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

["Hassan O. Ali" <hassan@xxxxxxxx>]

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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