I am trying to calibrate my RF daughter board in USRP software defined radio. I am trying to measure transmitted power for different frequencies at a certain set gain and transmitter amplitude. When I use a spectrum analyser, I get some voltage in dBm, so as dBm is an absolute scale I can measure the power by converting to normal MilliWatt units.

Now I connected a receiver USRP(software defined radio) and ran FFT script which is inside GNURadio. But I see that it shows a plot of frequency vs power in dB. Now I am unable to correlate between power value in spectrum analyser and power in dB given by GNURadio FFT script. I am of the opinion that you can't convert the dB value of FFT to normal watt because you don't know the reference by which the FFT is calculated. For example if there is a graph or spec which says 3 dB it means to say that when you give any input in the range specified, the output will be half of that value. So converting dB to normal units of power is dependent on what reference you divide it by. Am I right? In that case I need to know the reference that gnuradio script of FFT is calculated by. I would like to know other ways of correlating these two graphs that i am having.

  • \$\begingroup\$ Which spectrum analyser it is? is it superheterodyne.... \$\endgroup\$
    – user19579
    Commented Jul 17, 2013 at 4:42
  • \$\begingroup\$ Yes, it is superheterodyne. \$\endgroup\$ Commented Jul 17, 2013 at 8:43

1 Answer 1


The reference would be relative to the top of the dynamic range of the FFT block input, plus maybe a fixed offset.

If you want absolute power, you need to know the gain for all the components between your RF input and the FFT block, which is something you don't have readily available most of the time.

You can calculate the gain of the digital blocks, obviously, but for the analog components you need a calibration against a reference (which has conveniently been done already for the spectrum analyzer).

You have to take into account that the frequency response is flat for neither the RF parts (so you have to measure at different frequencies) nor for the IF parts (so you have to measure at different offsets from the center frequency.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.