Signal Integrity Characterization of Channels With Asymmetric Via Stubs

The characterization of the signal integrity (SI) performance of differential high-speed channels that have an imbalance due to mismatched via stub length is investigated. The impact of the asymmetric stubs are characterized with regards to impedance matching, differential-to-common mode conversion and intra-pair skew computation. Simulations for the short and long differential vias are carried out in both the frequency-domain and the time-domain with a back-drilling tolerance of ±10 mils for the residual stubs. It is shown that there is a 9 Ω differential impedance variation, over -40dB mode conversion loss above 5GHz and 2~3 ps of intra-pair skew is introduced. An example is illustrated to show the impact of the stub asymmetry for the high-speed channel with a time-domain eye simulation at 40 Gbps data rate. It is shown that the short imbalanced differential via with a 20 mil stub asymmetry has the eye diagram degradation in the peak-peak jitter that doubles the jitter for the channel with vias with equal-length residual stubs.