Direct Synthesis Method for Dual-Band Bandpass Filters With Wide Fractional Bandwidth Range and Center Frequency Ratio

This brief proposes a direct synthesis method for dual-band filters based on frequency transformations. This method can synthesize dual-band bandpass filters (BPFs) with a large range of fractional bandwidth (8.5%-56%) and at the same time, wide center frequency ratio (2-4). A novel circuit conversion scheme is proposed to ease the difficulty of implementing complex circuit and reduce the sensitivity of S parameters to the values of the circuit elements significantly (The influence of 20% fluctuation of inductance on S-11 is within 5dB). In order to prove the validity of the proposed method, a 3(rd)-order narrowband dual-band BPF with center frequencies of 1 GHz and 2 GHz (small frequency ratio) and a 5(th)-order wideband dual-band BPF with center frequencies of 0.5 GHz and 2 GHz (large frequency ratio) are designed and fabricated, the simulated and measured results have good agreement.

Automated summary

This brief proposes a direct synthesis method for dual-band filters based on frequency transformations, achieving a wide range of bandwidth and center frequency ratios. A novel circuit conversion scheme eases the difficulty of implementing complex circuits and significantly reduces the sensitivity of S parameters to circuit element values. Design and fabrication of narrowband and wideband dual-band BPFs demonstrate the validity of the proposed method with good agreement between simulated and measured results.