Re: [Discuss-gnuradio] Re: Maximum value of samples

[Josh Blum]

Markus Feldmann wrote:
> Firas Abbas schrieb:
> > Hi,
> >
> > > From: Sebastiaan Heunis <address@hidden>
> > >
> > > Can anyone please suggest a setup that I can use to test
> > > the maximum value of samples?
> > > Sebastiaan
> >
> >
> > The quickest way is to run grc, connect usrp to number sink and run 
> > the graph.
> I tried the Number Think, but does it show the current values or the
> maximum values ?

Numbersink updates slowly, its best used with signals that change 
slowly, like the average, rms, some of the SNR probe blocks.

The scopesink will also show the current values.

> I noticed that the values differ from the values shown in the FFT Plot.
> How to interprete the values in the Number Sink ?

Thee FFT plot is in decibels, its all relative. So if your signal is 
40dB above the noise floor in the fft plot, than you calculate SNR: 
http://en.wikipedia.org/wiki/Signal-to-noise_ratio#Technical_sense and 
life is good.

If you really want to know how many volts, how many dBm, how much is a 
count, then you must calibrate your receiver. Feed a signal of known 
amplitude into your receiver, look at the levels in scope and fft plot.

If it helps, this shows how the fft plot in dB is calculated from the 
fft: 
http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/python/gnuradio/blks2impl/logpwrfft.py#L56

-Josh

> In the FFT-Plot it is used dBFS as i think.
> Josh Blum:
> "USRP values are signed 16 bit values, so ± 32768 range."
>
> How can i compare the value from my FFT-Plot with this value ?
>
> Regards Markus
>
>
>
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