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

Spencer, David H (David.Spencer@usa.xerox.com)
Mon, 26 Jul 1999 13:03:54 -0400

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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