I shall keep & treasure this clip for future use, tho :>)
I agree with Larry on this
At 09:54 AM 5/26/99 -0700, you wrote:
>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
>From: Larry Smith [mailto:email@example.com]
>Sent: Tuesday, May 25, 1999 2:45 PM
>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
>> From: Mark Freeman <firstname.lastname@example.org>
>> To: "'email@example.com'" <firstname.lastname@example.org>
>> 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
>power planes. This rule states that the power plane should be smaller
>plane; The power plane edges should be back from the power plane a
>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
>26. I have not found any numbers - analytical, simulation or
>indicate the effectiveness of this technique over frequency. Intuition
>thing for this digital designer to rely upon) tells me that 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
>> Stratos Product Development, LLC
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