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

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: doug@xxxxxxxxxx,Scott McMorrow <scott@xxxxxxxxxxxxx>
  • Date: Fri, 21 Apr 2006 10:19:47 -0700

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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