Re: [Discuss-gnuradio] Measure the power using USRP

On 12/07/2015 10:25 PM, w xd wrote:

Thanks.

           What is the deviation between the result calculate by the formula and the result measured by the calibration source for the USRP N210?  For example,when I ues the formula:10*log10(var(x))+30 to calculate the power of the receiver data is -20dbm.What is the true range for the true result?

           Thanks.

I can't tell you that, because there's the whole bit about:

frequency/bandwidth/gain/sample-rate

The numbers you get out of the end of a Gnu Radio are (largely) linearly-proportional to the power as seen at the antenna inputs.
  The purpose of using an external calibration source is to determine what the proportionality constants are for each of your
  configurations of settings of frequency/bandwidth/gain/sample-rate.

The FFTs in Gnu Radio necessarily can only show dB relative to full-scale.  Gnu Radio has no way of knowing what those actually
  correspond to in terms of power as seen at the antenna terminals.

2015-12-08 10:53 GMT+08:00 Marcus D. Leech <mleech@ripnet.com>:

On 12/07/2015 09:34 PM, w xd wrote:

Hi,
    I want to measure the power.I'm now transmit the OFDM signal.

    I use the usrp to receive data,and I save it to a file.I read all the data.Then:
I use the formula to calculate the power:
    10*log10(sum(abs(data).^2)/0.001)   dbm
    Is it the right way to calculate the power under the ofdm?
    Thanks.

Best Regards,
z sw

Instantaneous signal power is proportional to:

(I**2)+(Q**2)

You can average that to whatever interval you want, and then scale (perhaps converting into dB, as above).  But to determine *absolute*
  power at the antenna terminals you'll need a known calibration source or two, and use that to come up with calibration constants for
  your particular setup of frequency/gain/bandwidth/sample-rate.




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