[SI-LIST] Re: Is a copper plane not tied to any net a reference plane?
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <ericsilist@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 4 May 2004 14:53:01 -0700
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The short answer is that I agree whole-heartedly with Lei: think return
current will probably flow happily on the floating plane.
Here's an excerpt of what I found when investigating something similar:
I found that, when TDR'ing a stripline trace that was referenced to both
power and ground, I got the same impedance whether decoupling caps were
populated or not. Actually, instead of a cap, I physically shorted power
and ground pins together at the launch point to keep even the parasitics
of a capacitor out of the equation. What I found was that, for the
stackup (5mil trace 7 mils above ground and 7 mils below Vcc), I saw no
substantial difference, regardless of whether I measured:
(1) with the probe referenced to GND,
(2) referenced to VCC, and
(3) with GND and VCC shorted together (at the launch). =20
Also, TDR'ing between the two planes shows a dead short.
The risetime was ~50pS (a TEK TDR), and I even slowed the risetime down
to 400pS, no change. I'm pretty sure rise-time is not a factor.
FURTHER INVESTIGATION (in case you're interested): I wondered if, by
definition of this symmetrical stripline, there isn't enough capacitance
between the planes that the return current has a low impedance path to
the reference plane. I.E., TDR'ing between the 2 planes shows a dead
short - no need for external caps (or a shorting bar, in my case).
This worked until I thought of the case of asymmetrical stripline -
would the impedance measured depend on which plane you were referenced
to? So, I built myself some crude asymmetric stripline (using a TDR
characterization board from TEK as a starting point).
I took a microstrip trace and added a layer of Kapton tape over it, with
a sheet of copper over that. This turned the microstrip into a
stripline, with the 2nd plane floating. I TDR'ed the trace relative to
Gnd, then relative to the floating plane, and with the planes shorted
together at the source (again, relative to Gnd and the floating plane).
I then added another layer of Kapton tape between the trace and the
floating plane, and repeated the measurements.
I did this until I had 8 layers of Kapton tape between the trace and the
floating plane.
Granted, this was a pretty crude experiment and there were clearly some
measurement errors, but some things were pretty obvious.
Findings:
1) Regardless of the Kapton thickness, the lower impedance measured
(referenced to Gnd or the floating plane) was approximately the same as
that as when the planes were shorted together.
2) With thin dielectrics (in the range that we typically use, < 7mils),
the impedance was approximately the same regardless of which plane was
used as a reference, and whether they were shorted together at the
source.
Conclusions:
1) When TDR'ing stripline, it probably won't matter which plane we use
as reference. If in doubt, I would TDR relative to whichever plane was
closest to the trace. If still not convinced, I would short the 2
planes together
at the source.
2) I would ensure that, when using stripline with both power and ground
planes, the trace is closer to ground than power. This is assuming the
signal is routed relative to ground elsewhere.
3) I believe that a correct model for what I'm seeing is - it's the
parallel combination of Trace-to-Plane1, Trace-to-Plane1, and
Plane-to-Plane impedances that makes up the final impedance for a trace,
relative to either Plane1 or Plane2.
Here's a representation of the experiment and the results (if it comes
through):
=20
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of eric steimle
Sent: Tuesday, May 04, 2004 2:16 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Is a copper plane not tied to any net a reference
plane?
I'm trying to settle an argument without saying which
side I'm on, and I was hoping someone could give a
quick and easy example to prove this.
You have a four layer board that looked like this
SIG1
GND
VCC
SIG2
And the maximum rise time on the board was about 2ns
(say a 100MHz clock), then if I ran a trace from one
IC to another on SIG1 the return current would flow
along the GND plane (assuming no splits in the plane
etc.)
What if I covered SIG1 with a plane of copper say
hovering 5 mil above SIG1, and then made the distance
between SIG1 and GND 100mil so..
Plane sheet of Copper
5 mil of air
SIG1
100 mil FR4
GND
VCC
SIG2
Now that plane sheet is not GND and it has no
association to any net it is just a continuous sheet
of copper about the size of the board. One of us
argues that the return current will continue to flow
along the GND plane as it did before. The other
argues that the return current will instead flow
mostly along that sheet of copper unitl it jumps back
to the GND plane as it gets closer to the IC. Any
help in this would be much appreciated.
Thanks
=20
=20
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