RC Servo Theory
The control signal for an RC Servo is a digital pulse train that looks something like this:
Here, the width of the “high” pulse, t, determines the angle of the rotor. The width, measured in time, will be in the range of
0.3ms ≤ t ≤ 2.5ms
And the frequency of the pulses, given by f=1/T, is around 40-50Hz (T ≈ 20ms).
For a practical circuit, see 555 Servo Circuit.