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

Gary L. Sanders (gsanders@ricochet.net)
Tue, 25 May 1999 19:00:10 -0700

I agree with Larry- also, if your power planes have a high-frequency potential with respect to ground, then they will radiate; but I would want to see proof first that the power plane is radiating. With a goodly amount of
interplane capacitance, and good decoupling practice to minimize the impedance between the power/ground planes, there should be no difference in how the edge of the board is treated. I've measured the impedance of one of our
boards recently, and although it had a number of resonant peaks, the impedance was always well under an Ohm. The impedance of the plane will be significantly less than this, because of its low inductance; but I would want field
measurements taken in a screen cage to prove that there is radiation off the edge of a good power plane design, and that there could be any improvement with this 20-H rule.
--
Regards,
Gary L. Sanders, Staff Analog Eng., gsanders@ricochet.net
L3 Communications, Inc. Celerity Systems   www.csidaq.com
Cupertino, CA USA   dir. (408) 861-7325 fx (408) 873-1397
    Ultra Fast Acquisition & Data Generation Systems


Larry Smith wrote:


> 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




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