Interestingly, if you look at the zoomed-in r(t) -- last attached screenshot -- it has some sinusoidal shape, which has another lower frequency sinusoid modulated onto it. Would the higher frequency be the carrier frequency, i.e., 48.68 MHz, or is it some alias or another frequency entirely? What would the signal be, that can be seen to be modulated onto it?
In comparison the I²+Q² and I/Q signals look much more random. Is it just noise?
- Most probably the higher frequency seen in the r(t) waveform is an artifact.
- Might be caused, for example, by the ADC taking the I and Q samples consecutive instead of simultaneous sampling.
- Yes, noise. Ideally it should be either zero or a constant value (about 0.4 in your II+QQ waveform)
In the zoomed-out and zoomed-in-a-little screenshots however, the I/Q waves clearly show sinusoids, which are 90° out of phase, as you would expect from an I/Q signal. Those sinusoids would have the frequency of the local oscillator (LO), right?
No, not the frequency of the LO. The LO ideally (to avoid any artifacts) must have the same frequency and be synchronous with the transmitter's oscillator, and the LO in the SDR certainly can not do that. My best guess is those "interrupted" sinusoids shifted at 90° are caused by a small difference between the frequency of the Tx oscillator and the frequency of the LO in your SDR, so they are an artifact, too, and if I am correct their frequency should be the difference between Tx and LO.
To probe if the apparent frequency seen in the I/Q "interrupted" sinusoids comes indeed from the difference between the Tx and the LO oscillators, try changing the receiving frequency just a little (from the SDR tuning). It is expected to see the frequency of the interrupted sinusoids in I/Q changing, too, when the receiveing frequency (the LO) changes.
Keep in mind that all this SDR and IQ thing is most probably not even used by your RC car, and you don't need it. Casual toys only have a Tx oscillator that is fully turned on/off, so the Tx is not modulated at all. Sure, one can say turning on/off the 40.68 MHz Tx oscillator is the same as a 100% AM modulation, which in theory it is correct, but it is much easy to just think about it in terms of just an on/off 40.68 MHz.
Most probably your remote is sending just a train of on/off pulses, and these pulses just turns on/off the 40.68 MHz Tx oscillator, this is what on/off keying modulation is. Nothing more.
The receiver is tuned on the same frequency, and it continuously listening on 40.68 MHz.
- When it detects 40.68 MHz oscillations, the detector in the Rx outputs a constant (let's say) 5V (in your attached pics, that would be the 0.4 seen in the I
2+Q
2 waveform).
- When the 40.68 MHz is missing from the antenna, the detector in the Rx outputs a constant 0V (in your attached pics, that would be the noise around 0, noise seen between other 0.4 periods of the I
2+Q
2 waveform).
In conclusion, SDRs and IQ modulations are very powerful techniques to have under your belt, but not necessary for this RC car.
To control your car, look at the I
2+Q
2 waveform. There are only 2 levels there, LOW (about 0) and HIGH (about 0.4 in your pics). You need to record the
duration length of each HIGH and LOW, and reproduce that as a 0/1 on a digital output pin. That pin will disable/enable (turn off/on) the 40.68 MHz oscillations that you put in your computer controlled Tx antenna.
So far all clues are pointing you are dealing with a simple on/off keying. Using an SDR to implement on/off keying will be an overkill, it can be done much simpler by just turning on/off the 40.68 MHz Tx oscillator, without using any SDR or IQ modulation.
DISCLAIMER: Other remote controllers might have some other more complicated modulation than on/off keying, but this is highly unlikely for this particular RC car of yours.