Re: gEDA-user: Eliminate separate Vcc planes?

[John Doty <jpd@xxxxxxxxx>]

Part of the conceptual difficulty here is that network theory can't  
tackle these issues very well. To really understand circuits you need  
to use physical optics: a circuit board is a collection of coupled  
waveguides. Network theory is the long wavelength limit of physical  
optics, but the trouble arises when the relevant wavelengths aren't  
so long.

I'm always amused by the notion of "voltage noise on ground".  
Relative to what? Sound of one hand clapping. Voltages are relative.  
Where is the electromagnetic noise field coming from, and how is it  
coupled to the port you want it to stay away from? Do you guide it  
away by configuring conductors? Better understand the leaks in your  
waveguide. Do you reflect it away with reactive components? Where  
does that unwanted energy go? Is there another reflector that just  
sends it back? Or do you try to absorb it? Decoupling resistors and  
lossy ferrites are the RF equivalent of black paint...

Inductance of capacitors? Magnetically, a capacitor is pretty close  
to a slug of metal of the same shape. It's not meaningful to quote an  
inductance for such a thing by itself: the return circuit has a great  
deal to say about that. Formally, the inductance per unit length of a  
straight conductor with magnetic field extending to infinity is  
infinite: the return circuit causes the field to fall off more  
rapidly and the inductance to converge. But that means that the test  
fixture is at least as important as the capacitor itself in  
determining the inductance: where does the return current flow?

One strange assertion I've recently heard is that one should avoid  
high K ceramics for bypassing, because their dielectric absorption  
raises their impedance. I don't buy it: I've never seen dielectric  
absorption (either on a data sheet or in a circuit) so bad it  
dominated the impedance, so a larger value high K capacitor is going  
to have lower impedance than a smaller value low K capacitor. For  
equal value, yes, the high K capacitor's impedance will be a little  
higher, but the point of going to high K is usually to get more  
capacitance.

John Doty              Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd@xxxxxxxxx




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