This may sound like a very dumb question and I apologise in advance.
I've gone through reading about PWM and working on some example sketches to control PC fan speeds using PWM and Arduino.
In all documents I read, it is mentioned that "PWM is a signal".
Is PWM really a signal? To me, it looks like an electric current turned on and kept on for a certain time, then off for a while repeatedly. What makes this a "signal"?
If PWM is a signal, why do we need to connect the Ground from Arduino to the ground of the device to which "PWM signal" is being sent to?
This is confusing me because in my mind, for example, radio signals do not need a ground connection. The signal comes and gets read.