[SI-LIST] Re: Question regarding return current in a differential pair
- From: Doug Brooks <doug@xxxxxxxxxx>
- To: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Fri, 21 Apr 2006 11:01:39 -0700
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
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
