A Compact Microstrip λ/4-SIR Interdigital Bandpass Filter with Extended Stopband

Abstract — This paper presents a new microstrip interdigital bandpass filter using λ/4 stepped impedance resonator (SIR) to achieve both size reduction and stopband extension. It is shown that as the low-impedance to high-impedance ratio is maintained smaller than unity, fundamental fO and the first spurious fS shift to the lower and higher frequency value respectively, thus extending the rejection band, as in the case of λ/2-SIR. Unequal length spurlines are embedded into the resonators where additional transmission zeros can be realized in the spurious passband such that the upper rejection band can be further extended. A 3rd order Butterworth bandpass filter centered at 700MHz with spurious response up to 4fo is simulated and measured to demonstrate the proposed design usefulness. Index Terms — Microstrip, microwave filter, spurline, spurious response, stepped impedance resonator