[SI-LIST] Re: Help Explaining Microstrip
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 23 Oct 2007 11:09:30 -0700
I've been thinking (and reading a bit) about this, so thought I'd throw
in my thoughts/questions...
Reference: http://www.physics.upenn.edu/~uglabs/exp68_doc.pdf, among
others
Two conductors close together, carrying the same DC current (connected
in series, resistors not shown), but in opposite directions.=20
V+ -------------------------------
|
|
|
-----<<<<<<<<<<<<<<<<<<<<<--------
|
----->>>>>>>>>>>>>>>>>>>>>--------
|
|
| =20
V- -------------------------------
Assuming the "<" and ">" sections are close together, they will repulse
following the formula: F =3D I^2 * (u0 * 2L)/(4 * pi * d0).
But, there's no mention of the currents in the conductors being affected
by this. I've only heard of the currents in the conductors remaining
distributed thoughout their entire cross-sectional areas to maintain the
smallest impedance (resistance, in this case). =20
Why aren't the DC currents influenced by the repulsive force? =20
If they are influenced by the force (and the effective cross sectional
area diminishes accordingly), the DC resistance would have to go up, yet
I've never heard of DC resistance going up because 2 DC conductors are
placed closed together. What am I missing?
Moving this to a PCB microstrip...
Start with the current we're talking about causing the repulsion: DC. I
wonder if we would measure some repulsion between microstrip traces and
the adjacent ground, if we had small enough strain gauges. I suspect
not, since the current in the ground plane would be distributed
throughout its entire area to minimize resistance. Force that ground
plane to be very small (such that it becomes a trace), and directly
below the microstrip trace, and I think you would have to see repulsion.
But again, I haven't heard of any change in current distribution due to
the repulsive force (and, it seems that this would apply to coplanar
traces).
Now moving to AC in a PCB microstrip...
As we move to AC, the current in the conductors distributes itself
differently to minimize impedance - the current in the plane bunches
under the trace. Again, we end up with 2 conductors close together,
carrying current in opposite directions. I suspect the conductors must
be repulsed, though I haven't heard of the distribution of the currents
in the conductors being affected. And, as was pointed out, the adhesion
to the substrate is strong enough to keep the traces from separating.
So: for the AC-case, very sensitive strain gauges would detect the
microstrip trace being repulsed by the ground plane, but why the current
distributions (and subsequent impedance) aren't affected isn't clear to
me.
Still left wondering...
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Paul Levin
Sent: Wednesday, October 17, 2007 1:44 PM
To: SI-LIST Reflector
Subject: [SI-LIST] Help Explaining Microstrip
Dear SI-LIST'ers,
I'm working on a presentation to explain transmission line to
non-engineers and I find myself stumbling over some of the basics.
(There's nothing like explaining something to bring out all of the
glitches in what you were sure you
understood!)
I'm hoping that one of you may be able to supply the missing link.
Nearly two hundred years ago Oersted and Ampere figured out that if you
have two conductors carrying current in the same direction, they would
would to pull in close to each other whereas if you had two conductors
carrying current in opposite directions, they would want to separate.
If one were to apply just these observations to microstrip, you would
expect to see all of the trace current bunched on the side away from the
ground plane and the return plane current in two bunches to either side
of the trace and as far away from the trace as possible, if not on the
bottom.
Of course, this is almost exactly opposite from what we know happens.
What is the force that overcomes Oersted and Ampere and causes the trace
and return currents to be so heavily attracted to each other?
Thank you in advance.
Regards,
Paul Levin
Senior Principal Engineer
Xyratex
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