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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: doug@xxxxxxxxxx
  • Date: Fri, 21 Apr 2006 14:07:52 -0400

Doug
If I have misunderstood your point, I apologize.  I thought we were 
talking about EMI from a differential pair above a plane. Is your 
contention that the energy contained in the bit intervals between the 
rising and falling edge is contained primarily between the differential 
pair traces (and not between the traces and the plane) and therefore the 
spatial distance between the traces in the pair will have a large effect 
on radiated emissions, because of the larger inductive loop area?

Scott

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

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



Doug Brooks wrote:
> Well, all this is true, of course.
> Just off point.
> Doug
>
>
> At 10:07 AM 4/21/2006, Scott McMorrow wrote:
>   
>> 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
>>
>> <http://www.teraspeed.com>http://www.teraspeed.com
>>
>> Teraspeed® is the registered service mark of
>> Teraspeed Consulting Group LLC
>>
>>
>> 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
>>>>         
>>> ____________________________________________________________________________-
>>> Check out UltraCAD's differential impedance and skin effect calculators 
>>> at http://www.ultracad.com 
>>>       
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