[SI-LIST] Re: Book - (Chassis - signal ground)

Hi Gary,
    You are indeed correct.  The difference between the two cases is only a
matter of degree.  What I failed to emphasize is that, in practice, I never
see a ground plane return path completely unperturbed and sufficiently
wide.  The consequence is that some energy, however small,  is usually
available to return via alternate routes like the chassis.  My point was 
that
if you do achieve a sufficiently well coupled return path in the return 
plane,
then other connections don't really matter.
John

GARY ROSEN wrote:

> John,
> 
> I have a question about one line you wrote:
> 
> 
> 
> John Howard wrote:
> 
>>   Alternatively if the high speed signal return path
>> is provided more intimately (read "smaller loop") in the PCB structure
>> then connecting the two grounds together increases your EMI liability.  In 
>> the
>> limit
>> case, where all signal routes are perfectly coupled to their respective 
>> return
>> paths, it actually doesn't
>> matter whether you tie the two grounds together or not.
> 
> 
> Isn't the first case just an example of the second?  If the
> high-speed signal return is in the PCB (ground plane I
> presume) then connecting the signal and chassis grounds
> outside this loop should not affect it, because
> mutual inductance will keep the return path in the same
> place and hence there will be the same radiating loop.
> Or am I misinterpreting what you are saying (or the
> topology I have outlined here)?
> 
> - Gary Rosen
> 
> 
> 
> 
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