A UHF 3rd order 5-bit digital tunable bandpass filter based on mixed coupled open ring resonators

This paper presents a third-order digital tunable bandpass filter based on digitally tunable capacitor loaded open ring resonator. Magnetic dominated mixed coupling is utilized to make the coupling coefficient meet the requirement of stable bandwidth response. Electric source-load coupling is designed to generate a transmission zero for improving the frequency selectivity. This filter is designed, fabricated and measured. The measurement shows that the filter can be digitally tuned by 5-bits, the fractional bandwidth is 9 ± 1%, the tuning range is from 410MHz to 820 MHz.