Re: [SI-LIST] : 20-H Rule for Power Planes

Michael E Vrbanac (mvrbanac@cisco.com)
Tue, 25 May 1999 18:50:40 -0700 (PDT)

Larry,

Interesting proposition....

The fields do exist and have been shown to be troublesome whether you
believe in the rule or not.

Judging from the assumptions that you have presented, you certainly
could have a bigger problem than fringing field radiation/coupling. You
probably won't need to worry about the 20H rule. That indeed won't be
the bigger issue in that case.

Which brings me back to the point I mentioned earlier... it depends
on "the design criteria"... "how much", how you implement your system
design, etc.

BTW, the plane that is larger in the 20H rule is the one connected to
system reference. For folks who do not reference planes to the system
reference, then the rule is not usable. The fringing field will
remain uncontrolled (the 20H rule applied in the PCB would be pointless
as the whole board assembly now becomes the energized element in the
"planar structure" generating the fringing field) and other
methods/intervention must be used to alleviate any problems arising
from the situation.

re: differential circuits, common mode current, et al
I think one would be hard pressed to define the typical digital signal
as purely differential unless the driver/receiver and transmission line
constructs are specifically built that way. The last time I looked,
most of that is "single ended or unbalanced" and not "differential
or balanced".

re: common mode radiation
The basic model for common mode radiation is an "energized element"
against a ground reference which typically happens when cable shields
aren't grounded to chassis and signal grounds do not have low
impedance attachments to the system reference (i.e. chassis)... hence
my comment above that the situation for a system like this does not
need to worry about the 20H rule in a PCB as a primary problem... it will
likely have more serious electromagnetically related issues elsewhere.
I will concede that controlling those issues in such a case is
possible but certainly more difficult and oftentimes more expensive.

Regards,

Michael E. Vrbanac

>
> I don't believe in the "20-H Rule". Suppose the power plane was at
> 3.3V and the ground plane was at 0V. It would be easy to reconfigure
> the system so that the "power" plane is at 0 volts and the"ground"
> plane is at -3.3V. Does this mean that the power plane should now be
> bigger than the ground plane?
>
> The only difference between the power and ground plane is that one is a
> 0V and the other 3.3V WRT (...thats with respect to, lest I start
> another discussion...) earth ground. But even this is not true in a
> battery operated system. In any modern digital system, the impedance
> between the power and ground plane is much less than 1 ohm well into
> the EMI frequencies.
>
> The ground plane probably has a path out to frame ground and eventually
> earth ground somewhere. But if that path is more than an inch long, it
> is going to be well over 10 nH. Ten nH is 1 Ohm at 15 MHz (Z=jwL) and
> higher impedance at higher frequencies. So, above 15 MHz, the voltage
> between the power and ground planes is insignificant compared to the
> voltage across the earth ground connection.
>
> The power and ground planes should be exactly the same size. To make
> one larger than the other will simply have the effect of turning nice
> diffential currents into common mode current and common mode
> radiation.
>
> regards,
> Larry Smith
> Sun Microsystems
>
>
> > From: Mark Freeman <msf@stratos.com>
> > To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
> > Subject: [SI-LIST] : 20-H Rule for Power Planes
> > Date: Tue, 25 May 1999 10:09:37 -0700
> > MIME-Version: 1.0
> > Content-Transfer-Encoding: quoted-printable
> >
> > Now and again I come across references to the "20-H Rule" for reducing radiation from
> power planes. This rule states that the power plane should be smaller than the ground
> plane; The power plane edges should be back from the power plane a distance of 20-times
> the plane spacing. This reduces fringing fields from the power plane and reduces
> coupling to adjacent planes and free space.
> >
> > Best I can tell, this rule originated with Mike King. The earliest reference I found
> is Mark Montrose's "Printed Circuit Board Design Techniques for EMC Compliance," pg.
> 26. I have not found any numbers - analytical, simulation or measurement - which
> indicate the effectiveness of this technique over frequency. Intuition (a dangerous
> thing for this digital designer to rely upon) tells me that the dimensions of the
> fringing fields are small, thus only affecting GHz-range signals. Is this technique
> currently only of interest to cell 'phone designers, or do we need to begin applying
> this technique to digital PBW design?
> >
> >
> > Mark Freeman
> > msf@stratos.com
> > Stratos Product Development, LLC
> >
> >
> >
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