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      www.elainecharton.com 2008年3月28日

          

           隨著科學技術的不斷進步和人類生活水平的逐步提高,家電設備、移動式和個人攜帶式電子設備日益增多,于是各電子設備間的相互影響和干擾問題變得日趨嚴重和復雜化。IBM公司對計算機電源故障進行分析后認為,近90%的故障源于電磁干擾(EMI);我國有關部門1994年對147家企業生產的不同型號汽車進行無線電干擾性能摸底檢測,達標合格汽車僅占1/4。電磁干擾還威脅著人類的健康和安全,海德堡大學的生物學家研究發現,甚至連微弱的電磁輻射也會通過眼睛侵入大腦,給人們播下癌的種子,因此呼吁人們要警惕“帶電磁的煙霧”?梢,EMI所造成的危害絕不遜于有形的污染。 
           而今,如何降低甚至消除電子設備的EMI已成為全球電子信息業及消費者普遍關注的問題,世界上為此成立了無線電干擾國際特別委員會(CISPR),美、德、日諸國也分別建立了專門組織FCC、VDE和電磁環境工程研究會,他們對電子儀器及測量設備的抗EMI性能要求越來越高,規定非經嚴格檢驗的產品不得出廠和上市。隨著我國經濟邁入國際范疇,電磁干擾(又稱電磁噪聲)問題已擺在我們面前。

           利用近年來發展的鐵氧體——陶瓷疊層共燒技術,可以將若干個鐵氧體疊層電感器和陶瓷疊層電容器集成在一起,經過共繞制構成截止特性銳敏的片式電磁干擾濾波器。根據電路特性選擇適當的濾波器可以得到令人十分滿意的抗EMI效果,特別適用于抑制數字電子設備及高速數據總線產生的噪聲。EMI濾波器按構成元件可分為電感器、電容器、LC復合型濾波器和電容性可變電阻器四種;按構成元件材料及生產工藝的不同,則可分為陶瓷型、石英晶體型、介質型和聲表面波型EMI濾波器;按具體用途又可分為電源線路用和信號線路用EMI濾波器兩大類。下表列出了有代表性的幾種EMI濾波器及其特點。

           LC復合型陶瓷濾波器是由鐵氧體電感器L和陶瓷電容器C按不同組合形式構成的一類多功能產品,其中心頻率、通帶、振幅、延遲等特性可根據電路要求靈活設計,所適應范圍相當寬,廣泛應用于音、視頻家電、通信及辦公自動化設備的EMI抑制,但最初其頻率較低、體積較大。進入90年代后,由于陶瓷疊層印刷技術的發展,國外先后開發出無繩電話用254MHz和380MHz帶通濾波器、便攜電話用700~950MHz濾波器、小功能對講機用430MHz低通濾波器以及數字無繩電話用1.9GHz帶通濾波器等LC復合型片式疊層陶瓷產品,其內藏屏蔽層抗干擾強,無需調整即可適應當前200MHz~1.5GHz高頻化需求。今后的發展趨勢是進一步高頻化(2GHz以上)、微型化、組件化(內藏延遲線、多層諧振器、與濾波器一體化的放大器和混頻器等)和低成本化。

           單片晶體濾波器(MCF)是在石英基片表面配置若干對金屬電極而構成的帶通或帶阻濾波器。它利用壓電效應的能陷入理論,選擇電極振子的幾何尺寸、返回頻率和電極振子間距控制超聲波的聲學耦合,從而達到濾波之目的。其特點是頻率選擇度十分陡峭、損耗低、穩定性好、阻帶衰減高,現已在移動通信設備中大量使用,是必不可少的初級中頻濾波器,對提高整機靈敏度和抗干擾能力有重要作用。國外MCF產品的實用化水平為中心頻率數MHz~150MHz,帶寬0.001~0.1%,頻道間隔12.5~25kHz,最小封裝尺寸為8×8×3.2mm,重僅0.4g。MCF的發展方向是開發新型壓電材料、擴展帶寬、減少帶內延時波動、增大帶外衰減、擴充和提高中頻點及線性度、封裝單片尺寸進一步小型化和片式化。

           利用介質陶瓷材料的低損耗、高介電常數、頻率溫度系數和熱膨脹系數小、可承受高功率等特點設計制作的EMI介質濾波器,由數個1/4波長型諧振器縱向多級串聯或并聯的梯形線路構成。其顯著特點是插入損耗小、耐功率性好、帶寬窄,特別適合便攜電話、汽車電話、無線電臺、無繩電話以及一體化收發雙工器等的級向耦合濾波。國外生產最多的是800~1000MHz范圍內的系列EMI介質濾波器,其技術指標為插損2~3dB,波紋小于1dB,電壓駐波比約1.5,帶外抑制在規定頻段內可達20dB以上,一些特殊要求點可達60dB左右。濾波特性優良。其開發方向是采用新型介質材料和其它諧振模式,設計制作新型的微型化、片式化低插損、高衰減、高性能的EMI介質濾波器。

           聲表面波(SAW)濾波器曾一度廣泛用于工作在20MHz以上頻段的次中頻濾波。它在抑制電子設備高次諧波、鏡像信息、發射漏泄信號以及各類寄生雜波干擾等方面起到了良好的作用。其特點是可靈活設計、模擬/數字兼用、群延遲時間偏差和頻率選擇特性優良、輸入輸出阻抗誤差小。近年來已將SAW濾波器片式化,重約0.2g;另外由于采用了新的晶體材料和最新精細加工技術,致使SAW器件的使用上限頻率提高到2.5GHz,這必將促進SAW濾波器在抗EMI領域獲得更廣泛的應用。低插損技術與片式化封裝是這類EMI濾波器目前和今后一段時間內的研究開發方向。

       
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