New low-loss tunable microstrip band-pass filter with two transmission zeros

Abstract

In this paper, a low-loss tunable microstrip band-pass filter with two transmission zeros (TZs) around the pass-band edge is proposed. Making use of exactly same structure, three different fractional-bandwidth variations (i.e. constant fractional bandwidth, decreasing fractional bandwidth and increasing fractional bandwidth) can be realize by adjusting the amount of coupling between microstrip pairs. A prototype filter with constant fractional bandwidth is fabricated on a er = 3.38, 32 mil Rogers substrate. The center frequency of the fabricated filter is tuned from 770 to 1300 MHz by two varactor diodes. The constant relative bandwidth and insertion loss of the filter are 11 ± 0.2% and less than − 2.2 dB respectively. S-parameters and performance of the proposed filter are measured by Agilent VNA which show a good agreement with simulation results.