[SI-LIST] Re: vias and current pathes
- From: "Antonis Orphanou" <orphanou@xxxxxxxxxxxx>
- To: "Antonis Orphanou" <orphanou@xxxxxxxxxxxx>, "afu@xxxxxxxxxxxxxxxxxx" <afu@xxxxxxxxxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Sat, 21 Jul 2012 00:17:17 +0000
...forgot to mention that there is another factor playing into this, how much
should the capacitance and inductance be in order to have a matching
transition.....
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Antonis Orphanou
Sent: Friday, July 20, 2012 5:13 PM
To: afu@xxxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: vias and current pathes
Hello Hebert,
The constituent low of current flow is divergence-less in the application you
have described.
In the vicinity of a ground return the current will close through the path of
least inductance; when this is not available the current will return through
the path of highest capacitance.
If you look at your structure I am sure it fits this model, even in the
presence of a finite skin effect in the metallic part of the path. Kirchhoff's
law is exactly derived from the conservation of charge continuity and describes
exactly the current continuity.
"Any" ground via, parallel to the signal via will potentially pick up some of
the current return. The signal via has capacitance to the surrounding ground
vias and also the ground has inductance which limits the spread of that return
current to the ground vias closer to your signal via; so the return will choose
low inductance & high capacitance via path. If that path is not readily
available you will see the degradation on your insertion loss.
Thanks
Antonis.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Herbert
Sent: Friday, July 20, 2012 4:44 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: vias and current pathes
Hello Antonis,
for the flow of current and my primary question, which deals with skin effect,
a return path with a distance of arround some inches does not affect the
influence of skin effect in the analszed wire.
To tell it in clear images, I assume a PCB with one trace in the middle of a 4
by 4 inches area and the return path, if insist in that, is on one or both
sides on the lower plane let the middle, below the trace with the via, be free
on the lower side.
Then we have a frame on the lower side and one middle trace going from one left
to right with a via in the middle, crossing the pcb in the center.
My interrest is only in the description of the current flow, through the via
and the way of this flow.
Herbert
--- In si-list@xxxxxxxxxxxxxxx, "Antonis Orphanou" <orphanou@...> wrote:
>
> Hello,
>
> At the charge rates of interest so far in the SI applications it is very fair
> to assume that current density is divergence-less, which means every current
> has to close upon itself in a loop (just how the magnetic field lines do).
> With that in mind you can not ignore the return currents; even in the example
> that you mention, the current in the wire closes the loop through
> displacement.
>
> At really high charge rates it is possible to have an accumulation of charge
> in the volume. For example for copper, the relaxation time is ~10*e-19 sec..!
> These frequencies are way beyond the current SI applications.
>
> Conservation of charge and charge relaxation is an interesting topic.
>
> Thanks
> Antonis.
>
>
> -----Original Message-----
> From: si-list-bounce@... [mailto:si-list-bounce@...] On Behalf Of
> Herbert
> Sent: Friday, July 20, 2012 3:51 PM
> To: si-list@...
> Subject: [SI-LIST] Re: vias and current pathes
>
> Hello Aaditya,
>
> I see that you are bound to a "near DC world".
> I am coming from the RF world an in this days I am between the frequency
> domain view and the time domain view.
>
> If you take a wire standing free into the sky and you send a pulse into that
> wire, then you will have a current flow into the wire without a limitation
> with the exception of the impedance, which will probably be near the
> impedance of the free room near 377 Ohm.
>
> At least for the time of the travel of the cureent forth to the far end of
> that wire and back, you will have a view similar to the normal transmission
> line.
> So.... the loop modell of the DC world does not fit allways.
>
> The deeper reason of my primary question was, how the skin effect influences
> the path of current and the time for that travell in the 3D labyrinth of the
> layer - via - layer path.
>
> Whenever the current path for RF is longer than the current path for DC, out
> of whatever reason, then a pulse sent on that way, for example with a TDR,
> will show that extended time needed for the current to flow to the end point.
> I confess that this a typical view of a designer with digital background and
> with additional knowledge of RF designs.
>
> Herbert
>
>
>
>
>
> --- In si-list@xxxxxxxxxxxxxxx, Aaditya Kandibanda <aaditya.kandibanda@>
> wrote:
> >
> > Hello Herbert,
> >
> > Current always flows in loops, any wire/trace floating in air
> > doesn't carry current, there must be a return path in a circuit for
> > the current to establish. If the wire is floating, there is no
> > current in it. If there is no return path there is no current in that
> > trace.
> >
> > What I meant by stitching ground planes is, connect the two ground
> > planes using vias placing them "around" the signal carrying via so
> > that the return current will flow through these ground vias.
> >
> > Thanks and regards
> > Aaditya.
> > On Fri, Jul 20, 2012 at 2:43 PM, Herbert <afu@> wrote:
> >
> > > Hello Aaditya,
> > >
> > > maybe, I did not make it realy clear, but I did not talk about the
> > > return path.
> > > Imagine the trace as a conductor with reference to the surrounding
> > > universe. Then you can imagine that the return path is of no
> > > influence for the forward traveling current.
> > >
> > > Am I right ?
> > >
> > > Herbert
> > >
> > >
> > >
> > > --- In si-list@xxxxxxxxxxxxxxx, Aaditya Kandibanda
> > > <aaditya.kandibanda@>
> > > wrote:
> > > >
> > > > Herbert,
> > > > Whenever a trace changes layers, the return current flows
> > > > through the
> > > vias
> > > > that stitch the adjacent ground planes of those two layers. If
> > > > those two planes are not stitched, there would be a
> > > > discontinuity for the return path.
> > > >
> > > > Thanks and regards
> > > > Aaditya
> > > >
> > > > On Fri, Jul 20, 2012 at 2:19 PM, Herbert <afu@> wrote:
> > > >
> > > > > Hello to all,
> > > > >
> > > > > I am new here and I have searched about the real current path
> > > > > for RF on a combination
> > > > >
> > > > > trace - via - trace
> > > > >
> > > > > On the trace there is current flow more or less identical on
> > > > > both sides of the trace ( top - bottom ).
> > > > > When flowing through the via , I would think, there is a more
> > > > > or less equal corrent on inside surface and on outside surface.
> > > > > That currents pass through the via and flow then on top and
> > > > > bottom of the trace on the second side.
> > > > > What about the interchnge of current between the sides and
> > > > > which
> > > influence
> > > > > has the pad ring of the via ?
> > > > > What about the idea of having less skinn effect when having
> > > > > that situation with a ground plane on the second side ( means
> > > > > a
> > > grounding
> > > > > via ) and using so called "thermal reliefs" ?
> > > > > I have the opinion, that the thermal releafs have a negative
> > > > > effect on the overall impedance of the via construction WITH thermal
> > > > > reliefs.
> > > > >
> > > > > Waiting for good ideas
> > > > >
> > > > > Herbert
> > > > >
> > > > > --------------------------------------------------------------
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