Design of a New Tri-Band Microstrip BPF Using Combined Quarter-Wavelength SIRs

A new tri-band microstrip bandpass filter (BPF) is designed to produce three passbands at the commercially practical frequencies through a single piece of filter circuitry. The basic component used to build the proposed filter consists of two quarter-wavelength ($lambda/$4) stepped-impedance resonators (SIRs). One of the$lambda/$4 SIRs is designed to operate at 1.57 GHz and 5.25 GHz, and the other at 2.45 GHz. The feed positions for the two$lambda/$4 SIRs are carefully located to have the same distance from the via-hole ground and meanwhile to give the required external quality factors (Qₑ's). In such a way, the microstripline sections between the via-hole and each of the feed positions can be combined into one to simplify the filter structure, and to reduce the BPF circuit size. Performance of the newly designed tri-band BPF is verified by measured results.