On 2/5/2010 11:48 AM, Martin Thompson wrote:
> rickman<[email protected]> writes:
>
>> BTW, you are aware that the power plane is just as effective as the
>> ground plane for determining the impedance.
>
> Yes, but the OP needs to be aware that care can be required when
> switching your signal trace from one layer to another. When you switch
> to a layer which references the other supply rail, then the return
> current has to also switch layers. If the way it has to do that is
> via a decoupling cap a long way away then the current loop can be
> quite large.
>
> I got Henry Ott's new book just this morning, and he has some
> discussion on p630... If you use Amazon's "search in this book"
> feature from here:
>
> http://www.amazon.co.uk/Electromagne...dp/0470189304/
>
> and search for "Changing Reference Planes", you can see pp 630-631.
>
> To the OP - ...and then buy it
>
> Cheers,
> Martin
>
Hi Martin,

It would appear Mr. Ott agrees that multiple ground planes with a big
centre core are a good idea, even on a four layer board. Fig. 16-15. He
must be a smart guy!! ;-)

Also, Fig. 16-16 he specifically says that

signal
signal
ground
power
signal
signal

is _not_ recommended.


Looks like he would do

signal
ground
signal
signal
ground
signal

with a thick centre core and routed powers. This way the internal signal
layers are shielded. I tend to agree. The ssggss stack I suggested
because I almost always use laser drilled micro-vias on my boards, so I
need two signal layers on the outside. Also, my enclosures do the EMC
shielding. With standard vias, sgssgs is probably better.

Cheers, Syms.