[SI-LIST] Re: Differential trace route question
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: James_R_Jones@xxxxxxxx
- Date: Sun, 20 Oct 2002 12:34:51 -0700
Dear All,
In trying to decide how much skew you can tolerate, consider this:
To the extent that you have un-matched the pair, you have converted
part of your differential signal into common-mode, which your
differential receiver can probably tolerate. Conversely, you have
converted part of your common-mode signal, whether due to transmitter
mismatch *or to external pickup*, into differential signal which is
now indistinguishable from the signal you are trying to receive.
This is the crux of the matter.
I hope that this insight helps.
Regards,
Paul Levin
Senior Principal Engineer
Logic Innovations, a Xyratex company
_____________________________
James_R_Jones@xxxxxxxx wrote:
> Doug,
>
> I might think about using a loosely coupled differential pair. Say you were
> trying to achieve 100 ohms differential, I might use two 50 ohm single ended
> lines such that there was little coupling between the pair. This would give
> you nearly 100 ohms differential. You can keep the traces close, but not so
> close that there is coupling. This way, you still get some benefit of
> common mode rejection, but you can insert 'pimples' or 'snakes' without
> worrying about the trace spacing affecting differential impedance. Also,
> you may experience less loss with this type of setup due to increased amount
> of 'skin'.
>
> Another thing that I might think about is the amount of skew that you are
> adding by turning your diff pair 180 degrees. If you calculate the length
> difference, how much length mismatch is there really? What is the amount of
> propagation latency that this introduces? Is this really a problem?
>
> If it is a problem, then I might consider making sure that the complementary
> signals remain in phase along the length of the trace. This could be
> achieved by the dicretionary placement of 'pimples'.
>
> I have found that for the digital differential frequencies that I am
> concerned with that none of the above really matters all that much. But I
> also realized that some designs are running much faster that what I am
> dealing with.
>
> James R. Jones
> Dell
>
>
>
> -----Original Message-----
> From: Doug Brooks [mailto:doug@xxxxxxxxxx]
> Sent: Friday, October 18, 2002 6:52 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Differential trace route question
>
>
>
> Hi all,
>
> This question came up in a seminar and I'd be interested in your inputs.
>
> Assume I have a differential pair extending "North" from a driver for a
> ways. The pair turns "East" for a ways and then "South" for a ways before
> terminating at a receiver. The inner trace is shorter than the outer trace.
>
> I am interested in controlling skew, so I want to equalize the trace
> length. I am also interested in controlling the differential impedance, so
> I want to keep constant trace spacing. These requirements are mutually
> exclusive! I will compromise what I have to on impedance to obtain equal
> trace lengths.
>
> I visualize that I have three options for increasing the length of the
> inner trace.
>
> 1. I can "snake" the extra length of the inner trace with the constraint
> that the added length must be added as close as possible to the receiver.
>
> 2. I can "snake" the extra length of the inner trace without that
> constraint. That is, the snaking can take place anywhere that is convenient
> along the trace.
>
> 3. I can add randomly placed little "pimples" along the inner trace (sort
> of like __/\__ ) (pointing away from the outer trace) each one of which
> would be small and insignificant but collectively they would add up to the
> total length I need to add.
>
> Does anyone have any strong opinions why any one of these is better than
> any other, or why there is an even better alternative?
>
> Doug Brooks
>
>
> ____________________________________________________________________________
> ___
> UltraCAD Design announces availability of its new book "Signal Integrity
> Issues in PCB Design"
> Details at www.ultracad.com
>
>
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