RFID tag in car/motorcycle key

Usually, these keys are powered from the radio signal emitted from the car.

They have RF circuitry on board and a little bit of intelligence - a low power tiny microcontroller. Or the two combined on a small flack of silicon.

I need to make hardware that captures this response and replays back, but if I don't know what going on, it is impossible....

Yes. 2 reasons. The main reason is that re-sending a previously recorded reply will not work. It's not a remote control.

2nd reason - not relevant because of the 1st one above - I believe your setup doesn't succeed in showing the signals from key to car.

I know that it uses RF for normal operation, so i would not see this on scope, but i am probing coil and i can't see anything, only data from ECU at very high voltage.

So I don't know whats doing on here. IS it really uses RF somehow ?

Data is static, does not change, does not have any roiling code, every single time is the same, so I can replay it and it should work. Problem I don't see any response from key, while motorcycle does see message from key. Maybe I have to use spectrum analyzer or something to get this working...

Very strange

It has to use RF. If there's no contact, and no infrared sender / receiver, it has to be a radio signal.

In essence the RF tag is more of a disturber than a radio. It will cause modulations on the signal emitted by the sender, by modifying the impedance of its own RF circuit.

The sender coil and token coil are closely coupled when near each other, so that impedance change in the tag antenna RF circuit impacts (agitates) the sender RF circuit.

The sender has to detect those modulations and interpret them.

If you are looking for a reply of the token, after the motorcycle sent its signal, you will not find it because it's not there.

The reply is imposed upon the original signal.

I would expect is is doing something to magnetic field with shorting or doing something similar. In this case I would expect signal like this :

In this case, you can clearly see that master is generating waveform, and tag is modifying quality of coil (and in this case resonating voltage will be lower that can be picked up) , and you can clearly see data transmitted.

But in my case, I see nothing like that. I see perfect 125kHz data burst and no response. If my key is modulating quality of exciter coil, signal WOULD NOT GO TO ZERO.

So how does key is communicating with motorcycle... Only thing that left is transmitting 315MHz RF that my scope can't capture in that timescale... I just don't see any other way

The key does not generate a clearly visible change. It modulates the original signal but that may be subtle. It would not be a amplitude modulation I'd expect.

The key does not generate a clearly visible change. It modulates the original signal but that may be subtle. It would not be a amplitude modulation I'd expect.