Tunable bandpass filters with constant absolute bandwidth and high linearity

This paper presents a frequency-agile bandpass filter with constant absolute bandwidth and high linearity. The proposed resonator is composed of an open-ended transmission line with back-to-back varactor diodes loaded at one end. A novel mixed electric and magnetic coupling scheme is proposed to contorll the coupling coefficients in order to keep the bandwidth constant when the frequency is tuned. Moreover, the back-to-back varactor diodes are used to enhance the linearity of the filter, which is better than that of the single varactor counterparts. For validation, a demonstration filter is implemented. The design methodology and the experimental results are presented.