On Feb 5, 8:45 am, Symon <symon_bre...@hotmail.com> wrote:
> On 2/5/2010 11:48 AM, Martin Thompson wrote:
>
> > rickman<gnu...@gmail.com> 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...ngineering-Hen...
>
> > 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.

In reply to both of your posts, I will say that there is a *lot* of
misinformation out there. There is *NO* one way to stack up PCBs. I
once took a class in "High Speed Digital Design" with Lee Ritchey.
Some "experts" in the field will explain the theory behind what they
say. Lee Ritchey not only gives the theory, he also shows detail
simulations and even builds test boards to verify that what he is
saying is how it works in the real world. That impressed me
greatly.

As to the return current having to "jump" between layers being a
problem, if you use the ssgpss stackup and have the power and ground
very close rather than widely spaced, the capacitive coupling allows
the signal to switch between them without issue. In fact, when
splitting a plane for multiple power sections, the return current will
switch from one power plane, to the ground plane and back to the next
power plane as if they were all one plane. This is because of the
capacitive coupling between layers. Of course this only works for the
highest frequency components of the signals, but that's all we really
care about, no?

-------+ +-------> Return Current
=======| |======== Power Planes
| |
+--+
=================== Ground Plane

The ascii art may not come out too well depending on your browser or
newreader, but I hope you get the idea.

I couldn't view the pages in Ott's book so I can't respond to that.
The one point I most learned from Lee Ritchey's course is that you
should never take any expert's opinion as fact. Many experts make
mistakes and a number of things look good on paper while the real
world works differently. Only trust an expert opinion if it is backed
up by reliable proof. How does Ott "prove" his analysis, or is it
just a paper analysis?

Rick