RE: [SI-LIST] : trace width for clock routing- wider/narrower?

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From: Weber Chuang ([email protected])
Date: Sun Mar 12 2000 - 18:34:07 PST

Hi all,
  I think John's question resides in that should 5 mils or 6 mils or maybe
8mils or 12 mils of trace width should be used, since we all know that
coupling is in inverse proportion(to some order) to the trace spacing,
especially in the PC platform design today, the clock generator usually
shows a Rout of about 30~40 Ohm, and to match the impedance , a trace width
of 8~15 mils should be used, or a series resistor(maybe 10~33 Ohm) has to be
used, why such a contradiction exists? For better real estate efficiency, 5
mils trace is the most popular one, yet the Rout is so low that we might
need to use series termination? Here I want to summarize my question also
and may have your kind and valuable comment and help.
1. will a wider trace cause more EMI issue than a narrower trace with a
series term if the Rout is small? (Assume that the return path is good).
2. If the return path is somewhat ruined because there are so many power
level in the PC design today, then which one would be impacted more than the
other one? I mean in SI regard and EMI regard.

  Best Regards
    Weber Chuang( e)
 Signal & Timing Integrity Engineer,
 VIA Technologies, Inc. Taipei, Taiwan, ROC
 TEL : 886-2-22185452 ext : 6522
  mailto:[email protected] <mailto:[email protected]> <>
 Very Innovative Architecture


-----Original Message-----
From: S. Weir [mailto:[email protected]]
Sent: Monday, March 13, 2000 9:29 AM
To: [email protected]
Subject: Re: [SI-LIST] : trace width for clock routing- wider/narrower?


Assuming stripline or microstrip construction, set the trace width to get
the required impedance and set the spacing to get the required crosstalk
isolation. The amount of attenuation for a given amount of spacing required
depends largely on the height to the nearest reflection plane, and the
parallelism to other traces. If 5/10 gives you 50 ohms, then 5/25 will give
roughly 6X the attenuation for any amount of parallelism beyond the critical
length that 5/10 gives you and still maintain 50 ohms.

The above does not hold true if you are using coplanar waveguide. There
were various discussions of CPW characteristics on this thread about a month


At 09:04 AM 3/13/2000 +0800, you wrote:

Dear All SI experts,

For clock routing, does wider trace width have more advantage than narrower
one ,in term of better electrical properties, under the condition of same
trace to space ratio and good match in impedance with source terminators?

For example, 5 mils trace width with 10 mils trace spacing versus 10mils
trace width with 20 mils trace spacing.

Thank you for your helps in advance.

John Lin
SI Engineer
Quanta Computer Inc.,Taiwan, R.O.C.
Email: [email protected]
Tel: 886+3+3979000 ext. 5183

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