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December 2004

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Subject:
From:
"Jack C. Olson" <[log in to unmask]>
Reply To:
(Designers Council Forum)
Date:
Thu, 2 Dec 2004 18:50:20 -0600
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Yes, that thought is what prompted me to ask the question.

Say we have a design that has separate planes for each
voltage that we need +5V, 3.3V, 2.6V, 1.2V (and also a +12V
section that is a partial split) and ALL of those voltages
are using the SAME ground plane. Wouldn't you think one
of the voltage planes would be a lot quieter than the busy
Ground plane?
That's just a speculation, I don't really know...

And as for routing, typically you will see either a routing
pair sandwiched between two planes, or you will see two
planes coupled together with routing layers "straddling"
them to either side. Think about the straddling one for
a moment, because it is easy to imagine... In fact picture
a 6-layer board that we have all seen before where the
outside layers are only used for fanouts and non-critical
junk, 3-4 are pwr/gnd, and 2 and 5 have all your "good"
stuff. (I know some of you will try to convince me to put
the routing inside the planes, but hey, there's a billion
boards out there with pwr/gnd in the middle, right?)
Ok, now picture a critical trace routed along horizontally
on layer 2, then pops down to 5 for a vertical segment,
then back up to 2 for awhile. Now what is happening to
the return current? Is it really going to jump back and
forth between the pwr and gnd plane to stay "under" the
trace? I don't think so... or if it does, its not a good thing.
So how are all those billion boards working?
Am I missing something?

Jack






             steve kingdon
             <steve.kingdon@AL
             LIEDTELESYN.CO.NZ
             >                                                          To
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             DesignerCouncil           [log in to unmask]
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             12/02/2004 03:37
             PM
                                                                   Subject
                                       Re: [DC] Return Current Path vs.
             Please respond to         Frequency
                "(Designers
              Council Forum)"
             <DesignerCouncil@
             IPC.ORG>; Please
                respond to
               steve kingdon
             <steve.kingdon@AL
             LIEDTELESYN.CO.NZ
                     >





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Hi there,

Good question, I knew that AC follows the path of least impedance, but
had know idea at what frequency this effect would become apparent. Bonus
question, same deal, as long as it's got the path of lowest impedance.

But I have an extension to the bonus question. Assuming you have
PWR-GND plane pairs in a multilayer stackup, is one inherently noisier
than the other? ie is it preferable to route next to the GND plane....or
does that depend? I guess a factor would be the coupling between the
planes, but who is to say the GND will be less noisy than a PWR rail?

Also when we have 3 or 4 plane pairs, but with each PWR plane being a
different voltage, with different noise levels, how does this effect our
"clean" GND rail? In the end we just do what the engineers ask and put
sensitive signals next to GND planes, it sounds obvious and safe to
route next to GND planes, but without further investigation I feel there
is a lot of "ASS U ME ing" going on.

Regards,

Steve

Steve Kingdon
PCB Design Engineer
Allied Telesyn Research

>>> [log in to unmask] 03/12/04 07:54:54 >>>
Return Current Path vs. Frequency

-=-=-=-
Easy question:

Imagine a "U"-shaped trace above a solid ground plane,
with a connection at each end.

Inject a signal through the "U", and vary the frequency.

At what frequency will you see the return current
follow the shape of the U rather than the shortest path?

-=-=-=-
Bonus question:

What if I replace the ground plane with a power plane?
(I say it makes no difference. )

thanks,
Jack (the "arguing with a new engineer" guy)

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