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

L D Miller (ldmiller@nortelnetworks.com)
Wed, 26 May 1999 11:16:01 -0700

Wrong guy--

I shall keep & treasure this clip for future use, tho :>)

I agree with Larry on this

Larry Miller

At 09:54 AM 5/26/99 -0700, you wrote:
>I agree with Larry on this and would add that at the frequencies of
>interest
>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
>the
>board becomes a slot radiator such that stitching becomes necessary.
>Lest
>you become lazy, stitching left to the layout designer and not defined
>by
>the SI engineer has been known to increase the slot radiation.
>Uniformity
>of stitching is not good. My point on all of this is to analyze on a
>use
>and technology basis and don't over generalize to a blindly followed
>rule.
>
>Tomm
>
>
>
>
>
>-----Original Message-----
>From: Larry Smith [mailto:ldsmith@lisboa.eng.sun.com]
>Sent: Tuesday, May 25, 1999 2:45 PM
>To: si-list@silab.eng.sun.com
>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
>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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>

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