[SI-LIST] Re: Question regarding return current in a differential pair
- From: Oscar Lang <design97@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 21 Apr 2006 11:12:52 -0700 (PDT)
Not really sure what you have in mind by *ideal*
trapezoidal shape?
Whatever it is, if we look from the frequency domain
view: one can take a Laplace / Fourier transform
(depending on periodic nature of the waveform), and
result will be a summation of sinusoidal waveforms
with various Amplitudes. So there is always AC
component to the signal.
--- Doug Brooks <doug@xxxxxxxxxx> 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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