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    無源交調的產生與測試

        隨著移動通信的飛速發展,對接收系統的靈敏度要求日益提高,使得同一傳輸信道內可能存在很多不同頻率的信號,如果傳輸線或連接傳輸線的射頻同軸連接器及其他無源器件的特性不良,就可能會在不同頻率之間產生無源交調信號(Passive intermodulationPIM),結果使有效的傳輸信號發生畸變,產生噪聲和雜波,影響信號傳輸速率及接收機的靈敏度。本文分析了無源器件產生交調信號的原因及預防措施,對無源器件的設計、生產制造以及通信系統中的選用有積極的指導意義和參考價值。

        1、無源交調產生的原因

    在全球移動通信和傳呼臺等蜂窩基站上,由于傳輸功率較大,通常會在系統中的無源器件中產生交調信號,這些雜波信號會嚴重影響整個系統的性能。在一個理想的線性傳輸系統內,其輸出相對于輸入是成正比的。但在實際中微波信號在無源器件中(如:濾波器、同軸電纜、各類接頭等)的傳輸往往是非線性化的,只是因傳輸的信號功率較小時非線性產生的無源交調信號不大而不引起人們的注意。但當所傳輸的信號大于40dBm時,這種交調信號的影響就尤為明顯。 這一交調的結果就產生了一些額外的頻率,即交調生成物,如圖A中的2F1F22F2F13Fl2F2 3F22Fl等。

        除了器件的非線性因素外,在無源器件中生成無源交調的還有以下幾個方面的原因:
        
    1)金屬零件電鍍過程中未清洗干凈的電鍍溶液。
         2)鍍層導電性不好,鍍層厚度不夠。
         3)表面銹蝕。
         4)中心接觸件的不同金屬材料。
         5)信通道內的磁線材料。
         6)較低的接觸點正壓力。
         7)表面粗糙度大。
         8)連接器及無源器件內的碎屑和灰塵。
         9)螺旋狀的信號通道。

        2、無源交調的影響
        GSM900DCS1800PCS1900為例我們通過下表來分析這些交調信號如何影響我們的基站和終端接收機:

        從上表1中我們不難看出基站下行發射的兩個大功率下行信號F1F2所產生的3階交調信號頻率正好落在上行信道接收機的頻段內,就會對接收機造成干擾,導致接收機的誤碼率上升,降低了接收機的接收靈敏度,嚴重影響終端的通話質量。而且產生的交調信號也是很多的,除了3階還有5階甚至7階。但又以三階交調信號的功率最大,5階和7階交調信號因離主頻較遠,其信號功率也較小,對接收機的影響也較小,所以大多數無源器件都會必測三階互調信號的大小,只有少數頻段的器件會測試五階和七階的交調信號的大小。
        因此在無源器件生產制造過程中必須要對器件所產生的交調信號的大小進行嚴格的測試,這已經是濾波器和同軸電纜等制造廠商必測的一個參數。

        3、無源交調的測試
        因基站和接收機對無源交調信號的要求比較嚴格,其對測試系統的要求也相對較高,首先要保證測試系統自身所產生的交調信號很小,這樣才能滿足測試要求。一般要求測試系統的交調信號至少要比被測器件的要求值小10dB。比如要測試一款塔頂放大器產品,其3階交調信號要求小于-110dBm,那么測試系統就必須要小于-120 dBm才能滿足測試要求。其次,要保證系統的穩定性。只有測試系統自身的穩定性高,才能保證測試結果的可靠性。
        如圖B所示是一個典型的 雙工器上行頻帶內無源交調信號測試的框圖: 

        測試臺各部件的作用:
        信號發生器: 產生主信道信號。
        功率放大器: 對信號發生器所產生主信道信號進行功率放大。
        隔離器:     信號單向傳輸,避免因開路或其他情況造成信號反射而損壞其他設備。
        合路器:     對從隔離器出來的兩路大功率信號合成到一個端口輸出.
        被測產品:   需要測試的產品。
        標準濾波器: 將主信道信號和互調信號進行分離。
        大功率負載: 將大功率的主信道信號消耗掉。
        頻譜儀:對交調信號的頻譜和功率進行分析。
        在這個測試系統中,標準濾波器是最主要的部件,其自身的交調信號的大小會影響整個測試系統的性能,必須加強維護和檢查其穩定性。對功放的輸出功率也是在連接合路器和被測器件的射頻電纜的輸出端口進行校準,其功率一般應達到43dBm~ 47dBm,如果功率太小,則被測器件的交調信號也會很小,無法反映器件在實際使用過程中的真實情況。
        在現代計算機和工業控制很發達的情況下,已經有國外的廠商做除了全自動的交調信號測試分析系統,其性能也完全滿足各濾波器、電纜、塔頂放大器對交調信號測試的要求,但其價格也比較昂貴,對一些小的生產制造企業來說還無法接受。但這些企業為滿足客戶對產品無源交調的要求,也可以按照測試要求自行搭建該測試系統。
        總之,無源交調是通信領域里一個非常重要的參數,如何最大程度的降低交調信號對通信系統所帶來的不利影響是研究人員面臨的一個新課題。本文僅是作者以前實際工作中的一些心得,僅供業界同行交流和參考。

     

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