In the context of transmission lines in high-speed circuits, which statement is true?

• A. Propagation delay has no impact on timing if the clock is fast enough.
• B. Propagation delay contributes to timing skew and must be accounted for in setup/hold constraints.
• C. Propagation delay can be ignored if impedance is matched.
• D. Propagation delay only affects analog signals.

Answer: (B)

Propagation delay is the time it takes for a signal to travel a transmission line. In high-speed circuits, different paths (data and clock) have different delays, which creates timing skew between when data arrives and when the clock samples it. Because setup and hold constraints define the valid data window around the clock edge, the line’s delay must be included in the timing budget to ensure data is stable when the clock latches it. Impedance matching helps prevent reflections but doesn’t change how long the signal takes to travel. A faster clock cannot eliminate the need to account for line delay, and propagation delay affects digital signals as well, not just analog. So the statement that propagation delay contributes to timing skew and must be accounted for in setup/hold constraints is the correct one.