AI from SAS Phy meeting

Jenkins, Mike Mike.Jenkins at
Wed Jul 11 13:54:18 PDT 2007

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Here is what I promised from my AI on dBmV, etc..  I have copied the
reflector to solicit review, as well, in case I've missed something.
*	dBmV (dB millivolts):  The decibel ratio of an RMS voltage value
relative to one millivolt.  Hence 20 mV(rms) is equal to
20*log10(20mV/1mV) = 26 dBmV.  Note that this does not depend on the
impedance level.
*	dBm (dB milliwatts):  The decibel ratio of a power value
relative to one milliwatt.  Hence, 20mW is equal to 10*log10(20mW/1mW) =
13dBm.	If this power were measured at a 50 ohm impedance level, 20mW
would be equivalent to sqrt(0.02W*50ohms) = 1V (equal to 60dBmV).
However, at a 25 ohm impedance level (the ref impedance for common mode
measurements), the same 20mW would be equivalent to sqrt(0.02W*25ohms) =
0.707V (equal to 57dBmV).
Below is the proposed Vcm spec with dBmV values on the left y-axis and
dBm values on the right y-axis:

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