Patent GB-1176639-A (GB)
Title: Inductive Transmission System.
IPC: H04B5/00
Filed: 1968-08-07
Abstract: 1,176,639. Inductive signalling. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 7 Aug., 1968 [10 Aug., 1967], No. 37693/68. Heading H4U. In a system for inductive signalling to road vehicles, of the type in which an inductive loop system is arranged along the track of the vehicle and the receiver on the vehicle is provided with an inductive receiving loop, a trigger signal is transmitted which is received before the main signal for vehicles moving in one direction, whereby the receiver is switched to permit the same inductive loop system to be used for transmission in both directions without interference between the signals intended for each direction of travel. The signals for each direction are transmitted on different carrier waves. General arrangement.-The road has a track 11, Fig. 1, for vehicles moving in one direction and a track 12 for vehicles moving in the other direction, a main loop 13 extending over both tracks and a trigger signal loop 14 being located only on track 12 and in front of loop 13. The loops are connected to a transmitter 15. Transmitter.-The transmitter 15, Fig. 2, includes oscillators 16, 17 producing carrier waves of 50 and 70 kc./s. respectively and a trigger signal source 25 producing an output at a frequency of 4500 c/s. The 50 kc./s. carrier wave is modulated at 18 with a signal from 19 and supplied to an adding circuit 20 where it is combined with the 70 kc./s. carrier wave modulated at 22 with a signal from 23. The output of adder 20 is amplified at 21 and supplied to the loop 13. The 50 kc./s. carrier is also modulated at 24 with the trigger signal and combined at 26 with the unmodulated 70 kc./s. carrier, the output of adder 26 being amplified at 27 and supplied to the loop 14.  Receiver.-The vehicle receiver, Fig. 3, includes a selective amplifier 29 having two tuning frequencies of 50 kc./s. and 70 kc./s. and normally tuned to 50 kc./s., the tuning being changed from 50 to 70 kc./s. by the application of a control signal on lead 30. Considering a vehicle travelling on track 12, the signals on loop 14 are received first and the amplifier 29 will pass only the 50 kc./s. carrier modulated by the trigger signal. The D.C. representing the carrier strength appearing at the output of detector 31 is detected by a threshold circuit 34 which operates a switch 37 to connect the output stage 35, 36 of the receiver and also supplies a signal to a gate 40. A filter 33 selects the trigger signal which is rectified at 38 and supplied as a second input to the gate 40, whereby the amplifier 29 is tuned to 70 kc./s. and now passes only the unmodulated 70 kc./s. carrier. After it has been activated by the coincidence of signals on leads 39 and 41, the gate 40 is arranged to remain open with an input on lead 41 only so that amplifier 29 remains tuned to 70 kc./s. as long as a 70 kc./s. carrier is received. Thus when the loop 13 is reached the 70 kc./s. carrier modulated with the signal from 23 is received, demodulated at 31 and supplied to the loud-speaker 36. A vehicle travelling on track 11 receives the modulated 50 kc./s. carrier via loop 13 and the carrier is detected by device 34 to operate the switch 32 so that the demodulated signal (from source 19) is supplied to the loud-speaker.