Electronically switchable microstrip bandpass filter with good selectivity

In this work, an electronically switchable cross-coupled microstrip bandpass filter using p-i-n diodes is proposed. The basic filter structure is based on the cascade of λ/8 parallel and anti-parallel coupled-line sections. Therefore, it occupies only about half the circuit size of conventional parallel-coupled filters. The magnetic cross-coupling is also introduced to achieve two transmission zeros in the lower and upper stopbands. In addition, by employing the four-line anti-parallel coupled microstrip structure, two more transmission zeros in the upper stopband are obtained. Two p-i-n diodes along with a common bias control circuitry are then introduced to allow the bandpass filter to be switched on and off. For design purpose, suitable equivalent-circuit for the filter is also introduced. Specifically, a switchable 4th-order microstrip bandpass filter with a center frequency of 1.5 GHz, passband insertion loss less than 2 dB, 3-dB bandwidth of 19.5%, and four transmission zeros is demonstrated. The ON-OFF isolation is better than 20 dB from dc up to 4.3 GHz.