[SI-LIST] Re: How to measure voltage drop on plane
- From: zhangkun 29902 <zhang_kun@xxxxxxxxxx>
- To: vrbanacm@xxxxxxxxxx
- Date: Fri, 23 Jul 2004 23:37:47 +0800
Michael
Could you tell me the detail about your method to find out the EMI source. I
agree with you that when the PCB is made, measurement is the best tool.
However, in EMI analysis, the near field and the far field do not coupled
closely.
Best Regards
Zhangkun
2004.7.23
----- Original Message -----
From: "Michael E. Vrbanac" <vrbanacm@xxxxxxxxxx>
Date: Friday, July 23, 2004 11:01 pm
Subject: [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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