Browse Prior Art Database

Electronic Line Pack with Modulated Current Source

IP.com Disclosure Number: IPCOM000088531D
Original Publication Date: 1977-Jun-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Pattern, 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 and similar applications.

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Electronic Line Pack with Modulated Current Source

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 and similar applications.

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. The latter is a phase splitting amplifying circuit, comprising transistors 18, 20 and 21, from which a differential signal is delivered through resistors 22, 24 to line terminals 26, 28. The line terminals have conventional 48-volt direct potential across them. Resistors 22, 24 also 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 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 setting of a potentiometer
38. 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 a pair of decoupling capacitors 46, 47.

The capacitors 46, 47 block direct current from the relatively high voltage line to the low voltage hybrid transmitting circuitry. Resistors 72, 73 are equal and twice the value of resistors 74, 75; therefore, ringing voltage on the terminals 26...