Re: gEDA-user: Edge ringing filtering

[John Doty <jpd@xxxxxxxxxxxxx>]

On Mar 6, 2006, at 2:31 AM, Karel Kulhavy wrote:

> On Thu, Mar 02, 2006 at 03:05:35PM -0800, joeft wrote:
> > If I understand your description correctly, you need to raise the
> > impedance of the 2.5cm wire at 200 MHz to keep it from conducting
> > interference to the outside of the box.  Is this wire used for power?
> > If so, get a ferrite core (toroid)  and put it over the wire; pass  
> > the
> > wire through it a few times if you can.
> >
> > Also, 100 nF caps may be beyond self resonance at 200 MHz  
> > depending on
> > their construction.  Check the manufacturer's data sheet.  It may  
> > help
> > to put a smaller cap (100 pF) in parallel with the 10 & 100 nF if 200
> > MHz is your problem.
>
> But together with the other caps they form a complex RLC network.
> What's the typical ESR of these caps so I can simulate it and look  
> what
> happens there?

Simulating this kind of thing is a tricky applied physics problem.  
Approaching it from an RLC viewpoint is difficult because the EM  
fields of interest are loose outside the components (and that's the  
problem ;-).

ESR of capacitors is usually negligible in problems like this.  
There's generally no concentration of magnetic energy in the cap, so  
to get parasitic inductance you need to consider where the return  
current flows in the circuit and calculate the inductance of that  
loop, with the cap considered as just a slug of metal. Beads are  
tricky to model as well: the impedance of a bead designed for EMI  
suppression is generally intermediate between an inductor and a  
resistor over a very wide frequency range. If you're just interested  
in one frequency an LR network with Q~1 and impedance matching the  
data sheet might work, but over a wide frequency range the bead acts  
like a fractal series-parallel LR network.

Since the stray fields overlap with other subsystems in the circuit,  
they're involved. Even getting a crude model may force you into a  
multimode transmission line approach.

Every engineer I know approaches these problems by experimentation  
guided by experience and "conventional wisdom". If that fails they go  
looking for a physicist (me ;-). Usually the "analysis" is a  
blackboard sketch, a very rough estimate of the main EM effects, and  
then I'll say "try a bead there", "add a return wire here" or  
something. The only time I went to the trouble of making a detailed  
model was for a wiring harness for a spacecraft when I didn't have a  
full scale prototype to experiment with. A model successfully  
reproduced the problem and correctly predicted the consequences of  
changing the cable construction. Twisted pairs and strategically  
placed beads solved the trouble, but finding this solution through  
detailed modeling was an enormous job, only justified by the very  
special circumstances.

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxxxxxx