Parallel-Coupled Stub-Loaded Resonator Filters with Wide Spurious Suppression

A compact bandpass filter with a large rejection band is designed using short-circuited Stub-loaded Resonators (SLRs). The fundamental resonance frequency and higher order resonant harmonics of the considered SLR are established. It is found that the first spurious resonance frequency can be conveniently controlled. For a proof-of-concept, a three-pole filter designed at 1 GHz, with a 6 % fractional bandwidth is fabricated and measured. A measured 2.3-dB insertion loss is achieved, with a first spurious passband located at four times the working frequency, i.e. 4 Ghz. Then, a U "corner" structure is considered to create a transmission zero at the upper stopband close to the high cut-off frequency leading to a deeper stopband rejection. Measurement and simulation results are in good agreement, thus validating the new concepts developed in this paper.