# [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 09:50:28 -0700

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 ------------------------------------------------------------------ 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: http://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: http://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

**Follow-Ups**:**[SI-LIST] Re: Question regarding return current in a differential pair***From:*Scott McMorrow

**[SI-LIST] Re: Question regarding return current in a differential pair***From:*steve weir

**[SI-LIST] Re: Question regarding return current in a differential pair***From:*Oscar Lang

**References**:**[SI-LIST] Question regarding return current in a differential pair***From:*Michael Kotson

**[SI-LIST] Re: Question regarding return current in a differential pair***From:*Doug Brooks

**[SI-LIST] Re: Question regarding return current in a differential pair***From:*Scott McMorrow

## Other related posts:

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