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I had some fun with an RTL-SDR dongle, and I noticed this weird signal with no noticeable pattern at 119.9 MHz. What's even weirder is that the signal is symmetrical. What might this be? Or is this simply a hardware error?

The signal in a waterfall diagram

EDIT: This picture is from an oscillating signal generator/MP3 tansmitter from AliExpress (2SC9018) and it looks pretty similar: That one

The only problem is, that when listening to the signal (In FM, AM, etc.) you cannot hear any music, any speech or any patterns that would sound digital.

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    \$\begingroup\$ Where did you put your antenna? \$\endgroup\$ – Ale..chenski Sep 3 '17 at 2:40
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    \$\begingroup\$ Is the signal always there and at similar level? If so, it's probably EMI from the spectrum analyzer itself or from the computer to which it's connected (e.g. a birdie from the receiver's oscillators.) \$\endgroup\$ – reirab Sep 3 '17 at 4:52
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    \$\begingroup\$ Make a multi-element directional antenna if you think this is some outside source. It will be more fun. \$\endgroup\$ – Ale..chenski Sep 3 '17 at 5:32
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    \$\begingroup\$ Are you near to an airport? \$\endgroup\$ – RoyC Sep 3 '17 at 7:43
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    \$\begingroup\$ Frequency range 108-137 MHz is used for aircraft communication (in AM). \$\endgroup\$ – Curd Sep 3 '17 at 12:27
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This signal is an extremely inefficient use of bandwidth, as you can see because there is an essentially unused area between the center and ±125 kHz. Therefore, I expect that it is almost certainly unintentional radiation (a.k.a. RFI/EMI) rather than a meaningful transmission.

The origin of the signal could be as follows — there are other ways it could arise but this is a simple plausible one:

  • There is an oscillator at 120 MHz, a nice round number probably chosen as a clock frequency. (Your 119.9 is either error in your receiver's oscillator or in the transmitting oscillator — it is likely in an application which does not need less than 0.1% error.) This oscillator is not deliberately connected to a transmitting antenna — it is just part of some circuit that isn't designed well enough to not radiate.

  • That oscillator is being amplitude-modulated by another signal at about 125 kHz (the distance from the carrier to the nearest sideband). This can occur many ways — one of the simplest being if something is varying the load on the common power supply at 125 kHz and the 120 MHz oscillator's output amplitude follows its supply voltage.

  • The 125 kHz oscillator's frequency is being modulated a bit by something else, causing the visible frequency changes. Again, this is fairly easy to have happen by accident.

  • Then if we look outward further to twice the frequency, we see a copy with twice the frequency variation but exactly the same shape other than that. Thus, this is just a higher harmonic of the 125 kHz signal. This is more evidence that this is not a deliberate transmission, as this doesn't efficiently add useful information to the signal.

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    \$\begingroup\$ It needn't definitely be AM, it could be narrow band FM. Also, looking at the harmonics as a snap shot picture doesn't really give much understanding to the basic nature of the source being square or triangle. I think you might be on the verge of "baffling with science" here. Also, there is no Y axis measurements at all. I'm not going to downvote because you analysis could be right but you are on shaky ground. \$\endgroup\$ – Andy aka Sep 3 '17 at 9:54
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    \$\begingroup\$ @Andyaka I've revised the answer to claim less. I think the description is still useful to illustrate that just because something is structured doesn't necessarily mean it's an intentional transmission. \$\endgroup\$ – Kevin Reid Sep 3 '17 at 14:03

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