[SI-LIST] Re: How to measure voltage drop on plane

Boris,

I've used the method to detect hidden ground plane problems that were 
the primary source of
an EMI issue that were otherwise difficult (or impossible) to spot on 
the CAD drawings after the
board has been built. It isn't always conclusive in itself but is 
usually helpful if one knows what they're
looking for and how to set up for the measurement.  I've heard the same 
argument (and others)
about the technique but in the end when I find the problem within a very 
short time (a few minutes to
a half an hour) people usually get quite interested in using it.

However, don't expect your instruments to just tell you in "ASCII text" 
where the problem is. You
have to train yourself both in the techniques and procedures for the 
measurement and the interpretation
of what you get as a result.  Although it takes a bit getting used to, 
what you need to know should
become obvious especially if you have some known issues and do some 
comparative studies against
known good "control" units.

You do, however, have a great start since you said.... 

"In my opinion the transfer impedance of a connection for signal transfer 
is more important: the lower this impedance the lower the induced voltage 
in the ground conductor."


... since that is the fundamental basis for how it works.  The rest of 
it is procedures, technique, and
interpretation of the results. 

That's why your statement about ...

"0 V is possible with a large distance between A and B as well as a high value 
when the distance is zero."


would not be correct since transfer impedance increases with distance on 
an imperfect plane.

It  would be helpful if  you do some experimenting with this technique 
and I'm sure the difficulties that
you cite will resolve themselves as you work with it. 

Michael E. Vrbanac



boris.traa@xxxxxxxxxxx wrote:

>Can somebody tell me for what reason you would like to know the voltage 
>drop on a ground plane. If this voltage could be measured it doesn't say 
>anything on what probably would appear at some inputs connected to this 
>plane nor does it say anything on what it means for EMI. The local fields 
>will mainly determine these voltage levels and emission level. Measuring 
>voltage drop between 2 positions (e.g. A and B) on a wire or plane will 
>result in arbitrary values: 0 V is possible with a large distance between 
>A and B as well as a high value when the distance is zero. It becomes more 
>complicated when current distribution is not homegeneous anymore  and 
>wavelengths are comparble to or smaller than the conductor dimensions. 
>In my opinion the transfer impedance of a connection for signal transfer 
>is more important: the lower this impedance the lower the induced voltage 
>in the ground conductor. 
>Kind regards
>Boris Traa
>
>System design engineer EMC
>
>PDSL/EMC3
>Building WDV, room 1.01
>Prof Holstlaan 4
>5656AA Eindhoven, The Netherlands
>Tel: ++ 31 40 27 43766
>Fax: ++ 31 40 27 42224
>E-mail:  boris.traa@xxxxxxxxxxxx Seri: btraa@nlsce1
>
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