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

[Karel Kulhavy <clock@xxxxxxxxxxxxx>]

On Sat, Mar 25, 2006 at 09:55:55AM -0700, John Doty wrote:
> 
> On Mar 24, 2006, at 7:41 AM, Steve Meier wrote:
> 
> >Ideally true but in reality not true. Put you oscilloscope probe  
> >tip to
> >the ground plane at a point further then the probes ground. More often
> >then not you will see some sort of noise. If the ground had zero
> >resistence and an infinite supply of available electrons this noise
> >wouldn't be there.
> 
> Not true. This usually isn't due to resistance. Taking skin effect  
> into account, at 100 MHz a copper ground plane has a surface  
> resistance of ~3 milliohms/square. On the other hand, a 3 cm diameter  
> loop has an impedance of ~50 ohms. Now this is just dimensional  

What is attenuation in decibels of an infinite copper plane 0.2mm thick
@100MHz (driven by a plane space wave perpendicular to the plane)?

CL<
> analysis, but the mutual impedances that cause the crosstalk tend to  
> be proportional to these numbers. So at this frequency you can expect  
> crosstalk due to mutual resistance to be something like 80 dB below  
> the mutual inductance crosstalk for circuits of typical dimensions.
> 
> The reason you see this on the scope is that the ground lead -> probe  
> -> probe tip -> ground lead loop picks up the induction from loops in  
> the circuit.
> 
> >Since ground is the return path of most circuits
> >(obviously not differential circuits) and two or more circuits can  
> >have
> >return paths that cross each other on the ground there will be cross
> >talk on the ground plane. In the case of digital circuits which often
> >have high frequency clocks that clock itself can propigate through the
> >ground and power and into the analog circuits where it can then be
> >amplified causing distortion.
> 
> Propagate through ground? No! The fields in a copper ground plane are  
> very low. The interference propagates through fields surrounding the  
> circuitry. The most common problem is coupled loops, as in your probe  
> experiment.
> 
> I wish EE profs would teach their students that the user of  
> Kirchoff's Voltage Law is pledging to account for all of the  
> induction in the circuit.
> 
> >
> >So it is often recomended using two grounds which are connected at a
> >single point. One for analog one for digital. The question is often
> >where and how to connect the two grounds.
> 
> Wherever you have net current flow between the circuitry on the  
> separate grounds, you want them connected at that point to provide a  
> return current path. Wherever wish to accurately carry a single ended  
> voltage between the ground systems, you want them connected at that  
> point to provide a return reference. If you're serious about EMI,  
> only differential signals may be allowed to cross between the ground  
> systems without a return connection between the grounds at the point  
> where they cross. Even there, you should be careful about common mode  
> excitation and CMRR: it may be better to provide a common mode return  
> connection.
> 
> The purpose of this is to collapse the loops that are the source of  
> the trouble. The return current has a strong tendency to flow as  
> close to the signal current as it can, so giving it a way to be  
> really close to the signal can make the effective loop area very  
> small. By reciprocity, similar considerations apply to voltage  
> transport.
> 
> Alexander Graham Bell attempted to patent the "return next to signal"  
> idea in 1878, but in 1881 the US Patent Office ruled that David  
> Brooks had beaten him to it (although Bell was successful at  
> obtaining a patent in England).
> 
> If coupling is through mutual resistance, the "single point ground"  
> idea is a good one, because in this case the coupling must occur  
> through a common conductor. For precision measurements below ~1kHz,  
> this may be the main story. But for mutual inductance coupling, no  
> contact between circuits is required. It's usually better then to  
> "let ground abound": you're much more likely to get into trouble by  
> being too clever.
> 
> John Doty              Noqsi Aerospace, Ltd.
> jpd@xxxxxxxxxxxxx
>