Re: gEDA-user: 3 questions on a 4 layer board

[Rick Collins <gnuarm.2007@xxxxxxxxx>]

At 11:48 PM 4/30/2008, you wrote:

>The difference is I think you're talking more about signal integrity
>issues around high speed digital and I'm talking about sensitive analog
>stuff.  The key phrase in what you wrote is "any significant degree".
>The particular board I had in mind had a coupling of perhaps -60 to -70
>dB which was too much for my application but would have been totally
>insignificant for a digital circuit.  And in this case, the coupling
>path was capacitance from a signal trace to the power plane and then the
>power plane back to a different signal trace.  This I was able to
>demonstrate conclusively in that case.

Yes, you are right, my information was in the context of digital 
designs, not low level analog.  So I stand corrected.


>Again, you have to careful of what type of design.  If you have single
>ended analog circuits, you have to define what the reference is
>("ground") and you'll almost always find that the supply rejection is
>better or worse depending on the exact circuit.  Yes fully differential
>circuits can help, but lots of practical RF circuits tend to be single
>ended.

I see how the power plane in this case can be an issue.  *Any* noise 
injected to the power plane will be a problem.  But even if you keep 
your signal from coupling to the power plane, how do you keep the 
noise from coming through the IC that is powered by that plane?  I 
guess that is what PSRR is all about.  Is PSRR effective up to the 
frequencies we are talking about?  I know in most LDOs, the feedback 
loop is mostly ineffective at 1 MHz or above.  I seem to recall that 
the output impedance of many opamp starts to rise significantly by 1 
MHz.  Does the PSRR also drop off?


>You just can't count on ground/power noise from one chip not causing
>major headaches for other unrelated circuits.  I helped someone fix a
>problem in an L-band receiver once where it was the 13th harmonic of a
>clock oscillator coupling through the power supply over into his RF
>signal chain.  The solution was easy, local series impedance in the
>supply and shunt bypassing with a well defined current path to the load.
>   Its been enough years now that I don't recall what the coupling
>mechanism was on the receiver end.  I also don't recall the details of
>his power routing (plane vs traces, how many supplies, etc).

I know that RF is tricky stuff to get right.  But I have seen some 
really stupid stuff designed in under the guise of "it was a problem 
once before".  In particular, we had a GPS module which needed 
digital and analog supplies.  The RF engineer wanted an LDO on the 
*digital* supply (from a switcher) to keep noise from getting into 
the digital side of the module and coupling into the module's RF 
analog section.  I mentioned that LDOs don't do diddly squat for 
isolation above 1 MHz.  He replied that he used an LDO to fix an 
audio frequency problem from a switcher.  I couldn't convince him 
that the LDO was not needed.  If you have audio frequency noise from 
a switcher, the circuit is oscillating and needs to be fixed.  An LDO 
is just a bandaid at that point. Of course bandaids are cheap, but it 
seems silly to use them on your hands because you once got a blister 
from a bad pair of shoes... :^o


>About 15 years ago or so, EDN magazine had a special issue all about EMI
>problems.  It was a pretty good read and one of the simple facts that
>they point out has proven very useful in practice.  To have an EMI
>problem, you need a source, a path, and a receiver.  You may not always
>be able to identify all 3, but if you can break at least one, you win.
>In cases where you can identify more than one and improve them, you win
>more.  It sounds simple, but it's the first thing I remind myself of
>when I have an EMI problem.

That is a good way to look at it.  Of course the hard part is to 
recognize these three elements.

Rick



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