Connect with us

How to change Vrms to dBm?

Discussion in 'Electronic Design' started by [email protected], Mar 18, 2008.

Scroll to continue with content
  1. Guest


    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?

  2. GPG

    GPG Guest
  3. To convert 1 Vrms / sqrt(Hz) to power, you need to specify bandwidth.
    If you just want power spectral density, that's in W / Hz. The figure
    1 Vrms / sqrt(Hz) at 1000 ohms is a PSD of 1 mW / Hz, or 0 dbm / Hz
    (by (E/sqrt(Hz))^2 / R).
    If your spectrum has a point specified at +10 dB relative to 1 Vrms /
    sqrt(Hz) @ 1Kohm, then at that point it should have +10 dBm / Hz, with
    the same 1 Kohm impedance, since dBs represent a power ratio.
    If you want actual power, and if the spectrum is flat, you can
    multiply the PSD by the bandwidth, e.g., +10 dBm / Hz is 10 mW / Hz.
    If BW is 1 kHz, power is 10 W, or +40 dBm.
  4. Tom Bruhns

    Tom Bruhns Guest

    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.

  5. Robert Baer

    Robert Baer Guest

    Depends on the impedance and standard.
    In audio there two standards: Western Electric which i think was 500
    ohms, and modern audio which i think is 600 ohms; if i am wrong,then
    switch the numbers.
    In RF, there are nominally two standards: 50 ohms and 75 ohms.
    If you did your work at a different impedance level (say 93 or 110
    ohms) then use that.
  6. Jim Thompson

    Jim Thompson Guest

    I have a little Windows utility that calculates dBm from VPP and Zo,
    and vice versa... written by oldest son Aaron.

    If you like a copy drop me an E-mail.

    ...Jim Thompson
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day