What's the Digital Servo Signal Wire For?
The receiver or flybarless system “talks” to the servo through this wire by means of a simple on/off pulsed signal. This is known as PWM, Pulse Width Modulation.
The "normal" frequency of this PWM signal is 50 Hz, meaning that 50 times a second, the position of the servo is "updated" or "refreshed". Time wise, that means once about every 20 milliseconds.
This frequency can vary quite a bit between brands and components (most fully compatible), and has no impact on the actual servo position, it only impacts how often that position is updated.
It is however important to understand that some devices (tail gyro's & flybarless units for example as will be demonstrated below), can generate much higher frequency refresh rates (upwards of 560 Hz). If you use such devices, you must make sure your RC servos are rated to handle those higher refresh frequencies. These refresh frequency numbers are generally given in the servo specifications in number of the maximum Hz the servo can operate at safely.
You can always run a fast rated servo at lower refresh frequencies, but you can't run a slow rated servo at fast refresh frequencies.
The actual servo position is determined by the length (pulse width) or "on time" of each of those individual PWM pulses regardless of the refresh frequency. The "nominal" pulse width range is about 1000 to 2000 mirco-seconds (us) with the center position set at about 1500us; however over travel is possible by going outside those numbers by increasing servo travel limits in your radio as one example.
IMPORTANT! Narrow Band RC Helicopter Tail Rotor Servos
You will often see high performance tail rotor specific RC servos that have shorter centering pulse widths (narrow band) to improve their resolution. The shorter the pulse width, the higher the refresh rate can be (again upwards of 560 Hz).
These specialized ultra fast tail rotor specific RC servos will have centering positions listed in the specifications of 760us or 960us. Fast 760us or 960us tail rotor specific RC servos can only be used with tail gyros or FBL systems that support these shorter narrow band centering pulse widths.
If you hook a narrow band 760us center servo up to a 1500us or 1520us output, the servo won't work at all. If you hook a 1500/1520us servo up to a narrow band 760us output, the servo can be damaged because it will overtravel and stall.