兩種新型微帶線帶通濾波器設計:具多重高階諧波抑制之鋸齒結構平行耦合級與具準橢圓函數響應之四模環形共振腔

Abstract

本論文提出兩種新型微帶線帶通濾波器設計。第一部分提出以鋸齒耦合微帶線結構為基礎，用傳輸零點和帶止響應的方式達成多重諧波抑制之特性；妥善設計的四分之波長耦合級可同時壓抑二階、四階和六階諧波，六分之波長耦合級可同時壓抑三階，六階和九階諧波；七分之波長耦合級可同時壓抑七階和十階諧波；於輸入輸出端使用擇定饋入產生的零點，可壓抑三階和五階諧波；再配合週期性結構所產生的帶止響應，十一階至十六階的諧波皆同時被壓制。如此可達到多重高階波的抑制，因而大幅增加截止帶的頻寬。 本文第二部分提出以雙環諧振腔設計多模態帶通濾波器。共振腔的內環是以傳統的單環結構配合兩段置於環左右兩側中央位置之開路傳輸線殘段，這殘段可等效成電容性負載，使得奇模的共振頻率降低，而偶模的共振頻率不變。以此結構之內環配以傳統結構之外環所設計的共振腔，可得到四個共振模態，合成之多模態帶通濾波器具有較寬的通帶頻寬，而且擁有很好的選擇性。本論文實作數個電路以資驗證設計，量測數據與模擬結果相當一致。

Coupled-line stages are designed with corrugated periods to accurately allocate inherent transmission zeros at three leading even harmonics of design frequency, fo. The corrugation is finely trimmed so that the eigenmode phase velocities at these frequencies can be equalized. Through adjusting number of corrugation periods of a stage, frequencies of the zeros can be arbitrarily scaled up to a certain extent. In this way, a parallel-coupled line bandpass filter with a cascade of several such stages can be synthesized to be free of spurious harmonics up to at least 10fo, with aid of tapped input/output coupling scheme. The spurious harmonics from 11fo to 16fo are further suppressed by bandstop characteristics of periodic structure. The measured data of the fabricated bandpass filter show a rejection level of better than 30 dB in the upper stopband up to 18fo. Double-ring resonator with two open stubs is proposed to design quadra-mode bandpass filters. Two open stubs are horizontally attached to the inner ring. By appropriately adjusting the length or width of stubs, it is possible to control an extra zero existing in the double-ring structure. In addition, perturbation patches are introduced to inner and outer rings to split up the four poles for synthesizing a bandpass filter with high selectivity and relatively wide passband. In order to validate the proposed idea, two quadra-mode bandpass filters with fractional bandwidth of 10% and 15% are fabricated. Measured results show that the in-band insertion losses are less than 1.1dB. Simulation data are also plotted for comparison.