Unless they have changed the definitition since they changed cycles per
second to Hertz; a dBm is "decibels referenced to one milliwatt at a specified
impedance." Back in those days, unless some other impedance was specified a
dBm
was taken to be 1 milliwatt into 600 ohms for professional audio work.
+ 67 dB is 2,000,000 times ( as close as I can read the scale on this
slipstick, or so close not to matter) the 0 dB level, and - 67 dB would be
one 2
millionth of the 0 dB level of 1 milliwatt at the specified impedance. If you
don't have a calibrated milliwatt meter, that's about 5 millivolts on an AC
voltmeter or 5 mv p-p on a calibrated o-scope.
73 Pete Allen AC5E
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