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 <[email protected]>
To: si-list <[email protected]>
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:[email protected] 170 W Tasman Dr
> voice: (408) 527-3460 San Jose CA 95134-1706
> fax: (408) 527-3460 (yes, the same)
>
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