pulse is sent, aka the pulse repetition rate) is in a range of 40 Hz to 200 Hz, the exact value of the refresh rate is irrelevant. The period of 20 ms (50 Hz) comes from the days where the signal was encoded in PPM (pulse-position modulation) format to be sent over the air. The PPM period was around 22.5 ms, and the conversion to PWM was trivial: the time of the PWM high state was the time position of the PPM pulse for that servo. Most RC receivers send pulses to the RC servo at some constant frame rate, changing only the high time. However, it is possible to command an RC servo to move over its entire range with a function generator set to a constant 10% duty cycle by changing only the frequency (frame rate). Force When these servos are commanded to move, they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position. The maximal amount of force the servo can exert is the torque rating of the servo. Servos will only hold their position for their timeout duration, though; the position pulse must be repeated, usually within 20ms, to instruct the servo to stay in position. Variations When a pulse is sent to a servo that is less than 1.5 ms, the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs. The minimal and maximal widths of pulse that will command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maxima and minima. Generally, the minimal pulse will be about 1 ms wide, and the maximal pulse will be 2 ms wide. See also PID controller Servo (radio control) Servo bandwidth Servomechanism References External links Hobby Servo Basics Radio control Control