多種抑制微波帶通濾波器二階及多階諧波方法之研究

Abstract

本文探討微波帶通濾波器虛假響應(spurious response)的產生原因，並提出數種有效消除虛假響應的方式，以突破虛假響應對於濾波器設計所造成的限制，使得加寬上截止頻帶的高性能微波帶通濾波器得以實現。依照處理方式與電路結構的不同，將論文區分為三個部分。在第一部份中，將探討虛假響應的形成原因，並提出懸浮基版(suspended substrate)結構。搭配所提出新的任意鏡像阻抗合成公式，不需任何額外調整，即可改善通帶響應的對稱性並且對於在二倍基頻(2fo)的虛假響應有近40dB的抑制,使得上截止頻帶的上限能夠從2fo延伸至3fo,增加一倍的上截止頻帶頻寬。 在第二部分中，提出了介電質覆蓋(dielectric overlay)的結構，在濾波器電路上覆蓋一層厚度均勻的介電質即可對二階諧波有至少40dB的抑制功能。第一部份中的任意鏡像阻抗合成公式與濾波器設計流程可以直接應用於此部分的設計。然而，當嘗試採用不同介電常數與厚度的覆蓋介電質來設計電路的時候，發現必須採用四分之波長阻抗轉換器才能完成濾波器的設計。為了解決這個問題，採用了擇定饋入(tapped input)輸入/輸出共振腔，並且推導出新的濾波器合成公式，使得電路設計的自由度大大提升。 在第三部分中，提出過耦合平行耦合微帶線濾波器架構，每一個耦合級在2fo附近會產生一固有零點，而此零點的位置可透過耦合級的耦合長度來調整。調整首末兩耦合級的耦合長度即足以抑制在2fo的虛假響應。此外，將每一耦合級的鏡像阻抗(image impedance)提高，可縮小奇模及偶模相位速度間的差異，使得虛假響應的抑制效果有15dB的改善。將此想法應用於中間的耦合級，調整耦合級的長度來抑制在3fo 及4fo的虛假響應，如此上截止頻帶可大大限延伸至5fo，而且通帶響應與採用傳統設計方式的一樣好。在每一部份中，都實做了數個電路來驗證理論的可行性，發現量測結果與模擬結果均非常吻合。

In this dissertation, the cause of spurious responses of microwave bandpass filters is investigated. Several methods are proposed to tackle these spurious responses in order to design microwave bandpass filters with wider upper stopband bandwidth. According to different methods and circuit structures, the dissertation is divided into three parts. In part one, the cause of spurious responses is studied, and suspended substrate is proposed to design coupled-line filters free of second harmonic resonance. Based on filter synthesis formulas for arbitrary image impedances, not only the passband symmetry can be improved, but also the spurious response at second the passband frequency (2fo) can have a suppression of nearly 40dB without any additional tuning. The upper limit of upper stopband can be extended from 2fo to 3fo, such that double upper rejection bandwidth can be obtained. In part two, the multilayered structure with a uniform dielectric overlay is proposed, and a suppression of at least 40dB to the spurious responses at 2fo can be achieved. The synthesis formulas and filter design approach in part one of the dissertation can be directly applied to the filter design of part two. However, the use of quarter-wave impedance transformer is inevitable as attempting to use overlay dielectrics with different dielectric constants or thickness. In order to overcome this problem, coupled input/output stages are replaced with tapped input/output resonators and the filter synthesis formulas are rewritten, so that the freedom of filter design can be greatly enhanced. In part three, parallel-coupled microstrip filters with over-coupled stages are proposed. An inherent zero of a microstrip coupled stage near twice the design frequency (2fo) is found tunable by varying its coupling length. It is found that applying over-coupling to the end stages, rather than to every coupled stage, is sufficient to improve the filter characteristics in the upper stopband. Also, increasing the image impedance of the filter sections can further enhance the suppression. This idea is extended to design over-coupled middle stages for suppressing the spurious |S21| peaks at 3fo and 4fo of the filter, so that the upper stopband can be greatly enhanced up to 5fo. The passband preserves a response as good as the traditional design. In each part of the dissertation, several circuits are designed and fabricated to verify the proposed ideas and the measured responses agree very well with the simulated ones.