It's not the voltage per se, but rather the length of time of each pulse:




The PWM signal goes to the control wire of the three wire servo plug on a servo or ESC.

There needs to be a ground connection between the ESC signal and control source. It can be through the main battery lead or servo lead ground depending on the complete system details, but be careful with ground loops for best practice, I'd try with just the battery ground first.

A couple of many sites with info. Search "RC servo signal" for lots more, image search works well too. Robotics sites might be better sources than RC, as your type of control approach is not unusual in that realm.

R/C Control Signals

Servo control interface in detail

Hope this helps.