A Novel Microstrip Tunable Bandpass Filter with Constant Bandwidth

A new fourth-order cross-coupled band pass filter with a tunable center frequency and constant bandwidth is designed in this paper. Based on the electromagnetic coupling between the resonators, a CQ-type topology consisting of three magnetic couplings and one electrical coupling is adopted, and the coupling between the source and the load is introduced in the feed circuit to generate two transmission zeros on each side of the passband. By applying the same DC bias voltage to the varactor diode, the center frequency of the filter is tuned. Using cross-coupling theory and based on the target curves where the coupling coefficient and external quality factor vary with the resonant frequency, the design objectives of constant bandwidth, low insertion loss and good out-of-band suppression are achieved. The simulation and measured performance of the filter are basically consistent. The center frequency can be tuned from 1.05 GHz to 1.28 GHz, while the 3 dB bandwidth is maintained at (94 MHz±3.3 MHz) within this tuning range. The performance of the filter meets the design requirements and it has a certain application prospect.