[SI-LIST] Re: Does longer trace length cause jitter
- From: Joseph.Schachner@xxxxxxxxxx
- To: Mohamed Khaleel <gotokhaleel@xxxxxxxxx>
- Date: Thu, 27 Jan 2011 11:25:07 -0500
Traces (and cables, to a lesser degree) have loss that increases with
frequency. Therefore longer traces slow down the rise time of the signal.
Even if that was the only effect, since any noise translates into jitter,
for a slower edge the same noise will translate into more jitter. And that
is not the only effect: If the signal takes more than 1 UI (UI = unit
interval = bit time) to settle to its final value then a rising edge that
starts after one 0 bit starts from a different voltage than a rising edge
that starts after two consecutive 0 bits. If the transition rate is always
the same, then these two cases will cross the transition voltage at
different times. That is Data Dependent Jitter (or perhaps I should say,
it's Inter Symbol Interference, one form of DDJ). In severe cases the
final voltages might not be reached for several UI, and the eye might be
very small or actually closed - in which case you need some kind of
equalization to boost high frequencies relative to low frequencies. Note
that if the high frequency content needed to open the eye is attenuated so
much that it's really in the noise (SNR approximately 0dB), then
equalization at the receiver probably can't help.
So, long traces DO add jitter due to high frequency attenuation causing
DDJ. Try putting 8Gb/s down 24" of FR4 and you'll see.
While I'm writing I should point out that most PC boards are not just a
nice trace. Vias and connectors cause impedance discontinuities, which
cause reflections. That distortion can be another source of jitter. There
are many people on this list who know much more about this topic than I do.
I'm sure someone will point you at some good reference material.
--- Joe S.
LeCroy Corp
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