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.