Servo Signal Level
I've been curious of how critical the amplitude to the signal from the receiver to the servo is so I did the following checks:
I built an interface between a servo signal and it's source so I could control the amplitude of that pulse independent of the voltage on the servo from less than 0.5volts up to the voltage applied to the servo. Here are the results:
On all Futaba (and I checked many of their very old ones as well as the later versions) the amplitude required was only one volt no matter how high the voltage on the servo bus was. The same held true for a bunch of Tower Hobby servos, Hitec servos, and Appolo servos. All worked well with a signal amplitude of a little as one volt no matter what the voltage on the servos was as long as the servo voltage was above 4 volts. I tested from 4 volts to 6.2 volts on the servo bus.
Other brands of servos did not fare so well. The FMA all required at least 1.5 volts amplitude on the signal to reliably operate. The DAD servos required slightly over 2 volts on the signal. The AS-090 required 2,5 volts. The small GWS Micro Std's required 2 volts and the PowerUP's required 2.1 volts. Of all the miniature servos, the Hitec HS-55 worked very well and reliably worked with the signal as low as 1.0 volts all the time. The HS-55 seemed to be the most reliable small servo although my supply of them was limited so there may be other good ones.
The amplitude of the servo signal pulses were measured using an oscilloscope to be sure there was no inaccuracies due to form factor or pulse distortions.