《電子技術(shù)應(yīng)用》
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符合EMI/EMC標(biāo)準(zhǔn)的SerDes——基本測試策略和指南
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摘要: 電磁干擾(EMI)和電磁兼容(EMC)測試是汽車應(yīng)用領(lǐng)域中串行器/解串器(SerDes)設(shè)計必須驗證的參數(shù),,需要在設(shè)計之初謹(jǐn)慎考慮EMI和EMC問題,,避免不必要的設(shè)計修改。
關(guān)鍵詞: EMI/EMC 策略 SerDes EMI/EMC
Abstract:
Key words :

    摘要:電磁干擾(EMI)和電磁兼容(EMC)測試是汽車應(yīng)用領(lǐng)域中串行器/解串器(SerDes)設(shè)計必須驗證的參數(shù),,需要在設(shè)計之初謹(jǐn)慎考慮EMI和EMC問題,,避免不必要的設(shè)計修改,。以下應(yīng)用筆記詳細(xì)介紹了如何在SerDes系統(tǒng)中測試EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" 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title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC" title="EMI/EMC">EMI/EMC的基本概念和技術(shù)指南。

  引言

  LCD視頻顯示屏在汽車領(lǐng)域的應(yīng)用越來越普遍,,堅固的設(shè)計,、小巧的尺寸及低廉的成本使其非常適合安全、導(dǎo)航及信息娛樂系統(tǒng),。LCD顯示屏是數(shù)字裝置,,每個像素為離散的數(shù)字值。由于驅(qū)動這些顯示屏的媒體/圖像信號源通常也為數(shù)字格式,,因此,,數(shù)字鏈路是連接視頻信號源和顯示屏最簡單、性能最高的方式,。視頻鏈路的數(shù)字通道必須具有足夠的帶寬,,例如640 x 480像素的彩色顯示屏工作于30fps (幀/秒)。如果每個像素的紅,、綠,、藍(lán)信號僅有6位分辨率,對應(yīng)的數(shù)據(jù)率則為640 x 480 x 30 x 18 = 166Mbps,。實際傳輸速率必須略高一些,,因為正常工作需要消隱時間。許多顯示屏具有更多像素和/或更高的像素分辨率,,從而使碼率大大提升,。串行器/解串器(SerDes)器件接收并行數(shù)字?jǐn)?shù)據(jù),,然后對其進(jìn)行串行轉(zhuǎn)換,以便傳輸,。有些器件,,例如MAX9209串行器,保持紅,、綠和藍(lán)數(shù)據(jù)通道分離,,三原色中的每種顏色為一個獨(dú)立的串行通道,另外還提供時鐘第4個通道,。其它器件,,例如MAX9247串行器,則將這些數(shù)據(jù)組合為單個串行通道,,采用嵌入式時鐘信號,。這兩種方法都會明顯提高傳輸基頻。盡管頻率增大會產(chǎn)生問題,,但是為串行轉(zhuǎn)換后的信號提供具有適當(dāng)屏蔽和阻抗匹配的傳輸介質(zhì)相對比較容易,。

  EMI測試

  汽車應(yīng)用中必須進(jìn)行EMI測試,確保處于工作狀態(tài)的系統(tǒng)不會干擾到周圍其他系統(tǒng),。針對空間輻射和傳導(dǎo)干擾進(jìn)行測試,。空間輻射的測試主要使用天線,,檢測系統(tǒng)通過自由空間向其他系統(tǒng)輻射的能力,。設(shè)計不合理的SerDes系統(tǒng)無法滿足EMI技術(shù)指標(biāo)。相比之下,,傳導(dǎo)干擾測試主要采用系統(tǒng)電源線上的電壓和電流探頭完成,。由于SerDes系統(tǒng)很少直接連接到電源線,所以很少考慮傳導(dǎo)干擾,。

  EMC測試

  與EMI測試類似,,汽車應(yīng)用領(lǐng)域通過EMC測試確保系統(tǒng)不會被周圍其他系統(tǒng)干擾。由于新型汽車車體內(nèi)部存在大量電子系統(tǒng),,所有這些系統(tǒng)都存在電流,、電阻并且工作頻率占據(jù)很寬的頻譜,因此很有必要進(jìn)行這項測試,。EMC測試采用大電流注入(BCI),,對被測系統(tǒng)來說是個難題。盡管BCI測試規(guī)范和方法依汽車廠商的不同而異,,但通常涉及到頻率從幾MHz到1GHz較強(qiáng)的外部電磁場,。

