Design of a Single-Dual Band Microstrip Reconfigurable Bandpass Filter Based on Pin Diodes

A single-dual band microstrip reconfigurable bandpass filter (BPF) using PIN diodes is proposed in this paper. The reconfigurable filter consists of two pairs of inductively coupled quarter-wavelength uniform impedance resonators (UIR), with PIN diodes loaded at the open end of the resonators. As an RF switchable device with variable bias states, the PIN diodes can independently control each passband in both single band and dual band states of the filter. Due to the introduction of source-load cross electrical coupling and inductive magnetic coupling between resonators, a transmission zero (TZ) is generated on both sides of each passband, greatly enhancing the filter's selectivity. To validate the effectiveness of the design, a single-dual band reconfigurable bandpass filter is designed, which can switch between a single band response with a center frequency of 1.8 GHz and a dual band response with center frequencies of 1.57 GHz and 2.4 GHz. In the dual band state, the insertion losses (IL) of the two passbands are 0.83 dB and 1.07 dB, while in the single band state, the IL of the single passband is $\mathbf{0. 6 2 ~ d B}$. Two states exhibit a wide fractional bandwidth (FBW) and high selectivity. The overall size of the reconfigurable filter is $0.225 \lambda g \times 0.117 \lambda g$, where $\lambda g$ is the guided wavelength at the center frequency of the first passband in the dual band state.