RE: [SI-LIST] : Q: Plane-jumping return currents

Chris Cheng (hycheng@3pardata.com)
Thu, 23 Sep 1999 19:13:49 -0700

this is only true if you have the two planes in different potential.
even if the other plane is of different potential, if you convert
the signal on that side to stripline by adding another plane with
the correct potential, it should be fine also.
chris

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Jin Zhao
> Sent: Thursday, September 23, 1999 2:09 PM
> To: si-list@silab.eng.sun.com, Eric Goodill; si-list
> Subject: Re: [SI-LIST] : Q: Plane-jumping return currents
>
>
> Hi Eric,
>
> The problem you raised on jumping reference planes can sometimes really
> become a serious signal integrity issue. Currents in vias between planes
> (such as those in stackup2) and passing through planes (such as those in
> stackup3) generate parallel plate-mode fields between planes. The
> parallel-plate mode fields propagate away from vias in the radial
> direction
> and can cause power and ground voltage fluctuations throughout the board.
> As been pointed out by Larry Smith in his yesterday's reply to your email,
> when a large number of nets switch simultaneously, significant damage to
> signal integrity can be caused by vias passing through planes. Actually,
> we've recently done a series of simulations, using SPEED97, on the effects
> of vias in the case of a large number of buses switching
> simultaneously, and
> on the effects of capacitors placed near vias. The report is at
> http://www.sigrity.com/infos/ApplicationExamples/app7.pdf. You can also
> download the report as well as reports of several other examples from our
> web site (www.sigrity.com). After entering our web site, click Support,
> then click "SPEED97 Application Examples".
>
> Regards,
>
> Jin Zhao
> Sigrity, Inc.
> 2105 Hamilton Ave. Suite 310
> San Jose, CA 95125
> Tel: 408-377-2180
> Fax: 408-377-2565
> ----- Original Message -----
> From: Eric Goodill <ericg@cisco.com>
> To: si-list <si-list@silab.eng.sun.com>
> Sent: Tuesday, September 21, 1999 10:56 AM
> Subject: [SI-LIST] : Q: Plane-jumping return currents
>
>
> > Hi,
> >
> > Consider a few different partial stackups each with a via:
> >
> >
> > Stackup 1
> > | |----------- trace B
> > plane =========== | | ==========
> > trace A ------------| |
> >
> >
> > Stackup 2
> >
> > plane =========== | | ==========
> > trace A ------------| |
> > trace B | |-----------
> > plane =========== | | ==========
> >
> >
> > Stackup 3
> > | |----------- trace B
> > plane =========== | | ==========
> > plane =========== | | ==========
> > trace A ------------| |
> >
> >
> > In stackup 1, the return currents for trace A and trace B just need to
> > migrate to the other side of the the plane which is fairly easy
> and has a
> > low impedance. However, this stackup is not preferred for PCB
> > manufacturability reasons.
> >
> > In stackups 2 and 3, the return current for trace A moving to
> trace B has
> > to jump planes. The only place that can occur is via a
> capacitance. Some
> > capacitance is provided by the interplane capacitance which works better
> in
> > stackup 3 than it does in stackup 2. Otherwise, a nearby
> bypass cap must
> > be found. The farther away the cap is, the larger the inductance (and
> > impedance) of the return current path.
> >
> > My system is running pretty fast (> 1 Gbps).
> >
> > My questions:
> >
> > 1. Is what I've described generally true?
> >
> > 2. How could one analyze how far away a "nearby" cap can be and not
> degrade
> > the signal too much?
> >
> > 3. How does the value of the cap affect this? Clearly we want a low
> > inductance package. Do I just go for the largest capacitance
> that fits in
> > a low-inductance package?
> >
> > 4. How could one analyze if the interplane capacitance is sufficient for
> > this purpose?
> >
> > -Eric
> >
> > --
> >
> > Eric Goodill Cisco Systems M/S SJ-N2
> > mailto:ericg@cisco.com 170 W Tasman Dr
> > voice: (408) 527-3460 San Jose CA 95134-1706
> > fax: (408) 527-3460 (yes, the same)
> >
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