I want to find out what is the real transmit power of my USRP 2922, unfortunately the only results I get when looking at the spectrum analyzer is the spectral power density.

These are the results of the my measurements (in LabView - transmitter parameters are: 1Mhz IQ Sampling rate, carrier: 2.4375GHz, 0 Tx gain, QPSK modulation; RX port on USRP receiver and TX port on USRP transmitter are connected with an attenuator):

enter image description here

Now, I'm wondering how to find the transmit power of the transmitter from this. The Gain control is for the RX Antenna gain, so it's not useful since it's not power of the USRP transmitter. Maybe I should integrate this value in the band? Alternatively for a sine of the signal, calculate the value from the amplitude? Any ideas?


1 Answer 1


You're right, to convert from a power density to a power, you integrate over frequency!


Well, if neither of your transmitter nor receiver are calibrated, then you simply can't find the "real" transmitted or received power. The plots you have are "amplitude relative to maximum ADC input". You don't know what power "maximum ADC input" corresponds to without calibration.

End of story - there's not a single absolute power in your system that you can use as reference!

The solution here is to calibrate; i.e. send a signal, measure it with a calibrated spectrum analyzer, measure the same signal with your USRP, and then calculate the factor between these two powers. Due to linearity, that factor (==corrective additive term in dB) stays the same for all powers.

  • $\begingroup$ Thank you for the answer. I know that USRPs are uncalibrated devices. Unfortunately I don't know if LabView has any calibrated spectrum analyzer. I also tried using niUSRP EX Tx/Rx Continous Sync and in the results I got this: i.imgur.com/qXEZMNt.png $\endgroup$
    – MagicMan
    Commented Jan 24, 2021 at 23:31
  • $\begingroup$ I understand that by integrating by bandwidth I will sadly not find out the real power of the transmitter anyway? $\endgroup$
    – MagicMan
    Commented Jan 24, 2021 at 23:34
  • $\begingroup$ labview is software. it can't have a calibrated spectrum analyzer – where would that calibrated signal capturing take place if not in the attached hardware. $\endgroup$ Commented Jan 24, 2021 at 23:42
  • $\begingroup$ Unfortunately in my case I only have access to two USRP 2922 SDRs and nothing more. I don't have access to external spectrum analyzers from NI :/ $\endgroup$
    – MagicMan
    Commented Jan 24, 2021 at 23:52
  • $\begingroup$ doesn't have to be anything from NI. Can be any other calibrated device, too. Also, a calibrated signal generator works, too. $\endgroup$ Commented Jan 25, 2021 at 0:11

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