> interesting article in the new 'printed circuit design' mag (june 2001)
> by Henry W. Ott (EMI consultant)
> title: 'partitioning and layout of a mixed signal pcb'
>
> in it he basically makes the case that split planes are generally a bad
> thing
>
> their best use he says is to correct a badly laid out board after the
> fact and also in some cases they are needed for safety isolation
<snip>
> i won't go into all that he has covered but as the split planes are so
> pervasive and enough of a nuisance to implement i thought i would throw
> this out there
>
> Dennis Saputelli
>
> What about mixed supply voltage boards?
<snip>
> On the other hand I do agree that splitting the ground plane in most cases
> is almost as bad as the stupid design we have where they eliminated the
> analog plane in one area to route analog input signals.
<snip>
> Rob
I think that there is a very good case to be made for saying that when split
planes are used, then they should be used intelligently. But even though an
expert has suggested that split planes are generally a bad thing, I consider
that there can still be times when users legitimately want Power Plane
layers to be used for more than one net, and as such, Protel should *not*
contemplate removing support for split planes (and should even *still*
contemplate acting on a previous suggestion of mine: that the capability be
provided for split plane polygons to reside *entirely* within other split
plane polygons).
Even if support for split plane polygons was removed, users could still
implement split planes on Power Plane layers by the use of primitve items on
these layers, i.e. arcs, tracks, and fills. However, the provision of the
split plane polygon feature permits the Protel application *itself* to be
aware of which net is associated with each (partitioned) region on each
Power Plane layer, and that aspect would presumably be lost if the split
plane polygon feature was ever removed.
I appreciate Dennis alerting other users about this article, but I for one
would be very unhappy if Protel lost its current ability to *fully* support
more than one net being assigned to each Power Plane layer.
On a slightly different note, I am pretty sure that there was a time when
users could *both* place polygons on Power Plane layers *and* define split
planes on these layers, and these were *different* beasts. As such,
"standard" polygons (also) removed copper from *within* their perimeters
(remembering that Power Plane layers are *negative* layers), while split
planes *only* removed copper on their (perimeter) *boundaries*. In some
instances, e.g. PCBs incorporating modems, the loss of the genuine polygon
feature is annoying, as users currently have to effectively "pour" their own
"polygons" in the regions concerned themselves. As such, it would be nice if
this feature could be restored.
There would be complications (including in all probability some legacy
considerations) in restoring *both* polygons *and* split planes to Power
Plane layers, but perhaps split planes could be treated as *special* cases
of polygon objects (so that on Power Plane layers, some polygons would be of
the "customary" sort, while others (that have an associated "SplitPlane"
Boolean variable set True?) would be split planes instead). That way, the
requirement for Protel to support a new type of Object (either
PowerPlane_Polygon or PowerPlane_SplitPlane) could be avoided.
The other desirable enhancement for Power Plane layers has been mentioned on
assorted previous occasions, but for the record, I will repeat it now. It
would be nice if the Design Rule checking procedure could detect (through
hole) pads and vias which are disconnected from *other* pads and vias
connected to the *same* net on each Power Plane layer (or in an extension of
this concept, whenever the "quality" of each such connection is not
compliant with a minimum standard defined by the user). But this is easier
said than done, and I think that it is realistic to say that it would be
more difficult to implement than supporting split planes being totally
surrounded by other split planes on each of these layers. (However, the fact
that it would not be straightforward to implement still does not alter the
desirability of having it provided.)
Regards,
Geoff Harland.
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