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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