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dB vs. dBm

Discussion in 'Electronic Basics' started by ssylee, Oct 24, 2008.

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  1. ssylee

    ssylee Guest

    Not sure if this is an appropriate group to ask this question.
    Basically, I'm trying to add 20 dB (from an attenuator) to a signal
    measured on a spectrum analyzer, which has the units of dBm. From
    reading some search results on google, I'm aware the dB is a measure
    of relative power where dBm is a measure of power relative to 1 mW. In
    order to convert between the two units, do I have to know about one of
    the power involved in the dB unit, which is the attenuator?

    Thanks in advance!
  2. terryS

    terryS Guest

    I agree. dBm is a measurement related to Zero dBm being one milliwatt.
    Decibels are a logarithmic ratio.

    So 100 milliwatts compared to 1 milliwatt.
    The logarithm of 100 = 2.0; so 100 milliwatts would be 2 Bels larger/
    louder/stronger etc.

    Since we are talking 'decibels', 2 bels = 20 decibels (tenths of a

    Btw a quick way to think about dBs is as follows.
    Every time a power ratio doubles. For example from one m.watt to two
    m.watts, or from 50 to 100 m.watts it is aratio of approx. 3 decibels.
    Derived from logarithm of two = 0.3xxx (approx).

    So power ratio:
    1 to 2 = 3 dB.
    2 to 4 = 6 dB.
    4 to 8 = 9 dB.
    8 to 16 12 dB.
    16 to 32 15 dB.
    64 18 dB.(Getting there!)
    20 dB.*This 20 dB is pwr. ratio 100 times more/less.
    128 21dB.
    256 24 dB.
    512 27 dB.
    & 1024 = 30 dB.
  3. Eeyore

    Eeyore Guest

    You also need to know the impedance of the circuit. Typically 50, 75 or
    600 ohms.

    dB is a RATIO (either power or voltage or SPL)

    dBm is a measure of POWER

    I think wikipedia covers it quite well.

  4. Eeyore

    Eeyore Guest

    One thing puzzles me here. Attenuators REDUCE signal level NOT add to it.

    Can you clarify what's going on here please ?

  5. SUREN

    SUREN Guest

    same as the friends aa toldd
    a c
  6. Eeyore

    Eeyore Guest

    Let's say it's advisable to know that the impedances have to be the same.

  7. Jasen Betts

    Jasen Betts Guest

    me guessing:
    OP wants to know how much to add to the readings his device gives to
    get the unattentuated value.

  8. Eeyore

    Eeyore Guest

    Could be. It wasn't clear to me.

  9. neon


    Oct 21, 2006
    DB Is a ratio of units. To me DBm could mean modulation as opposed to mw ma mv or whatever.
  10. Guest

    He's measuring something on a spectrum analyser through a 20dB
    attenuator and now has
    to add the 20dB to the values seen on the analyser to get the true

  11. neon


    Oct 21, 2006
    Take that signal and amplfy it by A=100 =20 db
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