At 10:17 AM 8/28/96 -0400, you wrote:
>GEE JOE 3584 wrote:
>> 
>> Thanks for all the insight into emi.
>> 
>> I have one additional question.
>> 
>> Bury signal trace between ground planes to shield the signals.
>> 
>> What if we bury signal traces between a ground plane and a vcc plane ?
>> 
>> Thanks,
>> Joe Gee

>This can be tricky depending upon alot of things, Joe. 
>Basically, keep your traces much further away from the 
>power plane than the ground plane if you can.  
>
>If you are running traces from the "power plane" side of the 
>board (I hope I'm not being vague) to between the ground and 
>power planes, referenced as close as possible to the ground plane, 
>you can run into other problems. 
>
>Changing from one common reference plane (say a power plane) 
>to another (say a ground plane) can cause problems.
>
>Doug
>-- 
>
>*******************************************************


See Printed Circuit Design Magazine, August 96
"The Effects of Vias on PCB Traces" where this
effect was explicitly looked for and NOT found.
In general, there is no practical difference in where
you reference to, assuming reasonable ac filtering on
the various power planes. This is especially true
if your trace is between a power and a ground plane.
But in the study referenced in the article, there
were no ill effects even when a trace moved several
layers and referenced to a completely different plane!

Nevertheless, it is good practice to try to keep the
trace confined between a single pair of planes.

Doug Brooks

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