GB704228A - Improvements in or relating to circuit-arrangements for use in automatic signalling systems - Google Patents

Abstract

704,228. Automatic exchange systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. April 7, 1952 [April 10, 1951], No. 8820/52. Class 40 (4) [Also in Group XXXVIII] In a system, e.g. for signalling digital information from a subscriber to a telephone exchange in which signals are transmitted to a receiver over a line loop by changing the value of the resistance inserted at the distant end of the loop, one leg a of the loop is connected to battery at the receiving end and the other leg b is connected to the cathode of a valve B1 the control electrode of which is fed with such a potential that the cathode remains substantially at earth potential irrespective of the value of the resistance inserted in the loop. This arrangement minimizes the errors due to leakage to earth in the line loop. Thus the current through the leak resistance of the leg b unlike that of the leg a passes through the signal receiver and may produce an erroneous operation therein. By maintaining the cathode of B1 at earth potential no current through the leakage resistance Rb of the leg b is permitted. The valve B 1 behaves as an earthed impedance equal to the reciprocal of its mutual conductance which is small compared with the values of the signalling resistors Rk1 ... Rk10 so that changes in current due to changes in these resistors produce only a small change in its cathode potential. This small change is however further reduced by the use of a negative feed backcircuit incorporating the valve B 2 . Changes in signalling current produce corresponding changes in voltage across the anode resistor R 1 which are detected by a known signal detector OI.