A Study on Imbalance Component and EM Radiation from Asymmetrical Differential-Paired Lines with U-Shape Bend Routing
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Component (thermodynamics)
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Recently, differential-signaling (DS) techniques such as low-voltage differential-signaling (LVDS) have been widely used in digital electronics devices in order to suppress electromagnetic interference (EMI). But in practical terms, a complete topologically and structurally symmetrical differential line is impossible. In this paper, we newly attempt to quantify the imbalance component and electromagnetic (EM) radiation when the structure and topology change from a symmetrical to an asymmetrical differential paired lines. Four different differential-paired lines structures are prepared for the comparison: PCB1 is a basic symmetrical structure taken as an "ideally balanced" case, PCB2 is an asymmetrical structure due to differences in bend and length, PCB3 is a symmetrical length structure with a bend region, and PCB4 is an asymmetrical topology with equi-distance and bend routing. Firstly, the differential voltages and mixed-mode scattering parameters are selected as a measure. The conversion parameter from differential-mode (balance component) to common-mode (imbalance component), Scd21, is dramatically increased by the difference of the length. Even if the differential paired lines have a bend-region, equi-distance routing can suppress Scd21. Secondly, spatial distributions of near magnetic fields are measured at certain resonant and out of resonant frequencies of Scd21. Although the differential paired lines are excited by the differential-mode, propagated magnetic field component at the end terminal of the differential paired lines at the resonant frequencies of Scd21 could be changed to the common-mode. Thirdly, the far-electric fields at 3 m are measured and calculated. Even if equi-distance routing is suitable for the improvement of signal integrity (SI) issues, it is not enough for the suppression of the far-field potential radiation. It is clear that Scd21 is one evaluator but it is not sufficient for predicting the EM radiation completely. The facts shown in this study suggest the basic characteristics of EM radiation from practical differential paired lines with asymmetrical structure and some significant problems in the design of a meander delay line for high-speed clock distribution.
Common-mode signal
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In a differential transmission line, a large common-mode current is excited due to its asymmetry. In this paper, the authors demonstrate experimentally the common-mode current and verify the imbalance difference model that was proposed for prediction of the common-mode current reduction. Experimental results show that the reduction of common-mode current of about 20 dB is achieved by changing the position of the transmission line. In addition, the differences that are calculated using the imbalance difference model are in agreement with the measured ones within 2 dB.
Common-mode signal
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For differential-signaling system, the ideal balance or symmetrical topology cannot be established due to discontinuities at turnoff point (90-degree turn), so called bending, and hence, an imbalance component is excited. Therefore methodology and geometrical-structure for satisfying both SI and EMI performances are indispensable. In order to provide basic considerations for the realization of methods for compensating the imbalance component and suppressing the EM radiation from asymmetrical differential-paired lines, the imbalance component and EMI from the differential-paired lines with discontinuities at turnoff point were studied experimentally and with numerical modeling. The differential-paired lines with different layouts were prepared for the discussion as typical routing. In the method, the distance between the paired lines at turnoff point is narrow, and hence the difference of length between Line 1 and 2 is compensated. Therefore the suppression of imbalance component by compensating the difference of the length and suppression of EMI by narrower separation can be achieved. Firstly the mixed-mode scattering parameters are discussed, from view point of SI performance. Secondly, frequency responses of electric-field and magnetic-field near a PCB are discussed, from view point of EMI. It is demonstrated that the proposed method is suitable for improving the SI performance and suppressing the EMI. This study has successfully reported the basic characteristics of imbalance component of differential-paired lines with turnoff point.
Classification of discontinuities
Common-mode signal
Component (thermodynamics)
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To provide the basic considerations for EM radiation from practical asymmetrical differential-paired lines with bend routing, this paper newly attempts to propose locally shielded differential-paired lines for establishing SI performance and suppressing imbalance component and EMI generated by differential-paired lines with bend (turnoff point) discontinuities. The conductive plane is placed on discontinuity region asymmetrically. The concept of locally shielded layout is based on compensation of phase-difference due to the bend discontinuities, by using electric-coupling between signal trace and a conductive plane for the shield. The proposed method is suitable for implementing on PCB, without changing layout of the differential-paired lines. To compare the effect of the locally shielded paired-lines on CM component, the |S cd21 |, which is defined as the conversion from DM to CM, are evaluated. The significant suppression effectiveness of |S cd21 | in the case study is achieved below 1 GHz by locally shielded differential-paired lines. Consequently, the validity of the proposed locally shielded layout is demonstrated in the case study. It is demonstrated that the proposed locally shielded layout is suitable for improving the SI performance and suppressing the CM in the case study. This study has successfully reported the basic method for suppressing imbalance component of differential-paired lines with bend discontinuities.
