高頻/高速差動電路電磁輻射之研究與解決方案

Designs and Solutions of the Electromagnetic Radiation from High Frequency/High-speed Differential Circuit Systems

陳泓銓(Hung-Chuan Chen)

指導教授 ： 吳宗霖

國立臺灣大學/電機資訊學院/電信工程學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.10615

摘要

本論文著重於差動信號在射頻系統與高速數位系統內造成的電磁輻射影響並提出電磁輻射改善策略。在差動信號介面下，由不理想效應造成的共模雜訊被視為是產生嚴重輻射問題的主因。因此，共模雜訊於差動射頻系統內的影響為首要探討的機制。經研究發現，當射頻系統內的天線受共模雜訊干擾時，會導致天線同極化場型的最大增益方向改變且交叉極化場型會大量增加，使得天線的輻射場型與原設計不同。為解決此問題，提出適用於差動射頻系統且具有高共模雜訊抑制功能的平衡式帶通濾波器。為求微小化，不以傳統半波長共振結構的方式設計，另提出兩種新穎的四埠集總元件架構，分別為電感式耦合與電容式耦合，達成平衡式帶通濾波器的設計，該濾波器於差模信號傳送時可提供帶通響應，而於共模雜訊操作下可達到抑制的效果，且其共模電路架構引入可任意調整的傳輸零點，使具有抑制平衡式天線的共模共振機制，而其所設計出的平衡式帶通濾波器面積僅有0.114 λg × 0.1 λg，並且可於差模傳送頻帶內量測到至少50 dB且最大值約有67 dB的超高共模雜訊抑制比，相較其它已發表的電路架構，是共模抑制能力最好的設計。只要將其平衡式帶通濾波器擺放在射頻系統的天線端前，即可確保天線的輻射場型不受共模雜訊干擾。
針對高速差動數位系統，主要研究來自於電路板模組的電磁輻射問題，經研究發現其中與各電路板相連的連接器，是產生高頻輻射的主要來源。傳統上，為解決連接器內因轉角造成的長度不等長而造成的共模雜訊，導致信號完整度的議題，長度補償技巧是常用的解決方案。經本研究指出，該技巧的確能改善信號的傳輸品質，且差模信號轉成共模雜訊的成分約略有10 dB降低，但卻無法改善連接器本身造成的電磁輻射問題。根據連接器的電流分布發現，連接器造成的電磁輻射與結構產生的天線模態電流有關，而非與結構不對稱造成的共模電流相關。進一步研究連接器本身的輻射特性，則發現造成電磁輻射峰值的頻率，是來自於連接器的側接地線與背接地線形成類似槽孔天線的輻射機制，依據天線模態電流分布，連結器於該頻段內產生半波長及其諧波的共振，產生大量的輻射。此外，當兩兩相鄰的側接地線或背接地線產生寬邊耦合效應時，會進一步產生新的共振機制，產生更多的電磁輻射峰值，惡化了連接器的電磁輻射響應。
為降低連接器於高頻的電磁輻射特性，本研究提出三種作法。首先，針對位處於連接器的最內側與最外側的非對稱差動信號對，提出額外引進側接地線，使其差動信號對變得對稱，改善差動信號對的模態轉換並且提供降低電磁輻射的功能。另一種降低輻射量的方法是在連接器的四周圍貼上損耗材質的貼片，該方式經實驗驗證可達到大約6 dB的輻射衰減量，且該作法並不會惡化連接器的信號傳輸品質。最後一種方式是針對產生輻射的槽孔結構進行改善，直接將側地線與背接地線以金屬導體相連，除去連接器內的槽孔結構，該作法可大幅降低輻射達到約25dB的抑制，並且亦可改善連接器的信號傳輸品質，達到信號完整度與電磁干擾的共設計。

