[SI-LIST] Re: Trace Width @ 3Gb/s

  • From: "Rudy Sterner" <rsterner@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>, "Hassan O. Ali" <hassan@xxxxxxxx>
  • Date: Mon, 10 Jun 2002 09:28:10 -0700


   The error occurs in the following type of pattern:  '0001010'.
It is at the first '1'.  The signal is NRZ.  We observed it on
the scope and noticed that the pulse resulting from the
first '010' transitions was smaller than the second pulse which
explains the error.

  As for the dimensions:  We buried the traces between two
ground planes.  We used 4-mil width, 7-mil spacing, half-oz.
copper.  Distance from trace to ground plane is 5 mils.  The
signals are AC coupled.  The individual trace impedance is
higher than 50 Ohms; however we control both the driver
and receiver impedance.  Neither seemed to help in this case.

  For the next round, we are going to run these traces on the outer
layer to minimize the effects of vias.


Rudy Sterner

----- Original Message -----
From: "Hassan O. Ali" <hassan@xxxxxxxx>
To: "Rudy Sterner" <rsterner@xxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Monday, June 10, 2002 9:00 AM
Subject: Re: [SI-LIST] Trace Width @ 3Gb/s

> Rudy,
> Could you be specific on the kind of issues you have with the SerDes link?
That way you
> can receive suggestions that are more specific to your situation.
> Yes, 4-mil tracks can be a problem. With those tracks, in a microstrip
format you need
> less than 3-mil height to achieve close to 50-ohm single-ended line
impedance (which
> might be your goal). With that small height and at that data rate,
component pads
> (especially AC-coupling cap pads, if you have any) start to dominate
heavily causing
> tremendous impedance discontinuities.
> In addition, narrow tracks give rise to higher loss - both line-loss and
reflection loss
> due to pronounced discontinuities at various locations (the connector
launch-point is
> one of the most important ones).
> Regards.
> Hassan.
> On Jun 10, "Rudy Sterner" <rsterner@xxxxxxxx> wrote:
> >
> > Hello,
> >
> >    We have some issues with a SerDes which we think
> > may be related to the trace width on the serial line.  The
> > serial  port is a differential pair passing data at 3 Gb/s.
> > The trace width is 4 mils.  Length is approximately 5 inches.
> > The differential impedance is 100 Ohms.  Other information
> > which may be of use is the board material is FR-4 with an
> > Er of 3.9.  It is specified at this data rate.
> >
> >   My question is:  Are the traces too narrow?  Should I
> > expect better performance with 8-mil traces?
> >
> > Thank You
> >
> > Rudy Sterner
> > LSI Logic
> >

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