A Miniature Selective Bandpass Filter With Frequency-Variant Couplings Realized Using Uniform-Impedance Quarter-Wave Resonators

In this paper, electromagnetic coupling characteristics of uniform-impedance folded quarter-wave resonators are explored for their application in high-performance inline bandpass filters. The folded quarter-wave resonators enable realization of two-types of frequency-variant couplings. Transmission zeros generated due to these couplings enhance the filter’s selectivity, improve out-of-band rejection level and also suppress a few of the harmonic passbands. These multiple merits, achieved by a simple fold, are demonstrated through the design of a fourth-order filter centered at 0.313 GHz with 11.86% fractional bandwidth. The fabricated prototype shows selectivity factor, defined as the ratio of 3-dB to 20-dB bandwidths, of 0.62 and roll-off rates of 1256 dB/GHz and 1156 dB/GHz at the upper and lower sides of the passband, respectively. Auxiliary open stubs are added to aid filter’s inherent harmonic suppression capabilities such that a stopband up to 10.2 times the center frequency is achieved with minimum rejection level of 20 dB. Moreover, the folds allow a miniature filter footprint of 0.048 by 0.129 times the wavelength at the center frequency.