FreeLists / si-list / [SI-LIST] Re: Q on Trace Width and Jitter for Diff Pairs

[Larry Smith <Larry.Smith@xxxxxxx>]

Charles - The width of the trace will affect the skin loss but not the 
dielectric loss as long as the trace remains at 50 Ohms.  A wider 
trace has less loss due to skin effect.  The skin effect loss usually 
dominates up to 1GHz or so, but is still present at higher frequency 
where dielectric loss dominates.

In order to maintain a 50 Ohm impedance with a wider trace, the 
distance (height) of the trace from the return plane must increase. 
For stripline dielectric material with dK=4, a 50 ohm trace has 3.39 
pF/inch and 8.47 nH/inch.  It has to or else the velocity would not be 
half the speed of light ( c/sqrt(dK) ).

The dielectric loss is often characterized by the loss tangent, which 
is determined by the real and reactive components of the current into 
the dielectric.  So with a constant loss tangent and constant 
capacitance/inch, all 50 ohm traces in a given material have the same 
dielectric loss, independent of their width.

regards,
Larry Smith
Sun Microsystems

Grasso, Charles wrote:
> Greetings!
>  
> I am stuck on understanding a contradiction - perhaps someone could
> "unstick" me?
> It is understood that (at high frequencies) the dielectric losses in a pwb
> dominates the
> losses of a transmission line. However, I have read in various publications
> that 
> increasing the trace widths improves the jitter performance of a high speed
> serial link.
> I would have thought that as a trace width increases the effects of the
> dielectrics
> would be exacerbated.
>  
> How does increasing the width of a trace improve the DJ of the link?
>  
> Best Regards
> Charles Grasso
> Senior Compliance Engineer
> Echostar Communications Corp.
> Tel:  303-706-5467
> Fax: 303-799-6222
> Cell: 303-204-2974
> Pager/Short Message:  3032042974@xxxxxxxx <mailto:3032042974@xxxxxxxx> 
> Email: charles.grasso@xxxxxxxxxxxx; <mailto:charles.grasso@xxxxxxxxxxxx; >
> Email Alternate: chasgrasso@xxxxxxxx <mailto:chasgrasso@xxxxxxxx> 
>  
>  
> 
> 
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