[SI-LIST] Re: Question regarding return current in a differential pair
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Doug Brooks <doug@xxxxxxxxxx>,Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Fri, 21 Apr 2006 11:28:16 -0700
Doug, when viewpoints differ widely, Lee's "show me" is a sound way
of getting to the bottom of things.
I am trying to reconcile your original statements with my
understanding of physics. If you have a reference that corroborates,
I would greatly appreciate seeing them, and I will be further
educated for the effort. If I have misunderstood your statements,
would you be so kind as to clarify?
Regards,
Steve.
At 11:02 AM 4/21/2006, Doug Brooks wrote:
>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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>>
>
>____________________________________________________________________________-
>Check out UltraCAD's differential impedance and skin effect
>calculators at http://www.ultracad.com
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