Re: gEDA-user: Eliminate separate Vcc planes?

[andrew whyte <ajwhyte@xxxxxxxxx>]

  I can't suggest much that hasn't been already stated.

  A good reference book "Printed Circuit board design techniques for
EMC compliance" (ISBN0-7803-5376-5) it has interesting advice about
different layre stackups and the effects on inductance and decoupling
(now 9 years old but still in print)  It doesn't suggest removing the
Vcc layres for RF design.

  I think that I would still use Vcc planes and decouple heavily
(nobody ever got the sack for adding more decoupling ;-) ), but
perhaps it is worth questioning this...

  At 3GHz can you assume that your Vcc and GND are shorted any more?
Probably not, but I suppose that depends on your power topology and
the inductance of the Vcc line.  Everyone seems to agree that the
amount of capacitance that comes from the planes is minimal and that
noise can be distributed by the power planes.

  I can see that your consultant didn't voice himself in the most
politically sensitive manner.  I've never seen it advised that Vcc
planes be completely removed, but, since they don't really add much
decoupling capacitance,  you might imagine times when routing thick
tracks as differential pairs with the ground plane on signal layres
would make the design less likely to have ground loops or mistakenly
violate moating.  Perhaps you'd be best to ask him why he see's Vcc
planes as categorically wrong?

I'd be interested to hear back what he says ;-)


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