Tunable Bandpass Filter with Constant Absolute Bandwidth Using Double-Sided Parallel-Strip Lines

A novel tunable bandpass filter with constant absolute bandwidth (ABW) using double-sided parallel-strip lines (DSPSL) was proposed. The constant ABW can be realized by selecting a specific coupling region to control the coupling coefficient between resonators. Both a single-ended and a differential signaling bandpass filters were studied. For the single-ended case, its 3 dB ABW (|S₁₁| <-3 dB) is about 80±4 MHz as the center frequency tuned from 0.97 GHz to 1.43 GHz. For its differential counterpart, the 3 dB ABW (|S_dd11|<-3 dB) is about 91±3 MHz, and the suppression of common-mode noise is less than -20 dB.