Call current detector for telephone equipment - has amplitude discriminator coupled via photocoupler to frequency discriminator

摘要

The call current detector, is used in telephone equipment. It consists of an input full wave rectifier connected to an amplitude discriminator responding to a minimum threshold amplitude. The output of the discriminator is connected to a photo-coupler producing a rectangular-wave signal which is passed to a frequency discriminator. The frequency discriminator consists of a differentiator that produces reset pulses for a monostable circuit. The mono stable circuit out put is passed over an integrator to form a means level and it is applied to a level comparator, which eliminates signals having a mean level less than given threshold.