Garage key fob cloning

[Burningskies202]

I've been playing around with an SDR and captured a signal off my garage key fob. It was sitting around 310mhz but after inspecting the signal I was a bit confused. I was expecting to see packets of easily distinguishable 1's and 0's instead I found this. Can anyone help me understand it?

 

[BradtheRad]

Software defined radio.
Looks like noise in the electromagnetic spectrum. The fob may transmit at 310 MHz but the signal proper is no doubt much slower.

My personal experience: My radio controlled airplane had an AM transmitter on 72 MHz. I placed it near my oscilloscope and could plainly see highs and lows (five pulses separated by 1/50 of a second, repeating).

My scope is antique 1 MHz Tektronix tube construction. I was surprised it picked up anything broadcasting at 72 MHz. Probably it detected the AM carrier turning On and Off I guess.

You may see your signal if you look in a lower frequency region. It may make a difference whether it transmits AM or FM. AM seems to be easier to detect.

 

[dick_freebird]

Many modulation schemes will not show nicely on a
time domain 'scope - like phase shift keying, you have
the carrier at constant amplitude and only the phase
changes, back and forth, relative to last symbol. Other
denser modulations like QAM change both phase and
amplitude, but you'd have a hard time figuring which
"symbol" is which by eye.

You need the same RX lineup as the host vehicle, most
likely, to be able to get anything sensible; modulations
are not cross- or backward-compatible pretty much.

 

[KlausST]

Hi,

If you have the key and the receiver ... it will be easy to locate the demodulated logic signal.
If you don't have the key ... or the receiver ... what are you trying to do? Copy the key of your neighbour? ;-)

Klaus