  像素時鐘頻率選擇

  正確選擇像素時鐘對EMI具有明顯影響,SerDes視頻鏈路類似于其它高速數(shù)字器件,,會產(chǎn)生時鐘頻率整數(shù)倍的諧波輻射,,由此檢測其EMI,。汽車應(yīng)用領(lǐng)域,EMI輻射限制隨頻率變化,。許多汽車制造商都在特定頻帶內(nèi)規(guī)定了相當(dāng)嚴(yán)格的限制。例如,,433MHz是用于無線門禁(RKE)的頻率,,往往是EMI規(guī)范中最嚴(yán)格的頻段。對于像素時鐘頻率為33MHz的系統(tǒng),,其13次諧波位于429MHz,,這會嚴(yán)重干擾433MHz RKE頻帶。選擇稍低的32.7MHz頻率,,可以將13次諧波移到425MHz,,形成更寬松的頻率裕量。

  SerDes PCB EMI/EMC測試的通用特點

  良好的接地對所有IC都是非常重要的設(shè)計準(zhǔn)則,,對于SerDes系統(tǒng)尤其重要,。所有接地引腳必須具有低阻,并連接到穩(wěn)定的接地區(qū)域,。不建議將PCB分為多個區(qū)域,。PCB元件層的覆銅區(qū)域,以及緊鄰下方采用連續(xù)覆銅區(qū),,這些都是常規(guī)準(zhǔn)則,。使最上層的覆銅遠(yuǎn)離阻抗匹配地走線,最好保留至少3倍于差分對布線的間距,。

  考慮每個接地點都采用了多個過孔,,過孔的寄生電感是造成非理想特性的關(guān)鍵。采用多個過孔有助于減小電感,,從而提高性能,。

  對IC的電源端進(jìn)行旁路非常重要,特別是對于SerDes系統(tǒng),。類似于接地的考慮,,電源引腳從電源側(cè)看必須具有低交流阻抗,尤其是低壓差分信號(LVDS)線路,、I/O電源引腳以及用于鎖相環(huán)(PLL)電路的電源引腳,。建議每個引腳連接兩個旁路電容,這兩個電容通常相差10倍至100倍(例如0.1?F和1nF),。最小電容應(yīng)距離需要去耦/旁路的電源引腳最近,。

  可以考慮在SerDes系統(tǒng)的電源引腳使用磁珠,特別是LVDS線路,、I/O電源引腳和PLL電源引腳,,當(dāng)然,,這一點適用于所有電源引腳。磁珠有助于減小高頻能量的輸入和輸出,,選擇峰值阻抗為100Ω至600Ω,、額定值至少為100mA的磁珠。

 

  圖1所示為MAX9247串行器PCB的放大圖片,,關(guān)鍵元件有FB4,、C6和C5,這些元件被布置在一列,,絲印參考點位于各自元件外框的右側(cè),。圖1底部為MAX9247的一角。FB4的右側(cè)端子通過一個過孔連接到嵌入式接地區(qū)域,。FB4的左側(cè)端子向下連接至C6和C5,,然后連接至MAX9247的引腳27,這是串行器的V*LL電源節(jié)點,。注意,,連接FB4、C5和C6的引線較寬,,使寄生電感降至最低,。由于該引線為了滿足MAX9247的引腳焊距而變窄,所以在C5和MAX9247之間使用了一個多邊形覆銅,,保持盡量寬的引線,,同時盡量靠近串行器。而且,,C5和C6的接地顯示每個電容自身都具有連接至接地區(qū)域的過孔(每個元件的右側(cè)),。最上層覆銅區(qū)域為接地層,為C6和C5至MAX9247的引腳26提供直接的低電感連接通路,,該引腳為串行器的PLLGND,。

