Patent GB-936592-A (GB)
Title: Improvements in or relating to signalling systems
IPC: B61L25/04
Filed: 1959-10-20
Abstract: 936,592. Inductive signalling; radio signalling; radar. GENERAL PRECISION Inc. Oct. 20, 1959 [Oct. 24, 1958], No. 35524/59. Classes 40 (5) and 40 (7). In a system for signalling between one or more interrogator stations and one or more responder stations, for example, for identifying aircraft, or, more particularly for determining the position and identity of railway vehicles, an interrogator station generates an interrogating signal having a plurality of components of different frequencies, each of which activates on reception by a responder, a different group of response signal generators of different frequencies. Either the responders or the interrogators may be stationary or in a moving vehicle. A radio link is preferably employed if the system is used for signalling to aircraft. A system is described in which a number of interrogators are placed at different positions along a railway track, and individual trains or railways trucks carry a responder, the coupling between interrogators and responders being inductive. A plurality of oscillators 104-108 in an interrogator are pulsed in succession by a modified ring counter 128 to produce an interrogating signal comprising a sequence of different frequencies F1-Fp. Counter 128 is controlled by a 400 c/s. oscillator 124 and bi-stable circuit 126 so that each oscillator is pulsed on for one half-cycle of oscillator 124, a half-cycle interval being loft between the operation of successive oscillators. The signal components are fed through a variable device 116 and amplifier 120 to transmitting means 122, for example a resonant circuit. A responder includes a plurality of sharply tuned resonant circuits 142-146, each corresponding to one interrogating frequency. Each of these resonant circuits is connected to a rectifier 150-154 whose output is connected to a group selected from response signal generators 156-160 operating at frequencies #1-#q. The outputs of generators 155-160 are each connected to a resonant circuit 166-170 comprising the transmitting means of the responder. The response signal comprises a sequence of different frequency groups, each group corresponding to a particular interrogating frequency, and is picked up by resonant circuit 180 at the interrogator receiver. The received response signal is fed to a number of filters 184-188 corresponding to frequencies #1-#q. The outputs of filters 184- 188 are passed through rectifiers 190-194 and thence to difference amplifiers 212-216 whose other input is a reference voltage from source 218. The receiver includes a plurality of storage channels 196-200, one for each interrogating frequency F1-Fp, and each channel has a number of stages, one for each response frequency #1-#q. Gates 202 are controlled by counter 128 and multivibrator 252 so that the outputs of amplifiers 212-216 are only fed to that storage channel corresponding to the interrogating frequency which produced the response signal being received. The code signal thus built up in channels 196-200 represents the identity, and possibly other information concerning the train or vehicle, and may be recorded, e.g. on punched or magnetic tape, or transmitted to a central traffic control station. Since it is desirable for the effective coupling between interrogator and responder to remain constant over a limited range of separation of the two equipments, an a.g.c. system is included. This system comprises a further interrogator oscillator 230, responder pick-up circuit 234, rectifier 236 whose output activates a further response oscillator 238, and transmitting circuit 204. Oscillator 230 is operated in the intervals between the operation of oscillators 104-108, and the received age response signal is fed through filter 242, rectifier 244 to difference amplifier 246 where it is compared with the reference voltage. The output of amplifier 246 controls variable gain device 116 so that the overall gain of the system remains constant over a given range of separation of the interrogator and responder. The output of amplifier 246 may also be employed to operate multivibrator 252 to provide the necessary gating pulses and also to clear the storage channels by way of conductor 250 after a completed interrogation.