Induction cancellation circuit

• US 3,932,713 A
• Filed: 05/15/1974
• Issued: 01/13/1976
• Est. Priority Date: 05/15/1974
• Status: Expired due to Term

• Current Assignee: Northern Telecom Inc (Nortel Networks Corporation)
• Original Assignee: Cook Electric Company
• Inventors: Shylo, Paul, Fleuchaus, Albert C.
• Primary Examiner(s): Robinson, Thomas A.
• Application Number:
  US05/470,074
• Time in Patent Office: 608 Days
• Field of Search: 179/174, 179/80, 179/78 R, 179/78 A, 179/79, 333/12, 323/22 T, 178/69 B
• US Class Current: 379/415
• CPC Class Codes: H04Q 1/44 using alternate current H04Q1/50 takes precedence

Abstract

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° 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.

Claims

1. An induction cancellation circuit used in minimizing the effects on a current sensor device of AC longitudinal voltages appearing on a pair of telephone lines, said induction cancellation circuit comprising:

an amplifier having an input coupled to one of said lines and having an output, said amplifier being biased so as to provide an output signal which is out of phase with any longitudinal voltages applied at said input,

a first resistor coupling said output to said current sensor, and

a second resistor coupling said other telephone line to said current sensor device so as to couple said AC longitudinal voltage to said current sensor, said first and second resistors having resistive values such that the output signal coupled to said current sensor through said first resistor is equal to but out of phase with said AC longitudinal voltage coupled to said current sensor by said second resistor.

2. An induction cancellation circuit as set forth in claim 1 wherein a first capacitor couples said input to said one telephone line and a second capacitor is in series relationship with said second resistor.

3. An induction cancellation circuit for use with a pair of telephone lines on which are induced AC signals, said induction cancellation circuit comprising:

a summing terminal external to said telephone lines,

an inverting amplifier having an input coupled to one of said telephone lines and having an output, said inverting amplifier converting the phase of said AC signals on said one telephone line substantially 180°

so as to supply out of phase signals to said output,

a first resistor coupling said out of phase signals from said output to said summing terminal, and

a second resistor coupling said AC signals on said other telephone line to said summing terminal such that said out of phase signal and said AC signal substantially cancel each other at said summing terminal.

4. The induction cancellation circuit as set forth in claim 3 wherein said inverting amplifier has an output impedance, said out of phase signals are equal in magnitude to said AC signals, and said second resistor is equal to said first resistor and said output impedance.

5. In combination with a conduction means which is normally in a first state of conduction, which is coupled by a first resistor to one of a pair of telephone lines having AC longitudinal voltages thereon and which is placed in a second state of conduction in response to a loop current flowing on said telephone lines, an induction cancellation circuit comprising:

a second resistor coupled to said conduction means, and

an amplifier means coupled to said other telephone line and to said conduction means by said second resistor, said amplifier means providing the conduction means by means of said second resistor with an AC signal equal to but 180°

out of phase with the AC signal provided to the conduction means by means of said first resistor.

6. The combination as set forth in claim 5 including

a first DC blocking capacitor coupling said amplifier means to said other telephone line, and

a second DC blocking capacitor in series relationship with said second resistor.

7. A current sensing circuit including an induction cancellation circuit for use with a pair of telephone lines having induced AC noise signals to sense the presence of a DC loop current flowing on said telephone lines, said current sensing circuit comprising:

a current sensor means external to said telephone lines, said current sensor means receiving an operating signal when said loop current is flowing on said telephone lines, and

operating means including amplifier means providing said current sensor with said operating signal, said operating means being coupled to said telephone lines such that the AC noise signals do not affect the production of said operating signal for said current sensor means.