A compact microstrip λ/4-SIR interdigital bandpass filter with extended stopband

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 f₀ and the first spurious f_s shift to the lower and higher frequency value respectively, thus extending the rejection band, as in the case of λ/2-SIR. Unequal lengths 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 3^rd order Butterworth bandpass filter centered at 700MHz with spurious response up to 4f₀ is simulated and measured to demonstrate the proposed design usefulness.