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

Tomm aldridge ([email protected])
Wed, 26 May 1999 09:54:43 -0700

I agree with Larry on this and would add that at the frequencies of
a server board with a setback of one plane vs another MAY constitute a
reasonably good slot antenna. In fact, at some conditions, the edge of
board becomes a slot radiator such that stitching becomes necessary.
you become lazy, stitching left to the layout designer and not defined
the SI engineer has been known to increase the slot radiation.
of stitching is not good. My point on all of this is to analyze on a
and technology basis and don't over generalize to a blindly followed


-----Original Message-----
From: Larry Smith [mailto:[email protected]]
Sent: Tuesday, May 25, 1999 2:45 PM
To: [email protected]
Subject: Re: [SI-LIST] : 20-H Rule for Power Planes

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

Larry Smith
Sun Microsystems

> From: Mark Freeman <[email protected]>
> To: "'[email protected]'" <[email protected]>
> 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
the ground
plane; The power plane edges should be back from the power plane a
of 20-times
the plane spacing. This reduces fringing fields from the power plane
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 -
indicate the effectiveness of this technique over frequency. Intuition
thing for this digital designer to rely upon) tells me that the
of the
fringing fields are small, thus only affecting GHz-range signals. Is
currently only of interest to cell 'phone designers, or do we need to
this technique to digital PBW design?
> Mark Freeman
> [email protected]
> Stratos Product Development, LLC

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