Balanced Quad-Band Bandpass Filter With Controllable Frequencies and Bandwidths

Abstract

A balanced microstrip quad-band bandpass filter (BPF) employing balanced microstrip/slotline (MS) transition structures and two pairs of folded asymmetric short stub-loaded resonators (ASSLRs) is presented in this paper. The center frequencies of the four differential-mode (DM) passbands can be controlled by changing the electrical length ratio of each ASSLR. Meanwhile, the fractional bandwidths (FBWs) can be adjusted by controlling the coupling gaps between the resonators and interdigital coupled line. High selectivity of the four DM passbands is achieved by introducing eight transmission zeros (TZs). Moreover, the DM passbands are independent from the common-mode (CM) responses, which significantly simplifies the design procedure. In order to validate the design strategies, a balanced quad-band BPF centered at 2.54/ 3.46/ 4.5/ 5.2 GHz was fabricated and a good agreement between the simulated and measured results is observed.

Keywords

• Asymmetric short stub-loaded resonators (ASSLRs)
• balanced BPF
• microstrip/slotline (MS) transition structures
• quad-band