GB1360301A - Receivers for plural f'equency signalling systems - Google Patents

Abstract

1360301 Data transmission: frequency shift keying active filters GENERAL ELECTRIC CO Ltd 27 Dec 1972 [6 Jan 1972] 713/72 Headings H4P and H3U Lower and higher band-pass circuits 6, 7, 8 and 9, 10, 11, Fig. 1A separate the two frequencies ("guard" and "trip") in an Input FSK signal and are respectively inhibited, to prevent erroneous operation by noise, by means of high and low pass filters 18, 14 also responsive to the Input. The latter is supplied to the band-pass circuits through a squarer 4 and main monostable 5 which produces a short pulse C (waveforms, Fig. 3, not shown) once per cycle. This pulse triggers guard and trip monostables 6, 9 which produce pulses D, G of duration respectively equal to an exact multiple of the period of the guard and trip frequencies. The trailing edges produce at 7, 10 short pulses E, H of which E coincides with C if the guard frequency is being received, and H coincides with C if the trip frequency is being received. This coincidence is sensed at 8, 11 to set or reset bi-stable 12 and operate (or not) a relay 13. The desired inhibiting is effected at these coincidence circuits, so that if for example a guard frequency is being received, the low pass filter will disable gate 11 to prevent its operation by noise which would give an erroneous trip indication. To warn that both frequencies are erroneously present, a coincidence circuit 23 operates an alarm 25 and clamps at 26 the bistable setting 12, a delay 24 allowing for non- persistence of the fault. Loss of input signal altogether is sensed at 27 to operate another alarm 30 and to clamp the bi-stable to whichever setting is selected by a switch 32. The reception of a signal whose two frequencies are incorrect is sensed at 33 by the absence of an output from either gate circuit 8, 11, the detector 33 being connected by OR gate 28 to the alarm 30.