3 dB error in "dbm" Command

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randyc
2013-04-30
2013-05-01
  • randyc
    randyc
    2013-04-30

    As I understand it, the syntax for producing the power level in dBm to a 50 ohm load for a peak AC voltage of "Vrf" is:

    Power = dbm(Vrf.v)

    However this expression is always 3 dB too high. Example: using the QUCS 50 ohm power source set to 0 dBm connected to a 50 ohm resistor will yield an output power level of +3 dBm.

    Replacing the above expression for Power with the following exact definition:

    Power = 10log10(((.707Vrf.v)^2/50)/.001)

    Yields the correct power level of 0 dBm

    I repeated this with an AC voltage source in series with a 50 ohm resistor and obtained the same 3 dB error.

    Is this a bug or am I using the expression incorrectly ?

    Cheers,
    Randy

     
  • Frans
    Frans
    2013-05-01

    Dear Randy,
    In this document http://qucs.sourceforge.net/docs/functions.pdf on page 5, you can read that all output voltages are peak values. the function dbm however (page 121) takes the rms voltge.
    To get the correct value for a sine you could use dbm(Vrf.v/sqrt(2))

     
  • randyc
    randyc
    2013-05-01

    Thanks, I suspected that was the case :)