A tunable 0.83-2.15 GHz bandpass filter with high selectivity
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摘要: 设计了一种宽频率调节范围的高选择性可调带通滤波器。宽调节范围由一对简单的加载变容管的平行耦合线谐振器设计实现。由于谐振器之间存在电磁混合耦合以及通过加载变容管引入的源与负载频变耦合，该滤波器最终引入了三个自适应性传输零点。而且，三个传输零点在整个调节范围内能够保持相同的相对位置，因此，设计的滤波器实现了在整个宽频率调节范围内的高选择性和良好的带外抑制。同时，在中心频率调节的过程中，通过选择合适的耦合系数，该滤波器可以实现恒定的相对带宽。最终设计出的可调滤波器的调节范围为0.83~2.15 GHz, 可调范围可达88.6%，并保持9%±0.3%的相对带宽不变。测试结果表明该滤波器具备了高选择性和良好的带外抑制能力。Abstract: This paper presents a tunable bandpass filter (BPF) with the wide tuning range of center frequency and high selectivity. The wide frequency tuning range is achieved by a pair of simple varactors-tuned parallel coupled line resonators. Since the electromagnetic mixed coupling between resonators and frequency-variant source-load coupling are incorporated in this configuration, three self-adaptive transmission zeros (TZs) close to the tunable passband are obtained. Also, three TZs can almost keep the same relative location of passband to achieve continuous high selectivity and good out-of-band rejection over the whole frequency tuning range. Meanwhile, by selecting a proper coupling region, a constant fractional bandwidth (CFBW) in the frequency tuning process can be realized. For demonstration, a tunable 0.83-2.15 GHz BPF with a 9%±0.3% CFBW is designed, fabricated and measured. The experimental results show the proposed filter has the advantages of high selectivity and good out-of-band rejection.
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图 5 变容管Cv3对传输零点的限制和作用
Figure 5. Effect and limitation of Cv3 on the transmission zeros
图 6 滤波器的仿真及实物测试结果
Figure 6. Simulated and measured results of the proposed tunable filter
表 1 列出的四个状态的性能参数及控制电压值
Table 1. Performance parameters and control voltage values of four states
state f0/GHz FBW/% BW30 dB/BW3 dB V1/V V2/V V3/V 1 0.83 8.72 4.86 0 0.9 0.9 2 1.10 9.09 4.35 2.1 2 1.5 3 1.60 9.17 4.20 6 4.2 3.5 4 2.15 9.30 3.90 19.5 5.8 7.3 
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表 2 与之前的微带可调带通滤波器的性能对比
Table 2. Performance comparison of the presented design with previous reported tunable microstrip BPFs
Ref. frequency tuning tuning range BW3 dB/MHz or FBW/% BW30 dB/BW3 dB return loss/dB insertion loss/dB circuit size/λg2 Ref.[2] 1.62-1.96 GHz 19.0% 4.8%±0.3% N/A ＞20 2.84-2.9 0.134 Ref.[4] 0.55-1.9 GHz 110.2% (92±6) MHz 5.7-9.4 ＞12 3.2-4.4 0.013 Ref.[5] 1.1-2.1 GHz 63% 40 MHz N/A ＞14 4.4-6.1 0.206 Ref.[8] 0.78-1.36 GHz 54.2% 103%±6 MHz N/A ＞12 1.68-2.9 N/A this paper 0.83-2.15 GHz 88.6% 9%±0.3 % 3.9-4.9 ＞23 2..3-3.2 0.037
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