GB1150997A - Selective Call System. - Google Patents

Abstract

1,150,997. Radio signalling. INTERNATIONAL STANDARD ELECTRIC CORP. 20 Dec., 1967 [29 Dec., 1966], No. 57891/67. Heading H4L. In a selective calling system of the type using a calling signal consisting of a sequence of elements comprising frequencies of specified duration which modulate a carrier, a station equipment includes a tuning device consisting of a capacitor and an inductor having a plurality of tappings which are connected successively into the tuning device circuit by a step-by-step switching means under the control of timing means, the equipment being operable either in the transmitting or the receiving mode. Received calling signals are amplified at 26, limited at 27 and supplied to a circuit 25 coupled to the tuning device 21, 22. In the normal (receiving) state a ring switch 37 is on position 1 and a bi-stable 30 is on position " O," so that a signal is passed to AND gates 36, 41, and transistor switch 40 is operative to connect tapping 3 of the inductor 22 into circuit, these tappings being connected to the switches 40 1 to 40 5 in accordance with the code allotted to the station. If the received frequency is appropriate to the tuning of the device at position 1 a signal is supplied via amplifier 32 and detector 33 to a shaper 34. The output of shaper 34 sets a monostable device 42 having a delay which exceeds the total signal duration and providing another input to the gate 41. The output of shaper 34 is also supplied directly and via a ¢-element delay 35 to the gate 36 so that when a signal has been present for half its normal duration the gate 36 is opened and via amplifier 64 steps the switch 37 to its next position. This process is repeated step-by-step until the last position is reached and this position transmits a signal via differentiator 43 to gate 41 to initiate a ringing signal. The subsequent resetting of monostable device 42 restores switch 44 to position 1. When transmitting, the switches 401 to 405 are connected to the tappings in accordance with the calling code of the desired station and a press button 30a is operated to set bi-stable 30 in the " 1 " position. This connects a reaction winding 28 so that the circuit acts as an oscillator and also brings into operation an amplifier 47 and an impulse generator 38 for stepping the switch 37. By this means the selected modulating frequencies are generated in sequence and supplied via the amplifier 32 to the transmitting amplifier 47. The first frequency transmitted sets the device 42 which at the end of its delay time resets the bi-stable 30 to normal via a differentiator 46 and resets the ring switch to position 1.