I/O Interfaces & Buses
Why does raising serial link speed make it harder to send signals over board traces of the same length, sometimes requiring retimers along the way?
- ATrace loss grows with frequency, so faster signals are attenuated more over the same distance
- BFast signals travel through traces faster than light
- CFaster signals make trace resistance approach zero and draw too much current
Show answer and explanation
Answer: A. Trace loss grows with frequency, so faster signals are attenuated more over the same distance
In traces, conductor loss from the skin effect and dielectric loss from the board material both grow with frequency. Doubling the speed weakens the signal much more over the same length, beyond what the receiver's equalizer can recover. So designers use low-loss board materials or insert retimers that receive the signal and retransmit a clean copy.
- A. Correct. A retimer receives and retransmits the signal afresh.
- B. Nothing travels faster than light.
- C. Losses actually increase with frequency.