Re: First tutorial

[Fabian Schwartau]

Hi Alexander,

that is obviously a "problem" in gnu radio ;)
It depends on the window chosen and if it is normalized or not. The 
problem with the window functions in an FFT is, that you spread the 
power of the signal over multiple bins (which also happens when you have 
spectral leakage). You can either normalize the window so that the peak 
power will be correct (what you would like to see) or that the total 
mount of power/energy in the complete spectrum is preserved (what gnu 
radio is probably doing).
You can choose a rectangular window, make the fft length an exact 
multiple of the signal period and you will get a perfect peak at a 
single bin with the correct amplitude. But rectangular windows suffer 
from a lot of spectral leakage.

Best,
Fabian

PS: Please always (also) respond to the mailing list, so that others can 
find the discussion as well :)

Am 30.03.22 um 09:47 schrieb Alexander Plotnikov:
> Greetings Fabian!
>
> Thank you very much for your response.
> I was talking about the main harmonic - the one I am generating with sin 
> source.
> I have attached a screen to be more exact.
>
> What I am expecting is to get the spectrum of a pure sin with the Qt 
> Freq Sink,
> For now the problem seems to be due to Qt Freq Sink small number of FFT 
> points.
>
> Maybe you can advice me with any other Gui spectrum block? As you have 
> probably noticed - I am very new to GNU Radio.
>
> Thanks in advance,
>
> Alexander
>
> On 30.03.2022 10:19, Fabian Schwartau wrote:
> > Hi Alexander,
> >
> > I see exactly what is shown in the tutorial and I don't see any 
> > harmonics. There are a two "shoulders" left and right to the signal, 
> > which are caused by the window function, which is applied by the 
> > Frequency sink to reduce spectral leakage. When I increase the lower 
> > boundary of the Frequency sink I can also see a lot of "noise" around 
> > -180dB, which is caused by numerical instability. Everything as 
> > expected... or did I get your question wrong?
> >
> > Best,
> > Fabian
> >
> > Am 30.03.22 um 08:19 schrieb Alexander Plotnikov:
> > > Greetings everyone!
> > >
> > > Can anybody please explain to me why in this example 
> > > (https://wiki.gnuradio.org/index.php?title=Your_First_Flowgraph) the 
> > > spectrum graph shows a harmonic far below 0dB?
> > >
> > > Thanks everyone in advance!
> > > Alexander