Re: gEDA-user: basic anti-EMI design q

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Thu, Mar 23, 2006 at 10:11:08PM -0500, DJ Delorie wrote:
> 
> > You're going to send this board out to a fab house, right?
> 
> Right.  The current choice is pcbpool - 4 layers 20 sq, 6/6/12, qty 1,
> for $104.  Unless find two other people who want a generic cpu/net/AC
> board, in which case it's about $160 for 3.
> 
> However, it's really hard to hack the inner layers, if needed.
> 
> > So why waste time and increase risk to save two digits of cost?
> 
> Well, it's more a question of "will it help with EMI"?
> 
> > The latest thinking on this is: Use one plane.  Keep analog and
> > digital components physically separated, but don't split the plane.
> 
> Problem is, the CPU/net is between the power supply and the I/O blocks
> :-P I figured for the I/O power I could run a trace around the edge,
> so no signal traces cross it, and have the FETs bridge the gap.
> 
> > If you split the plane, you run the risk of running tracks over
> > slots and other GND structures in your board which can
> > radiate/receive & can contribute to SI problems.
> 
> Actually, I think I can easily avoid this.  The I/O block is near the
> edge anyway, so the only things that go there are the things that need
> the isolation.  The 10baset is also not much of a problem, it's near
> the P/S anyway and the gap would only surround the analog half of the
> chip and the magnetics.
> 
> I just don't know if it will make a difference.  Remember, it's not
> *generated* EMI I'm worried about, it's *received* EMI.  Hence, I'm
> trying to isolate the I/O power - the 18g wires to the thermostats,
> the 10baseT wire, and the 24VAC power wire.  Those wires act as
> antennas to pick up crap from the rest of the furnace, I'm trying to
> keep the crap away from the CPU.

And how do they get into the CPU? Is it a special CPU with 24VAC I/O
lines dimensioned for 18g wires? Otherwise it's getting into the CPU
circuit through the power or through the air. If it goes through the
power, your filtering is bad. If through the air, the CPU is not
soldered up in a watertight shielding case. And also don't forget to put
"up to 60 meters" label on your device :)

CL<
> 
> > http://www.hottconsultants.com/techtips/split-gnd-plane.html
> > www.national.com/appinfo/adc/files/questweb_dec_2001.pdf
> 
> Ah, more late night reading :-)
> 
> > p.s. I will try your latest and greatest PCB version with the
> > autotools fix tomorrow.  Thanks for working on this!
> 
> You're welcome!