關鍵字

差動信號 ； 共模雜訊 ； 電磁輻射 ； 平衡式帶通濾波器 ； 高速連接器

並列摘要

This dissertation is concentrated on studying the radiated emissions from differential RF system and high-speed differential digital system and therefore presenting radiation mitigation strategies. At the differential signaling interface, common-mode noise has been considered as the source of producing server radiation issues. Thus, the influence of the common-mode noise on differential RF system is studied. As the antenna is coupled with common-mode noise, the peak gain of co-polarization is rotated and the cross-polarization level is significantly increased. In order to solve this problem, balanced bandpass filters with high common-mode rejection level are proposed and applied to the RF differential system. Without designing the filter using half-wavelength resonator, new four-port lumped-element circuit topologies with inductive coupling method and capacitive coupling method are presented. The newly circuit topologies can support bandpass response under differential-mode operation and exhibit bandstop filtering function under common-mode noise. Moreover, the proposed circuit topologies can have additional transmission zero to prevent the common-mode resonant mechanism of the balanced antenna. The implemented balanced bandpass filter occupies a compact size of 0.114 λg × 0.1 λg with superior CMRR of 50 dB within the differential-mode passband and approximately 67 dB in maximum, which show the best performance compared to the other works. As inserting the proposed balanced bandpass filter before the antenna, it can prevent the antenna’s operation from the common-mode noise.
For high-speed differential digital system, radiated emissions over GHz are mainly attributed to the connector across multiple circuit boards. Due to the bends along the connector, skew compensation strategies are commonly used to correct the imbalance length for reducing common-mode noise as well as improving signal quality. Although over 10 dB reduction on the mode conversion of |Scd12| can be observed, the total radiated power from connector is not significantly reduced. According to the current distributions, it shows that the radiated emission is strongly related to the antenna-mode currents, instead of common-mode currents. It is further discovered that the radiation peak is caused by the slot-like antenna, which is formed by the edge ground blade and back ground blade. Based on the antenna-mode current distributions, the radiation peaks are the integer multiples of a half-wavelength. In addition, as multiple wafers are considered within the connector, additional radiation peaks are produced due to the slots between wafers. It makes the radiation performance of the connector even worse.
Three radiation mitigation methods are proposed. One is to insert additional edge ground blade to the orphaned differential pairs, which are located at the wafer periphery. This strategy can make the radiated emission level lower and improve mode conversion, because the orphaned differential pairs with inserted edge ground blade become symmetrical. Another is to attach lossy material to cover the entire connector. Over 6 dB suppression on the radiated emission can be obtained. Furthermore, the attached lossy material does not degrade the signal quality through the connector. The other is to remove the slot structures inside the connector by connecting the ground blades together. Approximately 25 dB suppression on the total radiated power is given and the signal quality of the connector is also improved. This technique can benefit both SI and EMI issues for the high-speed connector.

並列關鍵字

Differential signaling ； common-mode noise ； electromagnetic radiation ； balanced bandpass filter ； high-speed connector

參考文獻

[1] D. E. Bockelman and W. R. Eisenstadt, “Direct measurement of crosstalk between integrated differential circuit,” IEEE Trans. Microw. Theory Tech., vol. 48, no. 8, pp. 1410–1413, Aug. 2000.

[2] H. Morishita, H. Furuuchi, and K. Fujimoto, “Performance of balanced-fed antenna system for handsets in the vicinity of a human head or hand,” in IEE Proc. Microwaves, Antennas Propagat., vol. 149, no. 2, pp. 85–91, Apr. 2002.

[3] R. Bourtoutian, C. Delaveaud, and S. Toutain, “Differential antenna design and characterization,” in IEEE Proc. of the third European conference on Antennas and Propagat., pp. 2398–2402, Mar. 2009.

[4] C. R. Paul, “A comparison of the contributions of common-mode and differential-mode currents in radiated emissions,” IEEE Trans. Electromagn. Compat., vol. 31, no.2, pp. 189–193, May 1989.

[5] B. Archambeault, S. Connor, and J. Diepenbrock, “EMI emissions from mismatch in high-speed differential signal trace and cables,” in Proc. IEEE Int. Symp. Electromagn. Compat., Honolulu, HI, Jul. 2007.