Compact electronically switchable parallel-coupled microstrip bandpass filter with wide stopband

In this letter, a compact electronically switchable parallel-coupled microstrip bandpass filter (BPF) based on quarter-wavelength resonators and p-i-n diode switch is proposed. The proposed filter occupies only half the circuit size of the conventional parallel-coupled filter. In addition, the type and length of each coupled-line section is suitably chosen such that the inherent transmission zeros associated with the coupled-lines can be used to reject the spurious passbands effectively. By adding only one p-i-n diode along with a simple bias control circuitry, the proposed filter becomes electronically switchable with no current consumption when the filter is turned off. Specifically, a switchable fourth-order microstrip BPF with a center frequency f(0) of 2 GHz, passband insertion loss less than 3 dB, and wide stopband up to 7.7 f(0) in the ON state is demonstrated. The isolation of the filter is better than 30 dB from dc up to 7.7 f(0) in the OFF state.