# Re: [SI-LIST] : Noise Voltage levels vs. EMI levels

Mon, 26 Jul 1999 22:06:24 +0100

Hven't we been here before in the last few weeks?

Coupling of noise to the big wide universe depends on how good an antenna your PCB and its housing are. Then there is conduction via power supply leads and signal connections. You can have a really "loud" PCB, so long as it can
live with itself, keeping the EM noise down for the neighbours is the clever bit. On the other hand, low noise plus a good antennna gain at the wrong frequency can give you real problems. Favourite approach is to get the circuit
(NOT the bit of flat material with conductors and devices on it) designer to design for minimum noise in the first place. How come we don't hear too much from those guys on this discussion forum???

Then do not forget the imunity issues, including magetic fields which I have referred to in the last week or so in the context of designs to be used in the European Union.

"Spencer, David H" wrote:

> I'm looking to correlate PCB ground noise voltage to an EMI levels.
>
> My understanding goes as far as: the voltage is measured from the PCB ground
> input to a point on the board differentially. That voltage drop is the
> noise voltage. The voltage drop tells us that there is common mode
> current. If we knew the impedance between the two points we could then
> calculate the common mode current.
> Since every PCB and has different geometry's and cables (power and I/O), the
> difficult unknown is the impedance.
>
> My questions:
> Is there is an absolute noise voltage level that should never be exceeded?
> Is there a way to "accurately" quantify the impedance's of the PCB ground
> plane and cables?
> Can noise voltage become a PCB design specification?
> Most importantly, when all is said and done, is there an "accurate"
> correlation between noise voltage and EMI levels?
>
> Thanks for the input
>
> David Spencer
> Xerox Corporation
>
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