GB1318015A - Radio-receiver - Google Patents

Abstract

1318015 Radio signalling TEKADE FELTEN and GUILLEAUME FERNMELDEANLAGEN GmbH 29 Nov 1971 [27 Nov 1970] 55352/71 Heading H4L A mobile radio receiver responsive to a calling signal consisting of a sequence of audio frequencies followed by an identification signal is provided with means dependent on the condition of the received identification signal to indicate whether suitable reception conditions prevail at the receiver. As described, the calling signal consists of six frequencies in the range 313 to 1063 Hz., each frequency being transmitted for a period of 100 ms. so that the complete code lasts 600 ms. and after an interval of 200 ms. the identification signal of 1153 Hz. is transmitted for not less than 200 ms. and the sequence repeated. The received signal is demodulated at D and supplied via a low pass filter TP to a limiting amplifier B, the identification signal being selected by a filter PZ, rectified at G (voltage U 0 ) and supplied to a comparator 1 operating as a Schmitt trigger. The audio frequencies forming the calling signals are utilized in conventional manner not described. If the voltage U 0 exceeds a reference voltage in the comparator 1, i.e. the reception field strength is adequate, the circuit triggers and the resulting voltage U 1 is fed to a timing circuit 2, a NOR gate 4, a differentiator circuit 5 and a second timing circuit 7. Timing circuit 2 provides a voltage U 2 which after the disappearance of voltage U 1 is maintained for the period of a call, i.e. 600ms. and is supplied to the NOR gate 4 which provides no output U 4 as long as at least one of the input voltages is present. If after completion of a call the identification signal is not received, the resulting voltage V 4 is integrated at 9 to operate a device via terminal K indicating that reception conditions are not adequate. Voltage U 2 is also supplied to a pulse forming circuit 3 producing a pulse U 3 which is supplied via gate 8, if it is conducting, to the integrator 9. At the start of voltage U 1 the differentiator 5 produces a pulse which via switch 6 causes circuit 7 to enable gate 8 for the normal duration of voltage U 1 (which must be at least 200 ms.). At the end end of this period gate 8 is blocked if the voltage U 1 disappears because a call is transmitted so that voltage U 3 cannot reach the integrator 9. If voltage U 1 disappears before the 200ms. has elapsed, e.g. due to fading when the vehicle is moving, gate 8 remains conducting and pulse U 3 causes the integrator 9 to operate the warning device.