[SI-LIST] Re: Question regarding return current in a differential pair

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: doug@xxxxxxxxxx
  • Date: Thu, 20 Apr 2006 14:16:33 -0400

Doug
I beg to differ.  If the signal has "stabilized" there is therefore no 
AC component.  If there is no AC component, there is nothing to radiate.

Scott

Scott McMorrow
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Doug Brooks wrote:
> In my humble opinion, and not counting common mode currents:
>
> During the signal rise and fall times, the return current tends to flow on 
> the reference plane, just as signals on single-ended traces do.
>
> During the time that the signal is "stabilized," there is no coupled signal 
> on the plane and the loop is around from one trace of the differential pair 
> to the other.
>
> It is during this latter phase of the signal that loop area (as in EMI) 
> might be an issue. During my signal integrity seminars I show some 
> animations that illustrate this pretty clearly.
>
> Doug Brooks
>
>
> At 09:46 AM 4/20/2006, you wrote:
>   
>> Hello Gurus,
>>     I have a question regarding the return current for a high speed
>> differential signal.  I've seen a couple of sources that state that all
>> of the signal return current is handled by the other half of the
>> differential pair. It seems more logical to me that there must be some
>> amount of return current (not just common mode) on the reference
>> plane(s) as well.  What's the truth?
>> Thanks in advance,
>>
>> Mike
>>
>> Michael Kotson
>> Project Engineer
>> VT Miltope
>> 4900 Pearl East Circle Suite 106
>> Boulder, CO 80301
>> (303) 473-0388 x132
>> http://www.miltope.com <http://www.miltope.com/>
>>
>>     
>
> ____________________________________________________________________________-
> Check out UltraCAD's differential impedance and skin effect calculators at 
> http://www.ultracad.com 
>
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