The transmitter circuit is very simple, I took a 20cm long twisted wire as an antenna. The microphone is electret, I connected the oscilloscope probes to pin 3 of the transmitter output to see what shape the signal will be at the output
Without sound in the microphone, the signal at pin 3 is a square wave. The frequency is fixed and nothing happens to the signal
And now I speak into the microphone and it is clear that the signal changes horizontally and its frequency changes. It looks like frequency modulation, because with FM the frequency should change. But as far as I know, the signal with FM should be a sine and here is PWM. I don't know what this modulation is called
The fact that this transmitter is the same quality as others, in principle, is really true. I fed sound from a telephone to pin 5 through a capacitor and on long waves you can receive a signal with fairly high-quality reception
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