Telephonic communications – Subscriber line or transmission line interface – Hybrid circuit
Patent
1991-10-25
1993-09-28
Dwyer, James L.
Telephonic communications
Subscriber line or transmission line interface
Hybrid circuit
379398, 379406, H04M 158, H04M 176
Patent
active
052492257
ABSTRACT:
A digitally programmable attenuator balances impedance across a two to four-wire hybrid to reduce echo. The hybrid can interface a two-wire bi-directional telephone line, used for both transmit and receive signals with four-wire circuits having separate unidirectional transmit and receive signal paths. The four-wire circuit interfaces the two-wire circuit through a transformer. Optimum isolation is obtained by setting impedance Z.sub.BAL equal to the impedance of the line including the transformer (Z.sub.LINE). The invention eliminates manual or customized individual adjustment of Z.sub.BAL by a set up technician by use of a digitally controlled attenuator to electronically vary the value of Z.sub.BAL in a highly precise fashion. The programmable attenuator provides an input voltage to a buffer voltage-follower amplifier. The output of the follower amplifier is coupled to a known impedance to limit current flow through the impedance and consequently, current flow through Z.sub.BAL. Data to the attenuator is provided from a DSP programmed to sense and automatically converge to a balanced condition.
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Coherent Communications Systems Corp.
Dwyer James L.
Fournier Paul A.
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