寬截止帶之雙寬頻濾波器設計

Abstract

本文提出利用耦合線和耦合三線作為共振腔，設計雙寬頻帶通濾波器，和傳統雙頻濾波器相比，此雙寬頻濾波器的特色是頻寬較大且介入損耗較低。此外，本文以具有兩開路殘段的十字型導納轉換子，取代雙寬頻濾波器中的傳統導納轉換子，以達到延伸截止帶之目的。十字型導納轉換子的兩開路殘段可產生兩個傳輸零點，其位置由開路殘段的長度控制，可藉以壓抑截止帶中的假性諧波。為了證明理論的正確性，實作兩個操作在900/1575 MHz和900/2000 MHz的雙寬頻濾波器。模擬結果和實際量測結果相當吻合。

Coupled-line and three-line microstrip resonators are used to design dual-wideband bandpass filters, which have the advantage of larger bandwidths and lower insertion loss compared with conventional dual-band filters. To extend the upper stopband, admittance inverter of a 90-degree transmission line section is changed to a cross-shaped inverter, i.e. a transmission line section with two open stubs shunt at its center. Each open stub generates a transmission zero, which can be controlled by its length, at the desired frequency. The transmission zeros are used to cancel the spurious response near the passband so that the upper stopband of the filter can be extended. For demonstration, two dual-wideband bandpass filters with cross-shaped admittance inverters operating at 900/1575 MHz and 900/2000 MHz are fabricated and measured. Measured results of experimental circuits show good agreement with simulated responses.