Circuit for maintaining the DC voltage on an electrically isolated telecommunication line at a reference level
US4551670A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | Apr 19, 1984 |
| Grant date | Nov 5, 1985 |
| Priority date | — |
| Expiry date | Apr 19, 2004 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH04Q1/38
- WIPO fieldTelecommunications
- WIPO sectorElectrical engineering
Abstract
Disclosed is a line interface circuit (103) having a negative feedback circuit (205) for maintaining the DC voltage on an electrically isolated communications line at a reference level such as ground for private branch exchanges using ground start supervisory signaling. The feedback circuit comprises a low-pass filter (301), a difference amplifier (302), and two voltage-to-current sources (303,304) each having a high output impedance. The low-pass filter passes only the voltage on the line below a predetermined frequency. The difference amplifier subtracts the passed voltage from a ground reference level to form a difference voltage. Depending on the polarity of the difference voltage, the two voltage-to-current sources source and sink current to and from the line to maintain the DC voltage on the line at the reference level. A load resistor (305) also included in the feedback circuit keeps the circuit from oscillating where the phase shift of any signal through the circuit is an integer multiple of 360 degrees. The high output impedance of the two voltage-to-current sources does not effect the impedance balance on the line.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.