Hi Markus, Thanks for the reply. The explanation is clear. I signed up to the GRC Wiki and I'll try to suggest something. Unfortunately, this may take a long time, as I'm just getting to know GRC.
Best regards, Maciej czw., 30 sty 2025 o 16:51 Marcus Müller <mmuel...@gnuradio.org> napisał(a): > Hi Maciej, > > yes, you're right that the frequency sink conflates "sampling rate" and > "bandwidth". It > always shows the full Nyquist bandwidth, and only scales the axis labels > to fit your > bandwidth setting. > > (Internally, what happens is simply that > 10·log_10(|DFT(Input[N*num_point…(N+1)*num_point])) are taken and plotted. > The DFT doesn't > actually care about any frequencies; it tells you something about > "something repeats every > so and so many samples", and what that means in "real-world" time or > frequency is only > defined by sampling rate.) > > This is "slightly" documented (read: probably not good enough) on > https://wiki.gnuradio.org/index.php/QT_GUI_Frequency_Sink, where it says > > > Bandwidth (Hz) > > bandwidth of signal (used to set x-axis labels) > > default: samp_rate > > The implication "used for nothing *but* to set the x-axis labels as > fraction of the > overall width" is not done well. > > Generally, that page could use some love: it's mostly a listing of the > parameters without > explanations, the example is outdated (and also a bad choice, because it > uses a "PSK Mod" > block, first of all, a deprecated block, and a block choice which to the > unsuspecting > reader would imply that PSK has a spectral shape, but that's of course > wrong, what has a > spectral shape is the pulse shaping done "hidden" in the "PSK Mod" block!) > > Would you think you could make a wiki account and help us improve that? > > Best regards, > Marcus > > On 1/25/25 8:06 PM, Maciej Zemło wrote: > > Hello, > > > > I recently encountered an issue while using the QT GUI Frequency Sink in > GNU Radio, which > > caused incorrect frequency readings when the displayed bandwidth was set > to a value > > smaller than the sampling rate (\(fs\)). > > > > My signal source was set to generate a cosine wave at 1000 Hz, with the > sampling rate > > 32k. The QT GUI Frequency Sink was configured with a displayed bandwidth > of samp_rate / 8 > > = 4 kHz. The frequency of the signal was displayed incorrectly on the > plot, showing around > > 120 Hz instead of the expected 1 kHz. > > > > After investigating, I think that this behavior was caused by how the QT > GUI Frequency > > Sink rescales and interprets the frequency axis when the displayed > bandwidth is set to a > > smaller range. When I increased the bandwidth in the frequency sink to > match the full > > samp_rate, the signal frequency was displayed correctly. Is this a > correct assessment and > > does it work like that for you too? > > > > Best regards, > > yabool2001 > > > >
