Patent GB-557359-A (GB)
Title: Improvements in or relating to arrangements for measuring or deriving currents from electrical signal amplitude changes
IPC: G01S1/02
Filed: 1942-04-14
Abstract: 557,359. Measuring current or voltage. STANDARD TELEPHONES & CABLES, Ltd., TERRY, V. J., and HARGREAVES, T. F. S. April 14, 1942, No. 4924. [Class 37] [Also in Group XL] An indication of, or a current dependent upon, the magnitude and sign of a repeatedly occurring voltage change, is obtained by an arrangement in which the voltage before and that after the change are caused respectively to charge two condensers, the difference between the charges controlling a valve circuit so as to produce in a branch of that circuit the current dependent on the voltage change. The invention is particularly applicable to aircraft landing systems using overlapping beams modulated by complementary dot and dash signals. When off course, either dot or dash signals of the forms shown respectively by (1) and (2), Fig. 1, are received. According to the invention, the voltage change v which occurs on the reception of a dot or dash is determined in sign and magnitude (the sign being opposite in the two cases) by measuring the difference between the voltages occurring during two short periods t 1  to t 2 , and t, to t, chosen, respectively, during and after the occurrence of the dot or dash. Fig 4 shows one form of receiver, in which the incoming signals are applied through a transformer T, to the control grids of two valves V 5 , V 6  in parallel, the amplified signals in their plate circuits being applied by a transformer T 2  to a full-wave rectifier W,. The rectified output, after smoothing, produces across the output load resistance R 21  voltage waves of the forms (3) or (4), Fig. 1, respectively, in the case of reception of dot or dash signals. These voltage waves are, by means of relayoperated switches h 1 , j 1 , q 1 , g 1 , g 2 , caused successively to charge two condensers K 11 , K 12  respectively during the periods t 1  to t 2  and t 3  to t 4  to the voltages v 1 , v 2 , after which these condensers are, by reversal of the switches g 1 , g 2 , connected across two equal condensers K 13 , K 14 in series, which are thereby charged to a voltage the magnitude and sign of which corresponds to v 1 , v 2 . The condensers K 13 , K 14  are shorted by leak resistance R 13 , R 14  and are in the grid circuits of the valves V 5 , V., the grid potentials of which are thus changed by equal and opposite amounts, resulting in similar changes in the anode currents and corresponding changes in their cathode potentials, produced by equal resisttances R 15'  R 16 . The difference between the latter is indicated by a meter M connected between the cathodes, which thus indicates whether dot or dash signals are being received and their magnitude. The contacts h 1 , j, &c. are operated in the desired sequence by a train of relays G, H, J, Q, which is determined by inter-control of the energizing contacts of the relays and by a time-lag in the operation of certain of them. The cycle of operation of the relays is initiated by closure of contacts f 1  by a mono-polarized relay F energized at the beginning of a cycle by a pulse produced by a valve V, to the grid of which the voltage across the resistance R 21  is applied through a condenser K 19 . The pulses, which occur in pairs as shown at (5), Fig. 1, are amplified by a valve V 8  and applied through a high-pass filter to the relay F. The first pulse of a pair, of whichever sign, thus causes initiation of a cycle, the closure of the contacts f 1 , by the second pulse being ineffective. Rectifiers W 4  and W 5  short-circuit the grids and cathodes of the valves V, and V 8  should strong signals tend to make the grids positive, and rectifiers W 6 , W, biased with respect to the cathodes by resistances R 20 , R 30  prevent the grids becoming excessively negative. In another illustrated form of receiver the signals are rectified without previous amplification by a bridge rectifier, and the condensers K 19 , K 14 , are replaced by a single condenser connected in the grid circuit of a single valve, to the cathode circuit of which the meter is connected. The pulses for operating the initial relay of the train are rectified in a second bridge rectifier so that the starting pulse is always of the same sign, and are applied to the grid of a valve normally biased to cut-off point, in the plate circuit of which is the first relay. It is not essential that the voltage change measured should be regularly repeated since as long as the grid leaks R 13 , R 14  are chosen with regard to the longest times between successive repetitions a steady reading on the meter M will be obtained. Measurements may thus be made during the passage of a code signal. The meter may be replaced by a relay or other device to be operated by the output current.