本論文著重在ALMA(Atacama Large Millimeter/Submillimeter Array) 第一頻帶(Band-1) 無線電波望遠鏡接收機前端之被動電路設計。接收機前端架構為由圓形號角天線(Horn Antenna) 再接至正交模態轉換器(OMT, Orthomode Transducer)，經由OMT將接收到的電磁波訊號拆解成兩正交之訊號，交由後級低雜訊放大器先經過濾掉鏡像訊號之帶通濾波器，在接至解決LO-to-RF訊號之吸收式帶止濾波器，最後傳遞到混頻器將訊號降至中頻。 首先，延續過去的研究，帶通濾波器部分選用Trisection為主體架構，此架構能在低頻側提供一傳輸零點提高選擇度，且有電路面積較小的優點，實作在砷化鎵製程以提高系統整合度。 再者，選用耦合線架構之帶止濾波器，並引入適當電阻值，能夠有效吸收止帶內之反射訊號，以達到吸收式止帶效果，並實作在砷化鎵製程上。 最後，隨著台灣加工廠技術的進步，能夠提供更精細而準確的加工精度，在OMT的Double-Ridge Junction、90-degree Bend-plane、Power Combiner能夠使用更平滑曲線進行設計，在本論文中，提出一適當設計流程，設計出適用於Q-band之OMT，並在最終實作中選用鋁金屬為製作材料。 ;In this study, we focus on ALMA (Atacama Large Millimeter/Submillimeter Array) Band-1 receiver front-end passive circuit design. First block of receiver is circular horn antenna, the second block is the orthomode transducer which could split the radio signal into two orthogonal signal, third block is bandpass filter which could block the lower side mirror signal, the fourth block is absorptive bandstop filter which could solve LO-to-RF leakage signal problem, and the final block is mixer which could down convert RF signal to intermediate frequency. First, carrying on the past studies, we select trisection as our bandpass filter main structure. This structure could provide a transmission zero to enhance selectivity in lower side and has advantage of small cell size. We could fabricate on GaAs process to improve system integration. Second, we select couple line as main structure of bandstop filter and introduce proper resistance which could absorb the reflection signal in stopband effectively to achieve absorptive stopband. We fabricate this circuit in GaAs process. Eventually, we could provide more refined and more accurate process than before with improvement of fabrication technique so that we could use smooth curve line to design our Double-Ridge Junction, 90-degree Bend-plane and Power Combiner of OMT. In this study, we provide a proper design process to design OMT application for Q-band and select metal Al as our fabrication material finally.
