本文主要闡述了一種中心頻率可調式雙通帶濾波器的設計過程和方法，並附述了幾個具體電路的模擬與實作的情況和響應結果。本設計通過整合雙通帶及可調式的功能於一個電路，達到縮小電路面積，提高整合度，提供更靈活的濾波器功能的目的。 本文設計主要通過適當設計共振器的兩個共振模態，實現在需要的兩個頻率上共振；另藉由在共振器傳輸線末端加載可變電容的方法實現共振頻率可調的效果，使用耦合係數設計方法，整合三階共振器耦合實現三階的通帶響應。 電路使用Rogers RO4350B板材以PCB方式實作，可調元件使用Skyworks SMV 1232 varactor diode實現，電路量測響應與模擬基本相符，但在第二通帶的穿透損耗上與模擬有一定出入，根據後續修正模擬認為，實作損耗較大主要由採用的可變電容元件自身損耗造成，模擬使用的元件參數在損耗上不是很準確，因而與實際量測不符。 本研究提出的可調式雙通帶濾波器，具有簡單明瞭的設計方法和流程，並已有實作驗證其有效性。後續可考慮採用損耗更小的元件或製程，進一步優化電路的響應特性。 ;This study mainly introduces the design method and procedure of a type of center-frequency tunable dual-band bandpass filter, and presents the results of fabrication and measurement. This design integrates tunability with dual-band features on a single filter circuit, aiming to miniaturize the circuit area, and provide more flexibility in application. This configuration of proposed filter is based on two-mode stub-loaded open-loop resonators, with each mode resonating at the frequency of the band which to be designed. Tunability is provided by the varactors loaded at the end of transmission lines of resonators. With coupling coefficients extraction and correction to help tune circuit, 3-order bandpass response of the filter is formed by 3 coupled resonators. Filter is fabricated on Rogers RO4350B substrate with ε_r=3.66, tanδ=0.0031, and Skyworks SMV 1232 varactor diode employed to provide tunability. For the final fabricated circuit, measurement matches simulation on band frequencies, but insertion loss does not match at second passband. With adding the equivalent circuit of the component into simulation, results observed indicates the extra insertion loss of measurement may be caused by inherent loss of varactor. This study proposes simple and clear design procedure of tunable dual-band bandpass filter and fabricated circuits with measurements. High quality factor components and process could be considered to be employed to lower the insertion loss of passbands in future.
