A constant fractional-bandwidth control scheme for tunable closed-ring bandpass filter

This paper presents a varactor reactance control scheme to sustain constant fractional bandwidth over the entire tuning range of a tunable closed-ring bandpass filter. The center frequency of the closed-ring bandpass filter is determined by the average of the odd- and even-mode resonance frequencies while the bandwidth depends on the difference of these modes. Therefore, the fractional bandwidth varies when the center frequency is tuned. To eliminate the fractional bandwidth variation, a proper control scheme of varactor reactances is proposed. A microstrip bandpass filter with fixed 15% fractional bandwidth is designed for demonstration. The measured fractional bandwidth is only ±0.2% across the center frequency tuning range of 0.8 GHz to 1.2 GHz. The insertion loss is less than 3.1 dB and the return losses are greater than 20 dB.