[SI-LIST] Re: Why return current is directly under the signal trace?
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Doug Brooks <dbrooks9@xxxxxxxxxxxxxxx>
- Date: Wed, 17 Apr 2013 11:43:33 -0400
To be even more correct we should use the term "path of least impedance"
instead of "path of least inductance" to quantify the signal/return path.
The path of least impedance is the path of lowest energy, which is the one
that all systems strive to achieve. This accounts for current spreading
all the way down to DC, and resonant phenomena.
On Wed, Apr 17, 2013 at 10:50 AM, Doug Brooks <dbrooks9@xxxxxxxxxxxxxxx>wrote:
> With all due respect, this is not (at least directly) related to skin
> effect at all.
>
> But within the topic of skin effect, there are effects called the
> "Proximity Effect": and the "Ground Effect." THESE effects are caused
> by exactly the same phenomenon that causes the return path to be
> directly on the plane directly underneath the trace: that is the
> point of lowest mutual inductance.
>
> The skin effect IS caused by inductance, but is not related to the
> return current path,
>
> I have a chapter on the skin effect in my forthcoming book "PCB
> Currents: How They Flow, How They React" (Prentice Hall) due out on
> 5/17/13. I will have a blurb on it posted on the UltraCAD web pages
> by the end of the week.
>
> Doug Brooks
>
>
>
> At 07:01 AM 4/17/2013, O Nithin Bharadwaj wrote:
> >Hi Bala,
> >
> >This is a resultant of skin depth phenomenon. Lower the frequency,
> >higher will be the penetration capability of charges in a conductor
> >and higher the frequency lower will be the penetration area. For
> >microstrip TL case, the current distribution will be spread to a
> >larger area beneath the trace when compared with that of higher
> >frequencies. This link below may give you better insight.
> >
> >http://www.rfcafe.com/references/electrical/skin-depth.htm
> >
> >Warm regards,
> >Nithin
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx
> >[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Balamanikandan K
> >Sent: Wednesday, April 17, 2013 5:58 PM
> >To: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Why return current is directly under the signal trace?
> >
> >Hi,
> >The return current for the high speed part of the signal will take
> >the path of least inductance. At high speeds, return current stays
> >tightly bunched under signal trace. Why is this directly beside the
> >signal? Please explain.
> >
> >--
> >Best Regards,
> >Balamanikandan.K
> >
> >
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--
Scott McMorrow
Teraspeed Consulting Group LLC
16 Stormy Brook Road
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
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