All, Be sure to check the articles written by Doug Smith (a regular contributor to this list). His articles on this subject are excellent reading. A link to one in the series on the effects of splits can be read here: http://emcesd.com/tt2002/tt120102.htm Regards, Bill > Several years ago I built a test board similar to Frank's example. There > was a line on one side and a 0.100" gap between two different ground planes > on the other. I built it so that I could place shorts between the two > ground planes at various distances from the signal trace. With no short > there was a large inductive spike as the signal passed over the gap as > viewed on a TDR. As I moved a short or cap closer and closer to the trace > the inductive spike both lowered in amplitude and shortened in time. > Finally, when the short was right below the trace there was a minimum > discontinuity on the TDR trace from the gap. If I had widened the short the > discontinuity likely would have disappeared. > Lesson learned, as long as you have a short return path for the ground > currents you should not have significant impact on your signal. If the > return path is long then you will see an significant impact. If the planes > couple to each other the amount of coupling between the two planes will > dictate the discontinuity seen by the signal. > > Tom Dagostino > Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC > 2926 SE Yamhill St. Device Modeling Division > Portland, OR 97214 13610 SW Harness Lane > Beaverton, OR 97008 > http://www.teraspeed.com 503-430-1065 > tom@xxxxxxxxxxxxx > > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Loyer, Jeff > Sent: Wednesday, September 24, 2003 3:01 PM > To: fdunlap@xxxxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx > Cc: Loyer, Jeff > Subject: [SI-LIST] Re: Impact of gap on stripline trace > > > Hi Frank, > My answer to your questions is conjecture based on my understanding of = > the physics, and on some experimentation done for a scenario not exactly = > like yours, but with some similarities. > > What is the impact of the gap? > I think there are at least 3 issues here: > > 1) The path the return current must take as it transitions from: > a) 2 ground planes to=20 > b) 1 ground plane plus 1 power plane to > c) 2 ground planes again > I believe the inter-plane capacitance between the ground and power = > planes will play the most significant role in determining the impact to = > Signal Integrity and EMI. If the spacing between the plane below the = > trace and the one above the trace is small enough that there is a lot of = > inter-plane capacitance, AND the gap is small, I would expect negligible = > impact. The return current will find an adequate path between the = > different reference planes through the plane capacitance. I would = > expect this to be the case for typical stripline topologies, with = > 15-20mils separating the planes. > If the spacing between the top and bottom planes is large and the plane = > capacitances are small, the return current will find a substantial = > discontinuity at the gap, causing signal integrity and EMI problems. > It's also possible that the entire length the trace spends traversing = > under the power plane is insignificant, relative to the risetime (though = > it doesn't sound like this is the case in your scenario). > I've pasted a synopsis of previous work concerning an issue similar to = > this (return path with various reference planes) below, under the solid = > line. > I don't know how effective capacitors between the planes ("stitching" = > caps) would be. For short risetimes, the effects of caps is diminished = > by their associated parasitics. I would rather rely on inter-plane = > capacitance. > > 2) The discontinuity of the gap itself. > I would typically assume this is insignificant, since the gap will = > probably be about 10mils, and that's only significant (using risetime/6 = > rule-of-thumb and 160pS/in Tp) for a risetime of less than ~10ps. > > 3) Another exotic effect - the gap becomes a waveguide > I have heard of traces passing over a gap having substantial amounts of = > energy propagating along the gap, causing much more significant = > crosstalk between adjacent traces than would be present without the gap. = > I believe the information I saw was for microstrip; I don't know how = > much difference your scenario would be. I don't have any details I can = > share about this - perhaps someone else has seen publications describing = > the effect? > > ________________________________ > > 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=20 > (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. > =20 > FURTHER INVESTIGATION: 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 (20 mils above ground plane) and added a layer = > of Kapton tape (2.5mils thick) 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. > > -----Original Message----- > From: Frank Dunlap [mailto:fdunlap@xxxxxxxxxxxxxxxxx] > Sent: Friday, September 19, 2003 6:46 PM > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Impact of gap on stripline trace > > > Consider a stripline signal trace that passes over a gap between a GND > plane and an I/O PWR plane. The stripline is covered above by a GND > plane. > =20 > What is the impact of the gap? Is it totally unacceptable for the trace > to cross this gap (there is a continuous GND plane on the other side of > the signal trace), or are there "speeds (edge rates)" for which the gap > may be okay? If there are some "speeds" for which it is okay, how does > one determine those acceptable speeds? > =20 > Does scale matter? In other words, if the gap is not acceptable for > feature sizes common in a PCB, might the gap be acceptable at the scale > of feature sizes common inside high-speed IC packages? > =20 > Regards, > =20 > Frank > =20 > -----------| |----------- > | | > | | > | | > GND | | I/O PWR > | | > ------------------------------ > SIGNAL TRACE > ------------------------------ > | | > | | > | | > | | > | | > -----------| |----------- > =20 > =20 > ------------------------------------------------------------------ > 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: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: =20 > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages=20 > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > =20 > > ------------------------------------------------------------------ > 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: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > //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: application/ms-tnef > -- File: winmail.dat > > > ------------------------------------------------------------------ > 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: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List archives are viewable at: > //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: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //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