RGMII Interface電磁干擾解決方案分析

Abstract

本論文探討網路晶片的MAC與PHY對接時使用的RGMII Interface 分析EMI問題常用的解決對策。藉由量測clock波形及FFT後的頻譜分佈，對於125MHz clock的基頻及其於250MHz、375MHz‧‧‧等諧波頻率EMI的抑制效果進行探討，本論文亦分析不同對策。

針對125MHz的Clock使用展頻及控制slew rate兩種抑制EMI的技巧。實驗結果顯示，展頻可透過將基頻及各階諧波能量分散到較寬頻寬的作法而達到降低峰值輻射的目的，而在clock trace並聯10pF電容來降低slew rate，亦可有效的抑制125MHz及諧波的EMI。雖然這兩種技巧都可以抑制EMI，但是使用時則需滿足timing spec的規定。

This thesis investigates the effectiveness of two techniques commonly used to improve EMI performance of networking products RGMII Interface. By measuring clock waveform and obtaining its spectral power characteristics via FFT, the EMI suppressing effectiveness of the two techniques are compared.

Spread-spectrum clocking (SSC) and slew rate control techniques are employed to suppress the EMI associated with the RGMII’s 125MHz clock signal. Experimental results indicate that SSC reduces EMI by spreading power contained in each clock harmonic over a wider bandwidth. On the other hand, reducing the slew rate by shunting the clock trace to the ground via a 10pF capacitor is also effective in suppressing clock signal’s EMI. Although both techniques can be used to suppress EMI, they have to be constrained not to causing any violation in timing specification.