A few weird things regarding an OOK modulated remote (newbie's question)


Recently I’ve played with LimeSDR and an OOK modulated garage door opener with an internal DIP switch like this one:

And I’ve noticed a few weird things. I’m hoping you could help me to understand them.

The first weird thing is that when I set a frequency to 433 M is see three spikes on the FFT Plot:

However when I change the frequency to 433.9 M there is only one spike:

So I’m a bit confused - how many carrier waves do I really see? Why the number of the spikes changes?

The second weird thing is about a signal itself. Here is how it looks like after passing it through the “Complex to Mag” filter:

On the DIP switch I’ve set 1 1 0 1 0 0 1 0, plus there are 4 buttons. As you can see the remote sends every bit twice. But if I remember the tutorials regarding the OOK modulation I’ve red previously correctly every bit should be sent only once. So I wonder whether a) I did some mistake or b) my remote just differs from the remotes used in these tutorials? How can I determine whether it’s a) or b)?

UPD: here is the GnuRadio Companion’s scheme I’m using:

Admittedly I’m also new to SDR too, but maybe this can help.

First, If you have an RTL-SDR laying around you can use RTL_433 to give you some of idea of the OOK attributes of your signal. This can be a good starting point to gather info about the modulation type, baud, etc. on an unknown signal.

As for the sending every bit twice, your remote may be using Manchester encoding. This means that each individual symbol is determined by a pair of bits (I think) and this can make your OOK packets to appear as if they have twice the number of symbols of within each burst.

If you haven’t tried Inspectrum, its is a free program that can display your captured recordings as amplitude over the time domain. This way you can visually analyze the signal.

Along with Inspectrum, I’d encourage you to also download DSpectrum. Its like an add-on that works with Inspectrum to give you useful tools to quickly analyze data exported from Inspectrum in various ways. Its fairly easy to use and I’ve had luck using it.

If your goal is to be decode the signal and transmit your recreated data, keep in mind that some OOK devices require the correct signal to be sent 3 to 5 times to react.

If you’re in the US make sure to check FCC numbers for more info too.

Good Luck!

[ Edited details for clarifcation ]

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