1771-OFE1
1771-OFE1
無(wú)論電壓型還是電流型的干擾源,在耦合到被干擾對(duì)象時(shí),既可能通過(guò)電路傳導(dǎo)耦合,也可能通過(guò)空間電磁場(chǎng)耦合,或者二者兼有。然而一般的仿真分析工具,往往由于功能所限,只能分析其中一種。例如在傳統(tǒng)的SPICE電路仿真工具中,只考慮電路傳導(dǎo)型的干擾,并不考慮空間電磁場(chǎng)的耦合;而一般的PCB 信號(hào)完整性(SI)分析工具,只考察空間電磁場(chǎng)耦合,將所有的電源、地都看作理想DC直流,不予分析考慮。耦合路徑提取的不完整,也是困擾數(shù)?;旌显肼暦治龅闹匾颉?/p>
數(shù)?;旌显O(shè)計(jì)中,電源和地的劃分,是業(yè)內(nèi)爭(zhēng)論的焦點(diǎn)。傳統(tǒng)的設(shè)計(jì)中,數(shù)字模擬部分被嚴(yán)格分開;然而隨著系統(tǒng)越來(lái)越復(fù)雜,數(shù)模電路集成度不斷提高,分割又會(huì)造成數(shù)字信號(hào)跨分割,信號(hào)回流不完整,進(jìn)而影響信號(hào)完整性,另外,電源的分割還造成電源分配系統(tǒng)的阻抗過(guò)高;有人提出“單點(diǎn)連接”:還是做分割,但是在跨分割的信號(hào)下方單點(diǎn)連接以避免跨分割問(wèn)題;但是如果數(shù)模之間信號(hào)很多,難于分開,這種“單點(diǎn)連接”也存在困難,因而又有人提出不分割,只是保持?jǐn)?shù)字和模擬部分不要交叉;還有一些資料介紹,在跨分割的信號(hào)旁邊包地線或者并聯(lián)電容,用來(lái)提供完整回流路徑。無(wú)論哪種方法,似乎都有一定道理,而且都有成功的先例,然而所有這些分割方案的有效性以及可能存在的問(wèn)題,一直沒有檢驗(yàn)的標(biāo)準(zhǔn)。
數(shù)?;旌想娐返姆抡?,還存在模型的問(wèn)題。業(yè)界普遍接受的模擬電路仿真模型還是SPICE模型,數(shù)字電路信號(hào)完整性分析使用IBIS模型。多家EDA公司的仿真軟件已經(jīng)推出支持多種模型的混合模型仿真器,然而擺在設(shè)計(jì)師案頭的主要困難是器件模型,特別是模擬器件模型很難得到。在數(shù)字設(shè)計(jì)看來(lái),時(shí)域的瞬態(tài)分析,即某一時(shí)間點(diǎn)上確定的電壓值,是仿真的主要手段,就像調(diào)試中的示波器那樣直觀。沒有精確的模型,瞬態(tài)分析就無(wú)法實(shí)現(xiàn)。然而對(duì)模擬設(shè)計(jì),特別是噪聲分析,激勵(lì)源在時(shí)間軸上難于描述或很難預(yù)測(cè),只知道他的頻率帶寬范圍和大致幅度,這時(shí)候我們通常會(huì)引入頻域掃頻分析,考察掃頻信號(hào)在關(guān)注點(diǎn)的變化,如同頻譜分析儀的作用。或者干脆如網(wǎng)絡(luò)分析儀(NA)那樣考察信號(hào)或噪聲通過(guò)的通道的頻域SYZ參數(shù),進(jìn)而預(yù)測(cè)干擾發(fā)生的頻率和幅度。可見,數(shù)?;旌显肼暦治觯刃枰С只旌夏P偷姆抡嫫?,也需要仿真器同時(shí)支持時(shí)域分析和頻域分析。
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