Timing Margin
High-speed serial receiver compliance standards specify allowable timing jitter tolerances using horizontal unit interval measurements. The parameter designated as eye width defines the horizontal duration where the receiver can reliably sample data without clock timing errors. Phase jitter and inter-symbol interference narrow this timing window during signal transmission across physical channels.
A narrow horizontal opening increases susceptibility to timing skew introduced by trace length variations or thermal drift.
Jitter Susceptibility
Dielectric losses and impedance discontinuities spread signal transitions across adjacent time slots, shifting the zero-crossing points of high-speed differential pairs. When signal transitions wander relative to the system clock, the available sampling window narrows, increasing the probability of bit sampling errors. Measurement algorithms measure eye width at specified threshold voltages, typically at the differential zero crossing or specified amplitude percentages.
Board layout practices control trace length matching and differential impedance to prevent timing degradation caused by cross-mode conversion and reflections. Routing differential traces over splits in reference planes disrupts return current paths, generating skew that reduces horizontal timing margins. Advanced equalization techniques in the receiver, including decision feedback equalization, restore timing margins by suppressing deterministic jitter caused by prior bit patterns.
Timing Boundary
Compliance testing uses real-time oscilloscopes to construct eye diagrams from millions of signal cycles, verifying horizontal clearance against strict mask templates. Margin testing determines how much phase noise or jitter the physical link can absorb before transmission errors occur. Receivers operating with insufficient horizontal opening suffer elevated bit error rates during burst transmissions.
Physical channel design requires maintaining clear horizontal timing margins under nominal and extreme operating conditions.
