[SI-LIST] Re: Does longer trace length cause jitter
- From: Mohamed Khaleel <gotokhaleel@xxxxxxxxx>
- To: "Joseph.Schachner" <Joseph.Schachner@xxxxxxxxxx>
- Date: Thu, 27 Jan 2011 23:51:44 +0530
Thanks a lot everyone.. I wasn't thinking on the lines of
noise susceptibility of signal due to the increased rise times. Thanks for
the quick reply. Also thanks for some good pointers.
Regards,
Khaleel
On Thu, Jan 27, 2011 at 9:55 PM, <Joseph.Schachner@xxxxxxxxxx> wrote:
> 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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