A tunable third-order bandpass filter (BPF) with constant fractional bandwidth (CFBW) is presented. Only two quarter-wavelength resonators have been used in comb-line fashion to design the third-order BPF. The physical dimensions of the resonators have been decided to satisfy the filter specifications by synthesizing the coupling matrix. Using modal voltage waveform analysis, locations of the varactors to be connected are identified to tune the first three modes of the resonators to form the BPF. The varactors connected symmetrically between the two quarter-wavelength resonators can only control the odd modes, while the varactors loaded at the resonator ends can control the odd and even modes simultaneously. Moreover, the capacitance ranges of the varactors have been chosen to achieve the desired tuning of the filter. The center frequency of the filter is tunable from 640 to 840 MHz with CFBW of approximately 32%.

中文翻译：

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使用两个四分之一波长谐振器的具有 CFBW 的三阶可调滤波器

提出了具有恒定分数带宽 (CFBW) 的可调三阶带通滤波器 (BPF)。只有两个四分之一波长谐振器以梳状线方式用于设计三阶 BPF。谐振器的物理尺寸已通过综合耦合矩阵确定为满足滤波器规格。使用模态电压波形分析，确定要连接的变容二极管的位置，以调整谐振器的前三种模式以形成 BPF。两个四分之一波长谐振器之间对称连接的变抗器只能控制奇模，而加载在谐振器两端的变抗器可以同时控制奇模和偶模。此外，已选择变容二极管的电容范围以实现滤波器的所需调谐。