微小化寬頻微波濾波器與耦合器

Abstract

本論文將介紹三種微波電路，包括了薄膜濾波器、寬頻岔路環耦合器與寬頻垂直安裝之平面基板架構帶通濾波器。 文中薄膜濾波器設計將利用薄膜介電層非常薄的特性來構成特性阻抗極低的傳輸線，並與原先的傳輸線相結合，組成一諧振腔來縮小尺寸，接著使用耦合係數法來分析外部品質因數與內部耦合係數；再利用電磁模擬軟體來求出相對應的濾波器共振腔尺寸。最後，設計出中心頻900MHz，頻寬10%的二階與三階微小化薄膜濾波器，長度縮小至原本10%左右。 而寬頻岔路環耦合器設計將利用四分之一波長雙端短路耦合器來代替傳統四分之三波長傳輸線的部份，此種雙端短路耦合器可以等效成四分之一波長傳輸線與理想的一百八十度相移器，但是此種耦合器必須要滿足耦模阻抗遠大於奇模阻抗的特性；這種緊耦合器將利用垂直安裝之平面基板架構來設計，這種架構的耦合器能達到所需的奇偶模阻抗值。最後，設計出中心頻2GHz，頻寬77.5%的寬頻岔路環耦合器。 接著將繼續使用垂直安裝之平面基板架構耦合器所具有的緊耦合特性來設計寬頻濾波器；利用平行耦合線濾波器來作為設計的架構，並將二分之波長的諧振腔改為歨階阻抗諧振腔，利用歨階阻抗諧振腔的共振特性將二次諧振頻率推遠，改善上止帶抑制效果。最後，設計出中心頻3GHz，頻寬66.67%的寬頻濾波器，且二次諧振頻率在中心頻率四倍處。

This thesis introduces three kinds of microwave circuits including thin-film filter, broadband rat-race coupler, and broadband vertically installed planar (VIP) bandpass filter. In this thesis, a thin-film filter is designed by utilizing the characteristic of thinner thin-film to realize the transmission line with very low characteristic impedance and combined with original transmission line to form a resonant cavity to minimize the dimensions. Besides, coupling coefficient method is used to analyze external quality factor and internal coupling coefficient, and then electromagnetic simulation tools are used to find out the corresponding dimensions of filter resonant cavity. In the end, second order and third order minimized filter with center frequency at 900MHz and ten percent bandwidth is designed and the length is minimized to the ten percent of the original dimensions. A broadband rat-race coupler is designed by utilizing λ/4 short-end coupler to replace the traditional 3λ/4 transmission line. This kind of short-end coupler can be equivalent to a quarter wave transmission line and a ideal 180 degree phase shifter under the request that even mode impedance is much greater than odd mode impedance. This tight coupler is designed by using VIP to reach the wanted odd and even mode impedances. At last, a broadband rat-race coupler with center frequency at 2GHz and 77.5% bandwidth is implemented. Then, a wideband filter is designed by using VIP coupler to get the characteristic of tight coupler; parallel coupled line filter is adopted as the structure and theλ/2 resonator is replaced by step impedance resonator(SIR). Utilizing the resonance characteristic of SIR to put the second resonance frequency farther to improve the upper stopband rejection. Finally, a broadband filter with center frequency at 3GHz and 66.67% bandwidth, and second resonance frequency at the four times center frequency is presented.