GB551951A - Improvements in or relating to electric signalling systems - Google Patents

Abstract

551,951. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd., FAIRLEY, F., and WALSH, R. Sept. 15, 1941, No. 11782. [Class 40 (iv)] [Also in Group XL] An A.C. signal receiver particularly for ringing and dialling signals in a telephone system comprises means for comparing the received energy in a predetermined range or ranges with that in other ranges and for causing or preventing the generation of local oscillations for operating the receiving device when the energy ratio is above or below certain values. A Wheatstone bridge in the feed-back circuit of a normally quiescent valve-oscillator is used for the comparison, the energy being applied to resistances slowly responsive to temperature variations. The arrangement shown in Fig. 3 comprises an amplifying valve VI and a normally quiescent oscillator V2 coupled by a network comprising frequency discriminators N1, N2 and a bridge W forming the feedback circuit of oscillator V2 the output circuit of which is tuned by condenser C1 and feeds the receiving device R. The bridge comprises ordinary resistances R1, R2 and resistances T1, T2 dependent on temperature and heated by the outputs of N1, N2. The bridge is adjusted so that normally the feedback is negative or not sufficiently positive for V2 to oscillate. The desired signal band passing N1 changes the resistance of T1 to swing the feedback sufficiently positive for V2 to oscillate ; the undesired band passing N2 has the opposite effect and reduces the possibility of V2 being brought into an oscillating condition. Valve V2 may also be backcoupled to V1 and networks N1, N2 arranged to pass the frequency of the oscillator to such extents that (1) the circuit locks in a state of oscillation, or (2) the tendency to oscillation is reduced so that the circuit oscillates intermittently due to the slow action of the resistance T1, T2, or (3) the feedback is unaffected but the period of restoration of the circuit to normal, when the signal ceases, is prolonged. In a modification, the resistances T1, T2 may be directly heated by the output of valve V1, the resistances being shunted by suitable discriminating networks. A modification, Fig. 4 (not shown) comprises a single valve circuit functioning in a similar manner. Specification 489,204 is referred to.