Cook Electric Company

Albert C Fleuchaus

Paul Shylo

An induction cancellation circuit is used to minimize the effects of AC induced signals appearing on a pair of telephone lines on the activation of a current sensor by a DC loop current flowing on the telephone lines. An input of the current sensor is coupled to one of the telephone lines by a coupling resistor so as to receive energizing potentials from both the AC induced signals and the DC loop current. On the other hand, the having an cancellation circuit includes an amplifier having an amplifier input coupled to the other telephone line by a DC blocking capacitor. The amplifier is biased at unity gain and provides an output signal equal to but 180.degree. out of phase with the AC induced signals received at its input. The output signal is coupled through a balancing resistor and another capacitor to the input of the current sensor such that the out of phase output signal and the AC induced signal received at the input of a current sensor via the coupling resistor effectively cancel each other because the balancing resistor and the output impedance of the amplifier are substantially equal to the coupling resistor.

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Wheeling, IL, United States of America

Albert C Fleuchaus