Two port signalling voltages filter arrangement
Patent 5623543 Issued on April 22, 1997. Estimated Expiration Date: 
April 22, 2014. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.
April 22, 2014. Estimated Expiration Date is calculated based on simple USPTO term provisions. It does not account for terminal disclaimers, term adjustments, failure to pay maintenance fees, or other factors which might affect the term of a patent.Patent References 3828281 Voice frequency RC active filter Methods and apparatus for providing reciprocal impedance conversion Impedance generator for a telephone line interface circuit Patent #: 5258713 InventorAssigneeApplicationNo. 216875 filed on 03/24/1994US Classes:379/402, Hybrid circuit333/32, With impedance matching333/175, Resonant, discrete frequency selective type379/398, LINE EQUALIZATION OR IMPEDANCE MATCHING379/400For line length compensationExaminersPrimary: Brown, Thomas W.Assistant: Saint-Surin, Jacques M. Attorney, Agent or FirmInternational ClassesH04B 003/16H04M 009/00 Foreign Application Priority Data1994-02-01 EPAbstractA twisted-pair telephone line is connected to a conventional telephone via a low-pass filter and equipment for receiving signals in an upper frequency range, e.g., 25-1000 kHz via a high-pass filter. The low-pass filter is primarily passive (to allow passage of line power, ringing and signalling voltages) but, to improve matching to non-resistive line and telephone impedances, at least the main part of the filter is flanked by impedance convertors in which amplifiers having an appropriate transfer function feed voltages back into the line via transformers through which power, ringing and dialing may pass unimpeded.Other References
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