# M.data in usrp\_spectrum\_sense.py **URL:** \ **Category:** GNU Radio\ **Created:** [June 10, 2015, 9:51pm UTC](https://www.ruby-forum.com/t/m-data-in-usrp-spectrum-sense-py/242268 "2015-06-10T21:51:17Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [June 10, 2015, 9:51pm UTC](https://www.ruby-forum.com/t/m-data-in-usrp-spectrum-sense-py/242268/1 "2015-06-10T21:51:17Z")
I have been running usrp\_spectrum\_sense.py for hours now and I cannot figure out where it is getting its power\_db from. I narrowed it down to m.data. My end goal is to have the program scan frequencies from a desired list that I have created and output its power. usrp\_spectrum\_sense.py scans frequencies in a range and I have been successful in reading a file of known frequencies in usrp\_spectrum\_sense.py. I what to be able to pass a list of frenquencies (floats) into the program and let it scan for the power. But the program computes power from m.data and now from its own variable ‘freq’. Can someone explain to me what these numbers (m.data) are? ---
**Author:** ![Ashraf\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/e19adc/32.png) [@Ashraf\_Y](https://www.ruby-forum.com/u/Ashraf_Y)\ **Post date:** [June 10, 2015, 11:02pm UTC](https://www.ruby-forum.com/t/m-data-in-usrp-spectrum-sense-py/242268/2 "2015-06-10T23:02:34Z")
Hi Ashraf, that’s no real power – it’s the absolute magnitude of the FFT in each bin, in dB relative to 1. If you need numbers in real power (ie. numbers on your screen that directly relate to a signal power in real physics), you will need to calibrate your system with an external signal of which you know the (physical) power of. This will only apply for a short time, for a given configuration of frequency, sampling rate, gain. I don’t know what you mean with m.data, but I think you’re on to nothing correct. Best regards, Marcus ---
**Author:** ![Ashraf\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/e19adc/32.png) [@Ashraf\_Y](https://www.ruby-forum.com/u/Ashraf_Y)\ **Post date:** [June 11, 2015, 2:22pm UTC](https://www.ruby-forum.com/t/m-data-in-usrp-spectrum-sense-py/242268/3 "2015-06-11T14:22:15Z")
m.data is what they use to calculate power\_db (i.e. power\_db = 10\* math.log10(m.data[i\_bin]/tb.usrp\_rate) - noise\_floor\_db) from this code [https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/examples/python/usrp\_spectrum\_sense.py](https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/examples/python/usrp_spectrum_sense.py) line 303. How would I calibrate my system with an external signal? I don’t understand what you mean by this. On Wed, Jun 10, 2015 at 5:01 PM, Marcus Müller [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) ---
**Author:** ![Ashraf\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/e19adc/32.png) [@Ashraf\_Y](https://www.ruby-forum.com/u/Ashraf_Y)\ **Post date:** [June 11, 2015, 4:06pm UTC](https://www.ruby-forum.com/t/m-data-in-usrp-spectrum-sense-py/242268/4 "2015-06-11T16:06:14Z")
ah! the meaning of m.data is explained a few lines before, line 284[1]: > m.data are the mag\_squared of the fft output As I explained, the value that gets displayed is just the squared magnitude of the FFT – that means that values displayed are somehow proportional to the powers in individual frequency spans, but they are /just proportional/ – which means the relationship between the dB displayed (dB relative to the digital value of 1) and real world power (relative to for example 1mW, which could then be displayed as dBm) is something that you will have to calibrate yourself. Best regards, Marcus [1] [https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/examples/python/usrp\_spectrum\_sense.py#L284](https://github.com/gnuradio/gnuradio/blob/master/gr-uhd/examples/python/usrp_spectrum_sense.py#L284) ---
**Author:** ![Ashraf\_Y](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/e19adc/32.png) [@Ashraf\_Y](https://www.ruby-forum.com/u/Ashraf_Y)\ **Post date:** [June 12, 2015, 9:34am UTC](https://www.ruby-forum.com/t/m-data-in-usrp-spectrum-sense-py/242268/5 "2015-06-12T09:34:17Z")
Hi Ashraf, > Oh I see. That can be done, I believe. Thank you so much for helping > me understand. > You’re welcome! > I do have another question about modify usrp\_spectrum\_sense.py > And that would be? Greetings, Marcus