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In a differential transmission line, a large common-mode radiation is excited due to its asymmetry. In this paper, the imbalance difference model, which was proposed by the authors for estimation of common-mode radiation, is extended to apply to the differential signaling systems. The authors focus on a differential transmission line with asymmetric property, which consists of an adjacent return plane and two signal lines which are placed close to an edge of the return plane. Three orthogonal transmission modes, a normal mode, a primary common mode and a secondary common mode, are defined. Among these transmission modes, the secondary common mode is dominant in radiation, and a mechanism of the secondary common-mode generation is explained. The radiated emission which was calculated using the imbalance difference model was in good agreement with that obtained by full wave calculation.
Common-mode signal
Mode (computer interface)
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Multi-channel differential-signalling scheme is one of the key technologies for modern electronics systems. For actual system, the ideal balance or symmetrical topology cannot be established, and hence, an imbalance component is excited. To provide the basic considerations for establishment of predicting the EM radiation from practical differential-paired lines, this paper attempts to quantify the correlation between imbalance components and EM radiation from asymmetrical differential-paired line with different equi-distance routing. Finally, the dominant radiation factor is identified using our proposed model.
Transceiver
Line (geometry)
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For differential-signaling system, the ideal balance or symmetrical topology cannot be established due to bend (turnoff point) discontinuities, and hence, an imbalance component is excited. Therefore methodology and geometrical-structure for satisfying both SI and EMI performances are indispensable. In order to provide basic considerations for the realization of methods for predicting and suppressing the EM radiation from asymmetrical differential-paired lines, the imbalance component and EMI from strong and weak coupled differential-paired lines with bend discontinuities were studied experimentally and with modeling. Firstly differential-mode impedance and the mixed-mode scattering parameters are discussed, from view point of SI performance. Secondly, frequency response of electric field near a PCB are discussed, from view point of EMI. It is demonstrated that the differential-paired line with weak-coupling is suitable for improving the SI performance and suppressing the EMI. This study has successfully reported the basic characteristics of imbalance component of differential-paired lines with bend routing and demonstrates the dominant factor of imbalance component.
Classification of discontinuities
Common-mode signal
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Prediction of EMI from two-channel differential signaling system based on imbalance difference model
In a differential transmission line, a large common-mode radiation is excited due to asymmetry. To suppress the radiation, the differential line must be designed electrically symmetric. In this paper, the imbalance difference model, which was proposed by the authors for estimation of common-mode radiation, is extended to apply the differential signaling system. The authors focus on two pairs of differential transmission lines with asymmetric property, which consists of an adjacent return plane and signal lines which are placed close to an edge of the return plane. The authors define five transmission modes; two normal modes, two primary common modes and a secondary common mode. In these transmission modes, the secondary common mode radiation is dominant, and the authors evaluate the radiation using the imbalance difference model. To reduce the common-mode radiation, placing a guard trace which has the same potential as that of the return plane, we can control the imbalance and reduce common-mode radiation even the transmission line has asymmetry. The reduction of common-mode radiation can be estimated quantitatively by calculation of the imbalance of the transmission line.
Common-mode signal
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At a differential transmission line bend, a differential mode propagated in a differential transmission line is converted to a common mode due to asymmetry of the transmission line. The common mode conversion leads to problems with signal integrity. We propose a method to improve electric symmetry of the transmission line bend and thus reduce the common mode conversion. The self-capacitance and inductance of one line are different from those of another. In order to fit those parameters, the narrowing signal line for increasing self-inductance and placing a conductor shape which is like a stub are used to reduce the imbalance degree of the transmission line bend. We obtained a 20-dB reduction of common mode excitation by optimizing the signal line width and stub length. In addition, an equivalent circuit model for the transmission line bend is proposed to evaluate the reduction. The simulation results using the proposed equivalent circuit model are in good agreement with the results obtained by the 3D electromagnetic solver.
Common-mode signal
Stub (electronics)
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A study of the common-mode radiation behavior of differential signalling is presented, considering small current imbalances, which may originate from differential driver phase skew and circuit asymmetries. Two configurations are investigated, a solid ground plane and a ground plane with an open slit as an example of a ground-plane discontinuity. The external coupling voltage responsible for common-mode radiation is quantified through coupling inductances, for which closed-form expressions are derived and numerically validated. It is found that common-mode electromagnetic interference from differential signalling may become comparable to conventional single trace routing when traces are placed near the edge of the ground plane. For traces routed across a ground-plane discontinuity, differential signalling is only an effective means for reducing radiation when signal imbalance can be kept small.
Ground plane
Common-mode signal
Discontinuity (linguistics)
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