What could this signal at 119.9 MHz be?

Question

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?

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

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.

Answer 1

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.