Patent GB-1496945-A (GB)
Title: Interrogating and warning radio systems
IPC: G01S13/78
Filed: 1975-07-01
Abstract: 1496945 Secondary radar; interrogator-responder systems DAINI SEIKOSHA KK 1 July 1975 [1 July 1974 3 July 1974 (2) 8 July 1974 (2)] 27735/75 Headings H4D and H4L In an interrogating and transponding radio system comprising a main station for selectively transmitting any of a plurality of predetermined different signals, on a common carrier frequency, and a plurality of subsidiary stations each responsive to a different one of said signals to transmit a predetermined response signal upon receipt of said interrogating signal, said main station having means for providing an alarm on receipt of said response signal at a strength below a predetermined level. The interrogation signals may be frequency, amplitude or phase modulated by individual ones of a plurality of modulation frequencies or by a digitally coded signal. The invention is applicable to control of children by means of a single adult who carries a main station, each child carrying a subsidiary (transponding) station. A further application is the warning of theft of vehicles from a parking place or of vending machines.  Main (interrogating) station.-As shown in Fig. 7, a clock pulse generator 32 is coupled to a shift register 33 which is set to the binary coded number representing the required transponding station under control of a keyboard 30 and logic circuit 31. The number of the selected station is indicated by means of a decoder 34 and display 35. When a different transponder is selected, a monostable multivibrator 36 initiates operation of a modulating signal generator 37 which is modulated at 39 on to a carrier generator 38 which is coupled via a transmit/receive switch 43 to an antenna. Monostable circuit 36 is arranged to change over to the receiving condition after a predetermined interval. In the receiving condition, the antenna is switched to receiver 40, detector 41, filter 42 and a flip-flop 46. The coded received signals are compared at 47 with a signal representing the coded modulation transmitted and, if correct, the shift register 33 is reset and the display 35 erased. If no signals are received after a predetermined time, a second monostable multivibrator 44 operates an alarm 45 and the display 35 continues to indicate the number of the interrogated transponder. Such a condition could be indicative of a child carrying the particular transponder having strayed. The selector 30 may be a keyboard (Fig. 9, not shown) and the operation may be initiated by means of a key-operated switch (Fig. 12, not shown) and the system reset by inserting a coded plug into a jack. Subsidiary (transponding) station.-As shown in Fig. 2, the coded modulated signal from the interrogator is normally coupled to a radio frequency amplifier 2, demodulator 3, low frequency amplifier 4, frequency selective filter 5 (e.g. a tuning fork) and rectifier 6. A monostable multivibrator 7 is triggered by the output of rectifier 6 which operates a transmitter for a predetermined time, said transmitter including a switching circuit 8, carrier oscillator 9, modulator 10, 11 and high frequency amplifier 12. Monostable 37 also operates the transmit/receive switch 1. If no signal is received for a predetermined period, a further monostable 13 operates an alarm 14.