dB/GHz is simulated, fabricated, and measured. The wide stopband is extended up to 10 GHz with a rejection level better than 15 dB, and the overall dimension is 0.078\lambda _{\mathrm {g}} \times 0.08\lambda _{\mathrm {g}} . This physical size is comparable to more compact, prior art reconfigurable LPFs while featuring among the sharpest roll-off rate ever reported.