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Electronic Hybrid Telephone Line Pack

IP.com Disclosure Number: IPCOM000079833D
Original Publication Date: 1973-Sep-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 43K

Publishing Venue

IBM

Related People

Patten, MA: AUTHOR

Abstract

This low-cost telephone pack is small in size, but provides the necessary isolation and two-to-four wire conversion at audio frequencies. The circuit is designed for use in a pulse amplitude modulation, time-division multiplexing telephone switching system.

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Electronic Hybrid Telephone Line Pack

This low-cost telephone pack is small in size, but provides the necessary isolation and two-to-four wire conversion at audio frequencies. The circuit is designed for use in a pulse amplitude modulation, time-division multiplexing telephone switching system.

Audio frequency signal voltage from the switching components enters the line pack at terminals 10 and 12. The signal is amplified by amplifying circuits 14 and 16, and a differential signal is delivered through resistors 18, 20 and capacitors 22, 24 to line terminals 26, 28. The line terminals have conventional 48 volt direct potential across them. Two 560 ohm resistors carry operating current to the telephone and limit that current, in the event of a short circuit on the line.

The audio signal is also presented to the input terminals of a differential amplifier circuit 30. The output of the latter is coupled by a capacitor 32 to a balancing amplifier 34, to which an equal and opposite signal is added. The amplitude and phase of the equal and opposite signal is adjusted by varying the values of resistors 36 and 38, resistor 41 and capacitor 40, respectively. Thus, only an attenuated signal from the sources described reaches the terminals 42,
44. The total phase shift due to capacitive coupling is equalized for optimum hybrid balancing.

The audio frequency signal from the line terminals 26, 28 is amplified by the amplifier circuits 30, 34 and is delivered at the terminals 42, 44 with no cancelling input from the balancing network. No direct potential offset from the line pack input is present at the output because of the capacitors 32 and 40.

The capacitors 22 and 24 block direct current from the relatively high-voltage line to the low-voltage hybrid transmitting circuitry. Resistors 72 and 73 are equal and twice the value of resistors 74 and 75; therefore, the amplifier 30 will saturate when the telephone is "on the hook" and terminals 26, 28 are -48V and +/- 0V, respectively. However, with a saturated output of +12V from amplifier 30 the input voltages to this amplifier do not exceed the maximum allowable voltage. Ringing voltage on the terminals 26 and 28 would cause the inputs to the amplifier 30 to exceed the power supply voltag...