On Sat, 07 Apr 2012 09:27:53 -0700
JSL Internet <inter...@jsl.com> wrote:
> Thank you for the detailed post. It is clear that you have some
> understanding of best practices regarding digital PCB layout. However,
> you've made a number of assumptions and generalizations. Are you
> certain that there are no power and ground layers within the net5501
> multi-layer board? I would be surprised if this were true, but I'll
> leave it up to Soren to clarify.
Uhm? I've never said that there are no power planes. To the contrary
i i'm pretty sure the net5501 has power planes. If you look at the
board, sepecially at the layer beneath the surface, you dont see any
tracks, actually you dont see any structure. This is a 99% sure sign
that you are looking at a plane. Of course, something could block the
view down to the next layer and give the impression of a plane, but
that's very very unlikely
> The photo you posted shows four bypass
> capacitors surrounding the RAM chip in very close proximity. It would
> be difficult for any layout designer to get them any closer to the
> chip. Sharing capacitors is just fine as long as there are separate
> runs from the capacitor to each chip. The whole point of the capacitors
> is to offset the inductance of the PCB power runs. It is physically
> impossible to have zero-length runs from the capacitors to the chips.
> The chips have small internal capacitors anyway. Your photo
> demonstrates a good PCB design.
I did not say that the capacitors are far away. They are near enough.
But i critisized that there are not enough of them. There are several
power supply pins per chip for a reason. And you are advised to put
a blocking capacitor at each of them. And the board would have had enough
space for more capacitors. Hence, i say that the board shows clearly
_bad_ PCB design.
> If I were having problems with a small SBC (net5501) that only
> occurred when it was attached to an RF transmitter (your WLAN card), I
> would be looking at the RF susceptibility of the SBC, and the isolation
> of the transmission line and transmitting elements from the SBC. It's
> doubtful that Soren did any RF susceptibility testing or analysis of the
> computer. Most computer manufacturers do not bother with such things.
> The metal boxes he sells for the SBCs should take care of most of the
> problems anyway. It's entirely likely that the problems that you've
> "fixed" are related to the RF susceptibility of the net5501. Small
> pieces of carefully applied brass or mu-metal in selected circuit areas
> would probably have accomplished the same thing that you did by adding
> some capacitors.
No, RF suceptibility looks different. Beside, if the board would be
suceptible for RF emissions from a WLAN card with which it is sold
on the soekris website, i would say that something is very wrong.
But lets focus on technical arguments: As you said, you are an experienced
RF and digital engineer, then you know for sure that wires that run
unprotected by a nearby ground plane and/or run in large loops are
susceptible for EMI. I cannot see any such wires/tracks on the board
at all. All run straight and quite short. Beside, the signals are all digital
which means that the field strength to cause problems would have to be
large. Larger than a hermetically shielded RF module can produce.
And what really destroys that argument is, that soldering capacitors
at the power supply pins fixes the crashes. Power supply is something
very low impedance, to even get a microvolt of induced voltage into
such a wire (which runs a couple of mm from the plane up and to the pin),
you need a multiple watts of directed signal. That's something you dont
have in such a system.
Attila Kinali
--
Why does it take years to find the answers to
the questions one should have asked long ago?
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