Switching regulator noise coupled onto high speed differential links

Modern PCBs are increasingly subjected to real estate constraints. Often tradeoffs are made between ideal signaling conditions and placement feasibility. Traditional analysis of high speed signals has focused on the effects of through-channel characteristics such as insertion loss, return loss, transmitter/receiver equalization, and crosstalk due to adjacent signals of the same bus. This paper presents an issue in preproduction hardware in which a switching regulator's placement directly led to degraded margin in nearby high speed serial links via noise coupling. The phenomenon was characterized in a lab setting and subsequent hardware revisions implemented an isolation technique that proved highly effective.