Reduce Common Mode Noise on Unshielded Twisted Pair Cable with Common Mode Choke
|關鍵字:||抗流線圈;非遮蔽雙絞線;共模雜訊;Common Mode Noise;Unshielded Twisted Pair Cable;Common Mode Choke|
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。
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.
|Appears in Collections:||Thesis|