Article ID: 22.20250497
This brief proposes two compact tunable bandpass filters (TBPFs) based on the switchable varactor-based resonators. Due to the difference in resonance equations between the two modes of the switchable resonators, the proposed filter can operate at lower frequency bands without significantly increasing its size. Varactor diodes and switching diodes are utilized at the resonator and the positions of feed lines, which remain the coupling coefficients and external quality factors almost constant throughout the entire tuning range. For demonstration, a two-pole Filter A with a frequency tuning range from 0.77 to 1.11 GHz and 1.83 to 2.24 GHz is designed and measured. 1-dB fractional bandwidth (FBW) remains at 4.5 ± 0.2%. The core circuit size is only 0.08λg × 0.17λg. Filter B, based on the optimization of Filter A, achieves continuous tuning within the 1.1 to 2.25 GHz range, with a fractional bandwidth of 4.7 ± 0.3%. The core circuit size is 0.09λg × 0.19λg.
