Compact microstrip bandpass filters using miniaturized slow-wave quarter-wavelength resonators

This paper proposes a novel λ/4 microstrip slow-wave resonator with interdigital capacitive fingers. The interdigital fingers provide a strong capacitive loading so as to miniaturize the resonator size. The resonant frequency is easily adjusted by varying the number of fingers and the spacing between fingers. To demonstrate the feasibility of the proposed structure, one four-pole Chebyshev and one four-pole generalized Chebyshev filter with 0.05 dB equal-ripple bandwidths of 7.5% and 5%, respectively, were designed and fabricated. An extra transmission zero can be close to the passband for high selectivity and stopband attenuation by using the proposed slow-wave resonator and the tapped input/output structure. Both filters are compact in size and exhibit good stopband rejection.