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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: Doug Brooks <doug@xxxxxxxxxx>
  • Date: Fri, 21 Apr 2006 13:07:52 -0400

Doug
If you do the Fourier analysis of a repetitive waveform, what you are 
indicating as the "stabilized" section of the waveform, would be the 
area of the waveform that contains the maximum low frequency content.  
If you do an analysis of any planar interconnect system, you will find 
that coupling from trace to plane extends down to as low a frequency as 
you desire to extend the analysis.  There is capacitive coupling all the 
way down to DC.  Inductive coupling is undefined at DC, but is defined 
at any frequency above DC.  Inductive coupling coefficients do change at 
lower frequencies, as the fields penetrate the metal conductors, 
however, these changes only have major implications below 1 MHz, when 
fields tend to fully penetrate 1/2 and 1 oz copper planes. Then we are 
worried about leakage through metal structures. These sorts of issues 
have a great deal of impact on analog video and analog audio signals,  
and low frequency EM Pulse susceptibility, but as for being an EMC 
concern ... I doubt it.  Far field cancellation, due to mutual coupling 
between traces and planes still exists at low frequencies, which is why 
plane referenced traces have such low radiated emissions. 


Best regards, and have a happy day!

Scott


Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

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Doug Brooks wrote:
> Mmmmmmm,
> Well..............
> Draw a picture of an *ideal* trapezoidal repetitive waveform and then 
> think about what you said.
>
> During transition time, there is a high di/dt component.  (hence a 
> high coupling component to the plane).
>
> During the part of the waveform where the signal has *stabilized*  the 
> instantaneous di/dt is very low or zero (hence a lower coupling 
> component to the plane).
>
> Nevertheless that part of the signal changes polarity twice a cycle. 
> If you want to think there is no AC component there, that is your 
> prerogative. I happen to think there is.
>
> Doug Brooks, PhD
>
>
>
>
>
> At 11:16 AM 4/20/2006, Scott McMorrow wrote:
>> 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
>>     
>>
>>
>> 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
>>>
>>>       
>>
>> ____________________________________________________________________________-
>> Check out UltraCAD's differential impedance and skin effect 
>> calculators at http://www.ultracad.com <http://www.ultracad.com/>
>>


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