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
