Patent GB-668298-A (GB)
Title: Improvements in or relating to signalling receivers
Inventor: RYALL LEONARD ERNEST
IPC: H04Q1/45
Filed: 1949-02-04
Abstract: 668,298. Voice frequency receivers. POSTMASTER GENERAL. April 12, 1950 [Feb. 4, 1949], No. 3191/49. Class 40 (iv). In an A.C. signal receiver comprising a volume limiter which provides a uniform output over a wide range of signal amplitudes, the harmonics generated by the limiter, which would otherwise affect the response of a relay to the signal frequency, are neutralized by utilizing part of a D.C. output corresponding to the signal frequency to bias off a D.C. output corresponding to a broad frequency band, this neutralizing bias being insufficient however if the incoming signal frequency is accompanied by other frequencies above a certain level. Two embodiments are described, relating to the reception of one and two signal frequencies respectively. Single frequency receiver, Fig. 1. Incoming signals are applied over a transformer 3 to a volume limiting valve V1 which may be supplemented by an additional limiting network 17. A bridge circuit comprising a transformer 8 tuned to the signal frequency and a resistor 7 which have equal impedances at resonance, is coupled by a transformer 4 to the anode circuit of the valve. Frequencies other than the signal frequency establish a voltage across bridge connections X and Y which is rectified by a voltage doubler 12, while the signal frequency establishes a D.C. voltage across series resistors 10, 11 over a voltage doubler 9. The D.C. voltage due to the signal frequency at doubler 9 is normally considerably greater than that across doubler 12 due to volume limiter distortion. These voltages are additive in resistor 11 and the resultant difference voltage across resistor 10 controls a relay 13 over valve V2. If other signal frequencies are present, the voltage across 10 is reduced and relay 13 disabled. Negative feed-back is provided by a resistor 14 which is balanced out in the quiescent condition by the voltage across resistor 15. The input transformer may be selectively tuned and the receiver used in a multi-channel V.F. telegraph system. Automatic volume control of the incoming signals to minimize inherent distortion may be effected by a circuit described in Specification 413,383, the control voltage being derived from a resistor-condenser combination of large time constant which is charged from a volume limiter connected across the tuned circuit 8. Receiver for two frequencies, Fig. 2. Each frequency controls a corresponding relay 34, 35 over series resistors 32, 33; 37, 38 and valves V4, V5 respectively and the circuit provides selective response to either or both the signal frequencies. The signal frequencies are selected in a bridge circuit, by series-connected tuned transformers 26, 29 and converted into corresponding D.C. signals by voltage doublers 31, 36. The unbalanced bridge voltages are applied to a voltage doubler 39 common to resistors 33, 38, rectifiers 40, 41 preventing interaction between the two relay-controlling circuits which function in the manner previously described. Each tuned circuit includes a limiter circuit 43, 42 which is operative at a lower signal level than valve V3 and adjusts the two signal frequencies to a common level when received simultaneously. Since these limiters, in operating, cause the output at doubler 39 to increase with a consequential effect on correct receiver operation, the voltage across each limiter is applied to the output circuit of doubler 39 corresponding to the other-signal frequency. According to the Specification the balance arms of the bridges may be omitted and the unwanted frequencies detected across resistors 7 or 30 in Figs. 1 and 2 respectively.