MAX9247的推薦旁路和接地設(shè)計
 

圖1. MAX9247的推薦旁路和接地設(shè)計

  關(guān)于串行器的建議

  抑制串行器的EMI輻射需要了解一些基本概念。通常情況下,,串行器對EMC測試不是特別敏感,;然而,其輸出需要具有阻抗固定的平衡傳輸線對,。大多數(shù)串行器IC針對100Ω阻抗進(jìn)行優(yōu)化,。如果設(shè)計中有不可更改的設(shè)計因素,接近該范圍的其它阻值也可以接受,。如果串行器輸出需要連接到機(jī)箱以外,,并入汽車線束,這些輸出就必須能夠承受對電池短路故障。最簡單的解決方案是采用0.1?F電容對每路輸出進(jìn)行交流耦合,。然而,,這樣需要一個直流平衡串行器,例如MAX9209,、MAX9217或MAX9247,。也可以使用非直流平衡器件,但系統(tǒng)設(shè)計必須保證所需偏壓由外部提供,,而這通常不可行,。最后,在串行器輸出接出PCB之前,,輸出端往往連接一個共模扼流圈。這對串行器套件之外的共模噪聲抑制有一定保護(hù)作用,。當(dāng)然,,共模扼流圈的作用很有限,當(dāng)插入損耗(標(biāo)稱為1dB)影響到鏈路的可靠性時,,則不應(yīng)使用,。

  關(guān)于解串器的建議

  類似于串行器,設(shè)計工程師也需要遵循一些基本的概念和原則抑制解串器的EMI輻射,。為避免解串器受EMC干擾,,需要掌握一些基本要素,因為解串器易受EMC影響,,同時也產(chǎn)生EMI輻射,。

  在解串器輸入端,靠近差分信號接入PCB的位置,,往往放置共模扼流圈,。共模扼流圈有助于減小共模噪聲的拾取,共模扼流圈在系統(tǒng)選定的工作頻率下必須具有較低的差分插入損耗,。解串器輸入需要具有阻抗固定的平衡傳輸線對,。大多數(shù)解串器針對100Ω阻抗進(jìn)行優(yōu)化,如果設(shè)計中存在不可更改因素,,接近該范圍的其它阻值也可以接受,。

  如果解串器輸入需要交流耦合,可在共模扼流圈之后進(jìn)行隔直流,。同樣,,這些電容只能用于直流平衡解串器,例如MAX9236和MAX9248,。差分對需要短接到100Ω差分阻抗,,盡量靠近接收器側(cè)IC放置。差分阻抗保持在100Ω時,,必須保持很低的共模阻抗,??梢允褂肨hevenin端接系統(tǒng),或者將與中間節(jié)點串聯(lián)的一對50Ω電阻旁路至地,,兩種連接方法如圖2所示,。對于EMI/EMC測試來說,最好采用一對50Ω電阻,,因為:

  它允許IC設(shè)置自身的直流偏置,;

  它不向終端注入VCC噪聲;

  它不消耗功率,。

正確的LVDS端接方法(左:Thevenin配置,;右:串聯(lián)50Ω電阻配置)
 

圖2. 正確的LVDS端接方法(左:Thevenin配置;右:串聯(lián)50Ω電阻配置)

  連接器和線束

  SerDes系統(tǒng)中使用的連接器和電纜是系統(tǒng)的關(guān)鍵部件,,對EMI和EMC測試的影響非常顯著,。汽車應(yīng)用領(lǐng)域的普遍做法是:規(guī)定鏈路兩端的PCB插座和電纜連接器由同一制造商提供。連接器必須保持固定阻抗,,并提供屏蔽接口,,以優(yōu)化性能。此外,,必須使用單極插入或正極性鎖連接器,,確保可靠性,。

  電纜也必須具有固定阻抗,,線束要求嚴(yán)格屏蔽以防輻射。如果使用多線對電纜,,每對電纜都需要單獨(dú)屏蔽,。對于汽車SerDes,一般CAT5電纜即可滿需要求,。

  有許多制造商提供連接器和電纜系統(tǒng),,建議使用Rosenberger、JAE或Hirose產(chǎn)品,。

  有些系統(tǒng)中,,連接器的屏蔽僅在鏈路一端接地,另一端則通過電容(典型為0.1?F)接地,。這種耦合有助于避免地電位在電纜屏蔽層產(chǎn)生直流電流,。

  其它EMI源

  SerDes視頻鏈路的另一個EMI源為解串器輸出。這些輸出為CMOS邏輯電平,,相對為高速沿,。如果CMOS邏輯輸出屏蔽不當(dāng),也會引起EMI輻射。減小來自于LCD面板邏輯信號的最好辦法是采用具有擴(kuò)頻技術(shù)的解串器,,例如MAX9242,、MAX9244、MAX9246,、MAX9248或MAX9250,。這些解串器提供各種工作模式、數(shù)據(jù)寬度和工作頻率,,以滿足大多數(shù)系統(tǒng)的要求,。


 

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