Compact fourth/third order dual‐band bandpass filter with independently controllable passbands

Abstract

AbstractThis article reports a compact microstrip dual‐band bandpass filter (DBBPF) using two pairs of λ/4 short‐circuited stepped impedance resonators (SSIRs) and a stub‐loaded triple‐mode resonator (STR). The first and fourth λ/4 SSIRs not only couple with the second and third SSIRs to form the lower passband, but also serve as the feeding structure and provide source‐load coupling for the higher passband, formed by the STR. As a result, a compact filter size can be achieved, and two extra transmission zeros are created for the higher passband to enhance the passband selectivity. Meanwhile, both the center frequency and bandwidth of each passband can be controlled independently. Furthermore, the impedance and length ratios of the two pairs of λ/4 SSIRs are dissimilar to achieve wide stopband. A fourth/third order DBBPF prototype centered at 1.575 GHz (GPS) and 3.5 GHz (WiMAX) is ultimately implemented to demonstrate the design concept, and the measurements match the simulations with good consistency.