SERDES With Dual Output Buffer to Reduce Common Mode Noise

The radiation of twice Nyquist frequency and its harmonics from SERDES link is a main EMI issue in high speed products such as core switch, it is caused by common mode current generated in SERDES link, which is due to the risetime and falltime mismatch between SERDES differential signals. The reason why risetime and falltime mismatch occur is demonstrated by analyzing the circuit of current mode logic (CML) output buffer of SERDES. A new proposal is presented in this paper to address the mismatch root cause in transistor level, followed by the illustration of the mechanism how to match the risetime and falltime at the very beginning of the noise source. This new method has very little impact on the chip real estate and power consumption, while can reduce the EMI noise significantly.