Most modern telephone sets, modems and facsimile machines will recognize the Ringing Signal at amplitude of 35Vrms and above. A few facsimile models, and some older telephone sets will require minimum amplitude of 40Vrms or even 45Vrms to ring. When calculating the required output amplitude of a Ring Generator, the objective should be to deliver more than the minimum ringing amplitude to the loop's end. The voltage drop on the loop can be calculated according to the typical line resistance, the loop length, connection resistance and the sum of the line's serial protection resistors. Higher ring generator output amplitude can support longer loops.
Telephone ringing elements include resistance, capacitance and in some cases inductance elements.
The PF is a function of the ratio between the real and imaginary elements of the ringing element's impedance. A PF of 1 will cause no phase shift, and indicate that the device has no capacitance and inductance elements, or that both exist and exactly cancel each other's effects. Higher phase shift between the Ring Generator's output voltage and current will cause its output stage to absorb more energy and to generate more heat.