Compact single-pole-double-throw switchable bandpass filter based on multicoupled line

A compact single-pole-double-throw (SPDT) switchable bandpass filter (BPF) composed of capacitively loaded multicoupled line is proposed. By replacing the quarter-wavelength impedance transformer at the common junction in conventional designs with a J-inverter, the circuit size is largely reduced and the design procedure is much simplified. As a demonstration, a proposed SPDT switchable BPF is designed and implemented with a center frequency f₀ of 1.0 GHz and a bandwidth of about 10%. Low insertion loss in the on state, good isolation in the off state, and good linearity are obtained. The filter size without bias circuits and input/output traces is only 2.5 cm × 2.7 cm, which is about 0.158λ_g × 0.170 λ_g at f₀.