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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?

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It is very hard to say definitely what this is, however it looks like the pattern from a switching powersupply.

Switching powersupplies are very common now-a-days, they are everywhere, from your laptop supply, your phone charger, LED lights, and many other devices.

First thing I would do (if you are running this on a laptop) to run the laptop on batteries.

Then it is a "trial and error" method.

Go to your switch/fuse board, and turn of the zone's one by one, to see if this improves/changes the patterns. Make sure you notify other people in the house that you are about to do so, ensure that washing machines and the like are finished their cycles.. and so on so forth.

Once you have found the offending zone or zones, then investigate what is powered through those circuits, and work on from there. Please note that there could be hardwired devices, such as security camera's, which you cannot individually switch off.

If you are in close proximity to your neighbours, the inteference may come from a next door propery. In which case a bit of friendly-neighbour politics comes into play. Proof that it is not your own devices, just switch off the main-breaker, and see what happens.

Once found the offending device or devices, then you need to apply various methods of getting rid of the interference, apply some filters... it is very hard to answer this in a general way.

Most radio clubs would have experts in EMI/RFI and related subjects, they may have tips/tricks as well.

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  • $\begingroup$ If it was PSU noise, would tuning from 25.680 MHz to 25.679 make it go away? $\endgroup$ – Phil Frost - W8II Feb 22 '18 at 2:32
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    $\begingroup$ When I unplugged my laptop they went away! Thank you for the useful tips on how to locate interference sources. They will come in handy down the road. $\endgroup$ – Jared Feb 22 '18 at 3:57
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The fact that the evenly-spaced signals are quite strong at 25.680 MHz and gone completely at 25.679 MHz strongly suggests that the signals are internally-generated. Spurious signals generated inside the receiver are often called "birdies", as described in this similar question.

An excellent way to test whether or not birdies are the problem would be to connect another receiver of a different design to your antenna, and see if the other receiver picks up the evenly-spaced signals.

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