Patent GB-582784-A (GB)
Title: Improvements in or relating to means for discriminating between complementary keyed electrical signals
IPC: G01S1/02
Filed: 1944-07-14
Abstract: 582,784. Radio navigation ; pulse generating circuits. STANDARD TELEPHONES & CABLES, Ltd., and WESTON, J. D. July 14, 1944, No. 13497. [Class 40 (v)] Relates to means for identifying the predominating signal in a mixture of two complementary-keyed signals, e.g. the A-N signals of an equi-signal radio approach-path systems similar to those disclosed in Fig. 2 of Specification 508,054. According to the invention a train of alternately positive and negative pulses is derived from the input signals and these pulses are separated into two sets of opposite polarity which are applied separately to two selective means tuned to different harmonics of the fundamental keying frequency, said harmonics being both present in one set of pulses and absent in the other, means being provided for comparing the outputs of the two selective means. As shown in Fig. 2, an A signal, Fig. 1, A1, applied to the valve V is differentiated by the transformer T which has a centre-tapped secondary to give a train of alternate positive and negative pulses, Fig. 1, A2, at the terminal S1 and a similar train with inverted polarity, Fig. 1, N2, at the terminal S2. The negative pulses in both trains are suppressed by the rectifiers W1, W2 and the resultant positive pulses A3, N3, Fig. 1, corresponding to the positive and negative pulses of Fig. 1, A2, are fed to the amplifiers V1 and V2 respectively, the inverted pulses in the outputs being applied to the circuits L1, C1 and L2, C2 which are tuned to different harmonics of the fundamental keying frequency of order 4n-2. When the dot and dash periods of the A signal are one and three units in length respectively, the N3 pulses are equally spaced and the A3 pulses have alternate intervals of two and six units; harmonic analysis shows that all odd harmonics of the fundamental keying frequency are absent from the N3 pulses and all harmonics of order 4 n-2 are absent from the A3 pulses so that in Fig. 2 only the L2, C2 circuit will respond for an applied A signal and the rectifiers B3 and 4 are connected to deflect the meter M in a given direction. If the N signal, Fig. 1, N1, were being received, the A3 and N3 pulses would be interchanged and the circuit L1, C1 would respond, the rectifiers B1 and 2 being connected to deflect the meter in the opposite direction. Alternatively the circuits L1, C1 and L2, C2 may be tuned to different odd harmonics of the fundamental keying frequency. The circuit may be modified by omitting the rectifiers W1 and W2 and suppressing the negative pulses by biasing the valves V1 and V2 to cut-off. In a modification Fig. 3 (not shown), the push-pull sets of pulses at the terminals S1, S2 are obtained by using the valve V as a phase splitter and differentiating the outputs from the anode and cathode circuits by CR networks. In Fig. 4 (not shown) the rectifier bridge B and meter M are omitted and the outer ends of the two tuned circuits are connected through rectifiers to a pair of cathode ray tube deflecting plates, the direction of deflection of the cathode-ray beam depending on which of the two tuned circuits is excited.