GB668629A - Improvements in alternating current signalling systems - Google Patents

Abstract

668,629. Voice frequency receivers. POSTMASTER GENERAL. March 17, 1960 [Jan. 6, 1949], No. 416/49. Class 40 (iv). [Also in Group XL (c)] A signal receiver providing selective response to a frequency liable to imitation in speech includes a phase-sensitive device which has a D.C. output dependent on the phase relationships between frequencies received over a circuit tuned to the signal frequency and frequencies received directly. The device may comprise a ring modulator or a pentode valve. As shown in Fig. 1, incoming signals are applied to a ring modulator M1 at centre tappings of a resistor R and a relay coil W1, and also over a tuned network N1 and a biased rectifier volume limiter LM1. The signal frequency is characterized by the lack of phase difference over the two paths and the consequential direct current in the relay coil W1 is therefore a maximum. When this current is reduced by the presence of other frequencies, the relay is rendered inoperative by a bias winding W2. The tuned circuit may alternatively be replaced by a band-pass filter. In a modified arrangement, Fig. 2, incoming signals are preamplified at a valve V1 with the two paths transformer-coupled to the anode circuit, the phase-displacing path being tuned by condenser C. The direct current established in a resistor R3 by incoming signals reduces the bias at a valve V2 and thereby controls a relay in the anode circuit. In an embodiment using a pentode valve, Fig. 3, incoming signals are applied over a valve V3 and a transformer T to a tuned circuit C1, L1 and a resistor R4 connected in series and to earth. The potentials across the tuned circuit and the resistor are applied to the suppressor and control grids respectively of a pentode valve V4 having a double wound relay PR1 connected in the anode and screen grid circuits. At the signal frequency, the voltages at the suppressor and control grids are in antiphase and relay PR1 is pulled up by screen current, no anode current being present. For frequencies other than the signal frequency, the screen current is decreased and anode current disables PR1. In the absence of any signal, the anode and screen currents are limited by a cathode resistor RC. An arrangement, Fig. 4 (not shown), is also disclosed in which the suppressor and control grid potentials are applied through a double triode valve-amplifying stage. In an embodiment comprising separate receivers for two different frequencies, Fig. 5 (not shown), the circuit shown in Fig. 3 is duplicated in the anode circuit of valve V3. Selective response of each receiver to a particular frequency without suppressive effects by the other frequency is obtained by shunting the secondary winding of each transformer T with a seriesresonant circuit tuned to the signal frequency which is not required.