High-Quality Compact Interdigital Microstrip Resonator and Its Application to Bandpass Filter

Abstract

A compact microstrip resonator based on the interdigital structure is proposed. The resonator has several times higher unloaded quality factor compared to similar resonators presented previously and can even reach the Q-factor of a regular λ/4 resonator. The size of the resonator can be significantly reduced with a substantial increase in quality factor by incrementing the number of fingers in the interdigital structure. In addition, for each gap between the fingers, there exist an optimal number of fingers that correspond to the maximum Q-factor. An extension of the upper stopband for a bandpass filter designed using the resonator can be achieved by the interconnection of the fingers in each of the comb structures. The simulation results are proven by fabricated resonators and a fourpole bandpass filter. For the central frequency of 2000 MHz and 16.2% fractional bandwidth, the lateral size of the filter is only 11.5mm × 3.8mm for alumina substrate (eps = 9.8). The filter has an upper stopband up to 5.8f0 at the level −40 dB.