[SI-LIST] Re: reference plane

Jeff,

This is the stuff of good engineering.  Thanks for sharing it.  I've made the
same kinds of measurements myself with the same results.

Lee

Loyer, Jeff W wrote:

> The short answer to your second question (Is this amount of AC decoupling
> achievable solely through the interplane
> capacitance?) is yes.
>
> The short answer to your first question is yes, you would get the same
> measurement if you measured a trace between 2 ground planes.  The following
> is a longer response to that question; it's an extract of an experiment I
> did:
>
> 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).
>
> 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.
>
> Jeff Loyer
>
> -----Original Message-----
> From: Ege Engin [mailto:engin@xxxxxxxxxx]
> Sent: Wednesday, June 26, 2002 2:19 AM
> To: jeff.w.loyer@xxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: reference plane
>
> Jeff,
>
> If the stripline has an equal distance from the power and ground planes,
> then
> logically the measurements with respect to ground or power would be the
> same.
> But your measurements also show that, a stripline between two ground planes
> (i.e., the two planes are shorted at measurement ports) would give the same
> results. Please correct me, if I misunderstood you.
> Is this amount of AC decoupling achievable solely through the interplane
> capacitance?
>
> Regards,
> Ege
>
> "Loyer, Jeff W" schrieb:
>
> > A few thoughts, extracted from my response to a similar question...
> >
> > The original question from last October:
> > An internal signal layer in a multi-layer stackup is situated between
> ground
> > and a power plane. If the power plane splits into two islands is the
> effect
> > significant on the signal layer?  (Dielectric thickness between layers = 8
> > mil.)
> >
> > My response:
> > Assuming your planes are very wide, and since the space between the planes
> > is kept small, there shouldn't be any significant effect to Z0 at these
> > geometries.
> >
> > I had the opportunity to measure impedances of stripline traces which run
> > between ground and power planes.  I found that the TDR was identical
> > regardless of which plane I measured relative to.  After some
> investigation,
> > I concluded that for the geometries involved (only 14mils between planes),
> > the coupling between the planes was strong enough that, AC-wise, they were
> > one and the same.
> >
> > The identical situation should hold true for your configuration, if your
> > planes are wide enough to have strong coupling between them and ground,
> and
> > the space between the 2 planes isn't wide enough to cause a discontinuity.
> >
> > In summary,
> > If there is strong coupling between the ground and power planes, they are
> > essentially the same for high-speed reference.  If they aren't strongly
> > coupled, there will definitely be a difference between which is the
> > reference.  People have used "stitching caps." (capacitors between the two
> > planes at the location where the signal transitions from one reference to
> > another) to remedy the problem, but I'm skeptical of the value of these.
> > For the rise-times we're at today, I believe those caps. would have little
> > real value (the parasitics would be too great).  I.E., a TDR done without
> > the caps. populated would look the same as one done with the caps. in
> place.
> >
> > I believe the best approach is to either keep referenced to the same plane
> > throughout the entire topology (and most chips reference signals to
> ground),
> > or only transition between planes which are strongly coupled.
> >
> > My 2 cents.
> >
> > Jeff Loyer
> >
> > -----Original Message-----
> > From: evillaf@xxxxxxxxx [mailto:evillaf@xxxxxxxxx]
> > Sent: Thursday, June 20, 2002 10:36 AM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] reference plane
> >
> > Can anyone explain the advantages and disadvantages of
> > using a power plane for a reference plane instead of a
> > ground plane?  I have always used both as a DC reference
> > in the past.  Now I am beginning to hear arguments that
> > only GND planes should be used for critical signals.
> > This becomes somewhat impractical for some boards.
> >
> > Thanks in advance.
> > Ellis
> > ------------------------------------------------------------------
> > 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 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
> >
> > ------------------------------------------------------------------
> > 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 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
> >
>
> ------------------------------------------------------------------
> 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 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




-- Binary/unsupported file stripped by Ecartis --
-- Type: text/x-vcard
-- File: vcard.vcf
-- Desc: Card for leeritchey@xxxxxxxxxxxxx


------------------------------------------------------------------
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 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

Other related posts: