以抗流線圈抑制非遮蔽雙絞線之共模雜訊

Abstract

近年來共模抗流線圈廣泛的使用在高頻資料傳輸系統上。通常共模抗流線圈可抑制之共模雜訊程度可由量測方式得知，但却不易有效分析影響抑制共模雜訊程度之主要因素，且須重複量測效率不佳。瞭解共模抗流線圈如何降低非遮蔽雙絞線之共模雜訊和電磁干擾對於設計或維護、架設整個線導通訊系統是非常重耍的。共模抗流線圈的數學模型之產生可使在瞭解共模雜訊抑制程度之因果闗係方面比完全經由量測方法更可有進一步理解。 文中首先以實例說明於非遮蔽雙絞線存在之不平衡電容及其如何形成共模電流迴路造成共模雜訊干擾。接著說明共模和差模電流之輻射電場效應。為了更精確瞭解共模抗流線圈，我們研究了共模抗流線圈的結構、材質、原理、擺設位置和推導其數學模型。又因共模雜訊是由在非遮蔽雙絞線上之不平衡元件造成，完整的分析必須包含此兩者的數學模型，文中亦推導兩者之傳輸矩陣。整個完整系統之模型是將各個方塊之傳輸矩陣串接而得。最後由系統之傳輸矩陣參數推導出縱向轉換傳輸損失比(Longitudinal Conversion Transfer Loss)。另外我們說明了輻射電場之模型。 我們模擬不同線長時共模抗流線圈和電阻電容兩種不平衡元件位置變化對共模雜訊抑制效果的影響。我們亦量測因電容性不平衡元件所造成之輻射電場強度來和模擬結果相互驗證。其結果為輻射電場強度和LCTL成正比且共模抗流線圈位置需在不平衡元件之後才可有效降低兩者。對使用非遮蔽雙絞線傳輸資料的通訊系統而言，本文提供一個提高訊號完整性和降低輻射電場的有效方案，其中推導之數學模型和驗證流程可作為設計共模抗流線圈時之分析工具之用，並可使工程師更明白瞭解共模抗流線圈在電子裝置應用上之貢獻。

Recently, a common-mode choke (CMC) has been widely used in high frequency data transmission systems such as LAN and ADSL. The understanding of how common-mode choke reduces noise effects on UTP cable is very important for the development, maintenance and the cabling of wire communication system. Generally, The ratio of noise reduction could be obtained by measuring of UTP, but which provide less information for understanding the main factors that affect the reduction ratio. On the other hand, it required repeated measuring through lack of efficiency. The development of numerical analysis model of CMC provides better information and higher efficiency than measurement method for understanding the reasons that affect the reduction ratio. At the beginning of this research, we introduced the radiated emission of different-mode and common-mode current and a simple example is then provided to explain why there exists a capacitor unbalances on the UTP cable. This example also explains how the common-mode current is formed and common-mode noise is generated. In order to reduce common-mode noise more precisely with a CMC so full aspects of CMC characteristics are examined including structure, mechanism, material, location and the analysis model. Because the noise on UTP cable is to be reduced so we derive the transmission matrix of noise sources and cable. We then derive the completed analysis model using a cascade transmission matrix. Finally, longitudinal conversion transfer loss (LCTL), which is an important characteristic about the unbalances ratio in a system, is derived. Additionally, We explain the emission prediction model for both different-mode and common-mode current. By using the transmission matrix method, we analyse an effective position of a CMC to reduce the common mode noise for the capacitive, resistance unbalances respectively along an unshielded twisted pair cable. To further correlate the effects of electromagnetic interference (EMI) caused by common mode noise. The radiated EMI is measured in the laboratory. As a result, we concluded that the EMI is proportional to the LCTL and a CMC should be located after the unbalanced position on the cable to improve LCTL and EMI characteristics in the local area network (LAN) or telecom interface circuits. We have provided a solution to improve the signal integrity on the UTP cable and to effectively reduce the EMI effects caused by the common-mode noise as well. The derived models and the developed verification flow could be used as a standard tool kit while designing a CMC. The contents of this paper could provide a contribution to engineers with a clearer understanding about CMC application in electronic equipments.