Note: modifying RF equipment is most likely illegal in all countries, unless you know your stuff!
I bought 3 radio-controlled toy cars for parts and for racing around the house, since they were really cheap.
Unfortunately, they all share the same RF channel (27MHz) which means you cannot use two at the same time. I mean, you could, but the fun of having two cars mirroring each other fades away after a few seconds.
So I've wired the oscilloscope ground probe around the antenna to check the frequency and modulation. No surprise for finding out it's 27Mhz with some sort of OOK.
Time to take the car apart to see what's inside:
Notice the first hole on the bottom-right of the board surrounded by 4 soldered leads. That hole is for the adjustment cap, we'll make use of that later.
The transmitter frequency is controlled by the 27.145 MHz oscillator, there's nothing to adjust there. The receiver frequency is in part controlled by a trim capacitor.
Now for the fun part, the TX oscillator is replaced with another one from the parts drawer:
After adjusting the trim cap on the receiver unit to one extreme I'm able to get the two cars to work independently. Remote range went down from ~10 meters to ~3 meters.
However, if the 27MHz transmitter is within 30cm of the ~16Mhz car it will also control that. Same thing happens the other way around.
I'd be curious to see what the new TX spectrum looks like and what's the typical RF power, before and after the mod.
Note: modifying RF equipment this way is probably illegal in all countries!
To be legal you'd probably have to choose one of the 13.56 MHz or 40.68Mhz ISM bands, those being the closest to this range. But don't quote me on that.