A highly selective tunable filter with switchable bandpass and bandstop frequency response
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摘要: 设计了一种高选择性可调带通与带阻可切换微带滤波器。在微带谐振器及枝节末端加载变容二极管实现中心频率的可调；在输入与输出端口馈线之间，利用PIN二极管实现滤波器阻带和通带特性的切换。采用奇偶模的方法对滤波器结构进行分析，通过对奇偶模频率的调节实现了频率调节过程中的恒定绝对带宽。同时，在输入与输出馈线间引入源与负载耦合，使得频带两侧各有一个传输零点，且传输零点在整个频率调谐范围内相对位置几乎不变。因此，在整个频率调节范围内，滤波器实现了高选择性及良好的带外抑制能力。最终设计出的可调带阻滤波器的频率调谐范围为5.58～5.89 GHz，绝对带宽80 MHz±5 MHz，阻带衰减优于14 dB；可调带通滤波器的频率调谐范围为5.42～5.79 GHz，绝对带宽120 MHz±5 MHz，插入损耗1.69～2.25 dB，回波损耗优于13 dB。同时，该滤波器具有0.28λg×0.62λg（λg是可调频率范围中心频率的波长）的紧凑结构尺寸。实验和仿真结果一致性较好。Abstract: A highly selective tunable filter with switchable bandpass (BP)-to- bandstop (BS) frequency response is designed. The frequency of the filter can be adjusted by loading a varactor diode on the end of microstrip resonator and branch; a PIN diode can be used to switch BP and BS characteristics. The filter is analyzed using the even- and odd-mode method, and constant absolute bandwidth (CABW) is achieved. Then, a source and load coupling is introduced so that there is a transmission zero (TZ) on each side of band, which can be almost keep in the same relative position throughout the frequency tuning range (FTR). Therefore, in the FTR, the filter achieves high selectivity and good out-of-band suppression characteristics. The FTR of the proposed filter is 5.58～5.89 GHz, the measured attenuation within the stopband is greater than 14 dB with CABW of (80±5) MHz under BS state; the FTR is 5.42−5.79 GHz, CABW is (120±5) MHz, the measured return loss and insertion loss within the passband are about 13 dB and 1.69−2.25 dB under BP state, respectively. Moreover, the fabricated tunable filter has a compact size of 0.28λg×0.62λg (λg is the wavelength of the center frequency during the FTR). The experimental and simulated results are in good agreement.
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Key words:
- bandstop filter /
- bandpass filter /
- switchability /
- tunable filter /
- constant absolute bandwidth
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图 3 变容二级管加载谐振器等效电路图
Figure 3. Equivalent circuits of resonator loaded with three varactor diodes
图 6 可调带通滤波器仿真及实物测试结果
Figure 6. Simulated and measured results of the fabricated tunable filter with bandpass frequency response
Note: short dot, state 1; dash dot, state 2; dot, state 3; short dash dot, state 4; short dash, state 5.
图 7 可调带阻滤波器仿真及实物测试结果
Figure 7. Simulated and measured results of the fabricated tunable filter with bandstop frequency response
Note: short dot, state 1; dash dot, state 2; dot, state 3; short dash dot, state 4; short dash, state 5.
表 1 列出的带通滤波器5个状态的性能参数及控制电压值
Table 1. Performance parameters and control voltage values of 5 states of bandpass filter (BPF)
state f0/GHz BW/MHz V1/V V2/V V3/V 1 5.42 115.4 6.34 9.83 0 2 5.49 117.5 6.65 10.05 0 3 5.56 120.9 7.12 10.34 0 4 5.71 117.4 7.37 10.56 0 5 5.79 120.1 7.51 10.79 0 
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表 2 列出的带阻滤波器5个状态的性能参数及控制电压值
Table 2. Performance parameters and control voltage values of 5 states of bandstop filter (BSF)
state f0/GHz BW/MHz V1/V V2/V V3/V 1 5.58 85.3 6.65 10.58 0.89 2 5.64 76.2 6.95 10.95 0.89 3 5.71 77.6 7.12 11.13 0.89 4 5.80 76.1 7.37 11.69 0.89 5 5.89 78.5 7.51 11.94 0.89
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表 3 与之前的微带可调带通-带阻可切换滤波器的性能对比
Table 3. Performance comparison of the present design with previous reported tunable microstrip BPFs and BSFs
design frequency tuning/GHz
(bandpass/bandstop)BW3dB/MHz or FBW/%
(bandpass/bandstop)insertion loss/dB
(bandpass)return loss/dB
(bandpass)rejection level
(bandstop)circuit size/
λ2gof Ref.[15] 0.68～0.985/0.725～0.995 N/A 2.2～5.9 ＞13 ＞20 N/A of Ref.[16] 1.9～2.3 2.5～3/3～7 3.3～6.5 ＞10 ＞15 0.188 of Ref. [17] 2.9～3.6 45～230 N/A N/A ＞20 ＞0.333 of this paper 5.42～5.79/5.58～5.89 （120±5）/（80±5） 1.69～2.25 ＞13 ＞14 0.178
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