A Compact Bandpass Filter with Widely Tunable Frequency and Simple Bias Control

This research paper introduces an innovative design for a compact bandpass filter, notable for achieving a substantial tuning range of operational frequencies using just a single bias voltage. The design is based on a planar microstrip approach, distinguished by its straightforward and efficient structure. Additionally, this filter is characterized by its excellent performance in suppressing out-of-band frequencies. To validate the practicality and effectiveness of this novel design, a compact filter was designed, manufactured, and measured. The testing results were impressive, showing that the filter’s measured center frequency could be tuned across a wide spectrum, from 1.1 to 3.1 GHz, achieving a relative bandwidth of up to 95%. Herein, the relative bandwidth is derived from the calculation of 2(f_high − f_low)/(f_high + f_low). Remarkably, the filter consistently maintains a return loss exceeding 19 dB throughout its tuning range. Additionally, it sustains a 15 dB return loss bandwidth greater than 90 MHz, with the insertion loss varying between a minimum of 0.9 dB and a maximum of 2.1 dB. The compactness of the filter is one of its standout features, with circuit dimensions measuring a mere 0.06λ_g × 0.09λ_g (physical dimensions is 13.8 mm × 21.3 mm), where λ_g represents the wavelength at the filter’s lowest center frequency.