Re: [Discuss-gnuradio] FFT plot unit

[Martin Braun]

This pops up a lot, and hence earned a spot on the FAQ a while back:

http://gnuradio.org/redmine/projects/gnuradio/wiki/FAQ#How-do-I-know-the-exact-voltagepower-of-my-received-input-signal

...although that section could surely be expanded.

M

On 03.08.2015 09:04, Marcus D. Leech wrote:
> On 08/03/2015 09:36 AM, Sylvain Munaut wrote:
>> Hi,
>>
>>> I was able to gather results, and I am really confused with it. I
>>> generated
>>> a -30 dB signal based on the fft plot shown and transmitted it using
>>> a usrp.
>>> My spectrum analyzer received a signal at -50 dBm (-80 dB) and my
>>> receiver
>>> which also uses a usrp received the signal and plotted it at -30 dB. My
>>> question is, what is the unit of the fft plot? Is it dB or dBm?
>> They're dBFS  or dB Full Scale ...
>>
>> So they're just relative to the "full scale" range defined in GR as
>> -1.0 ... 1.0
>>
>> USRP are not calibrated instruments, so you can't map this to dBm or
>> any absolute power measurement. Only relative measurements are valid.
>>
>>
>>
>> Cheers,
>>
>>     Sylvain
>>
>> _______________________________________________
>>
> In order for Gnu Radio to display calibrated power units, it would need
> to have a very vigorously-defined interface to each and every piece of
> hardware so
>   that power levels displayed in the FFTs are in calibrated convenient
> power units, like dBm.   This in turn, would require that every piece of
> hardware that
>   connects to Gnu Radio be vigorously calibrated over their entire
> operating parameter space, including sample-rate, tuning frequency, and
> gain setting.
>   This isn't, as you might imagine, practical.
> 
> So, what Gnu Radio receives are digitized voltage samples that are
> mostly-linearly-proportional to the voltage received at the antenna
> terminals
>   of the device.  These are in turn, for purposes of convenience and
> generality, converted into a floating-point number in the range {-1.0,+1.0}
>   within a flow-graph.  But without calibration on the part of the
> end-user, they are "unitless", and you have to determine the
> proportionality
>   in the context of your own application.
> 
> 
> 
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