频率可调带通-带阻可切换滤波器的设计

Design of Frequency Tunable Filter with Switchable Bandpass-Bandstop

该文设计了一种频率可调带通与带阻可切换微带滤波器,在距λ/2(λ为波长)谐振器开路端约1/4处加载变容二极管实现中心频率可调,利用PIN二极管实现带通与带阻两种状态的可切换。通过对耦合系数与外部品质因数(Q)值的分析,选取合适的参数可实现滤波器绝对带宽在调谐范围内保持恒定。带通状态时,由于源与负载间的耦合及谐振器间的混合电磁耦合,在滤波器的通带两侧各产生1个传输零点,提高了滤波器的选择性与阻带抑制。选用介电常数2.2的F4BM介质基板制作实物并用矢量网络分析仪进行测量。测量结果表明,当PIN管加正偏压,变容二极管加反偏压时,实现了中心频率可调的带阻滤波器;当PIN管不加偏压,变容二极管加反偏压时,实现了中心频率可调的带通滤波器。滤波器的中心频率调谐范围为3.45~3.90 GHz,调谐范围内绝对带宽保持恒定。该滤波器尺寸为44.4 mm×16.1 mm(0.59λg×0.21λg)(λg为调谐范围中心频率对应波长),符合小型化要求。

A frequency-tunable microstrip filter with switchable bandpass and bandstop is designed,the varactor diode is loaded at about 1/4 of the open end of theλ/2 resonator to realize the center frequency adjustment,and a PIN diode is used to realize the switch of the band-pass and band-stop states.Through the analysis of coupling coefficient and external Q value,the absolute bandwidth of the filter can be kept constant within the tuning range by selecting appropriate parameters.In the bandpass state,due to the coupling between the source and the load and the mixed electromagnetic coupling between the resonators,two transmission zeros are generated on both sides of the passband of the filter,which improves the selectivity and stopband suppression of the filter.The F4BM dielectric substrate with the dielectric constant of 2.2 is selected to make a filter prototype and measured with the vector network analyzer.The measured results show that the band stop filter with adjustable center frequency is realized when the PIN tube is added with positive bias and the varactor diodes are added with reverse bias.The band pass filter with adjustable center frequency is realized when the PIN tube is not added with bias and the varactor diodes are added with reverse bias.The central frequency tuning range of the filter is 3.45~3.90 GHz,and the absolute bandwidth within the tuning range remains constant.The filter size is 44.4 mm×16.1 mm(0.59λg×0.21λg),which meets the requirements of miniaturization.