Hello,
I have a power spectra in dB relative to 1Vrms / sqrt(Hz) and I want
to change units to dBm (dB power relative to 1mW). Signal was measured
at 1 kOhm load. How to change this units?
E.C.
John O' gave you a very nice concise answer with everything you need,
assuming you have only the spectrum to work with. If you have access
to the analyzer and can re-run the measurement, you should be able to
put the analyzer into a mode where it displays dBm directly, though
perhaps not at 1kohm impedance. An impedance change can be accounted
for through a constant dB offset: for example, since P=V^2/R,
doubling the resistance halves the power, which is nominally -3dB. In
general it will be 10*log10(Rref/Rload)--you add that to your measured
dBm at the assumed Rref load (commonly 600 ohms for audio, or 50 or 75
ohms for RF; settable to an arbitrary "user" value in some analyzers
that then do the math for you).
In general, if you are measuring a broadband signal, either a band
power or a power spectral density measurement is appropriate. If you
are measuring one or more discrete frequencies--carriers, tones,
sidebands resulting from single-tone modulation--it's better to use a
straight spectral measurement.
Cheers,
Tom
