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

  • From: Doug Brooks <doug@xxxxxxxxxx>
  • To: steve weir <weirsi@xxxxxxxxxx>, Scott McMorrow <scott@xxxxxxxxxxxxx>
  • Date: Fri, 21 Apr 2006 11:02:20 -0700

Gee at first I thought this came from Lee Ritchie
"Show me the data....."
Doug


At 10:19 AM 4/21/2006, steve weir wrote:
>Doug, I am amiss to find another reference that supports the idea that the 
>fields reshape concentrating around the two lines during the bit 
>interval.  If we are not talking about an audio frequency diff pair but a 
>digital one where plane penetration is not an issue then I see the 
>relative coupling line to line versus line to image plane as varying very 
>little across the signal spectra.  If you have references that show 
>differently would you share them?
>
>Regards,
>
>
>Steve.
>
>At 09:50 AM 4/21/2006, 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
>>
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>

____________________________________________________________________________-
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http://www.ultracad.com 

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