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I'm running GQRX on Mac OS X 10.11.6 hooked up to an RTL-SDR dongle. I noticed when I'm tuned anywhere between ~56 MHz on down to 25.680 MHz I see evenly spaced signals on the FFT. But when I tune to 25.679, they disappear completely.

On the higher end, the signals are faint (here I am tuned to 56 MHz):

enter image description here

They become more pronounced as I work my way down (here I am tuned to 43 MHz):

enter image description here

They are most pronounced when I'm tuned to 25.680 MHz. But when I tune to 25.679 MHz they disappear entirely. Here I am switching back and forth between 25.679 and 25.680 MHz:

enter image description here

What's causing this? Since I see signals below 25.3 MHz when I'm tuned to 25.680 but they disappear when 25.679 I tend to think this is a bug. If so, is it hardware or software related?

EDIT:

The signals are more pronounced tonight. Each one has a valley surrounded by two peaks:

enter image description here

The one I'm tuned to in the image above has peaks at 27.03 and 27.05 MHz. The first peak is at about -60 decibels and the second one is at -50.

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  • $\begingroup$ Is it possible to post a trace of the time domain signal from which the spectrum is derived please? $\endgroup$ – A_A Feb 21 '18 at 6:34
  • $\begingroup$ Thanks A_A. I'm new to this. What is the time domain signal and do you have any idea how I might get a trace of it from GQRX? (If it's this, I'm reading it's not a feature of GQRX, unfortunately: groups.google.com/forum/m/#!msg/gqrx/iV-kdxj0sYI/qiHLDQhaCQAJ) $\endgroup$ – Jared Feb 21 '18 at 6:49
  • $\begingroup$ Yes, that one. If the software does not provide that functionality, can you please tune to the frequency the artifact is more pronounced at and then look at the frequencies and ampitude of the peaks? Can you post them here? 5-6 pairs of values would be enough. $\endgroup$ – A_A Feb 21 '18 at 6:55
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These periodic spectral lines look like they might be caused by something like a sparking power supply, DC-DC converter (for LED lighting or USB power, etc.), or high powered digital drivers running at approx. 150 kHz. You might want to walk around the house or neighborhood with a pocket AM radio to see if you can DF this RF noise (determine the direction its coming from).

RTL-SDRs USB dongles based on R820T2 + RTL2832U also have a lower frequency limit somewhere around 52 MHz (where they go deaf or alias or won't lock). 26.68 appears to be near a folding point in the sample rate aliasing, e.g. right around Fs/2, where any spectrum plus its own complex conjugate can partially cancel.

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  • $\begingroup$ I think you're on the right track as they disappear when I unplug my laptop. $\endgroup$ – Jared Feb 22 '18 at 4:07

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