GB1395482A - Circuit arrangements for telephone-charges indicators - Google Patents

Abstract

1395482 Automatic exchange systems; pulse width and frequency discriminators SODECO SOC DES COMPTEURS DE GENEVE 5 Jan 1973 [10 Jan 1972] 764/73 Headings H4K and H3T Metering at a substation is effected by bursts of pulses whose authenticity is determined by a pulse discriminating logic which determines that the pulses of a burst have a mark portion that falls within predetermined limits before permitting the pulses to be applied via an integrating circuit to an electromechanical meter which registers one unit per burst. In Fig. 1, metering pulse trains applied between both live wires and earth are coupled via electro optic device 2, e.g. a photo-diode and photo-transistor integrated circuit, to the pulse discriminator 12. The pulse output of the latter is used to charge a capacitor 16 which thereby permits transistor amplifier 18 to conduct throughout the train and thus enable the meter coil 17 once. The circuit is energized by line current utilizing a bridge rectifier 22 to guard against polarity reversals. Diodes 8 and 21 protect their shunt components from overvoltages. Back-to-back Zener diodes 5, 6 serve as a pulse amplitude descriminator, i.e. they reject pulses of lower than a predetermined amplitude. Pulse discriminator, Fig. 2, comprises two digital "high-pass" filters 24, 25 whose effective action is akin to that of a band pass filter in that if the repetition frequency and mark/space ratio of an incoming train falls within its "passband" the train is transmitted through the circuit to output 17. Each filter 24, 25 comprises a monostable 26, 27 and a D-type flip-flop 28, 29 respectively. The pulses present at the outputs of the circuit components are shown in Fig. 3, from which it can be gathered that the astable period of monostable 26 is longer than that of monostable 27 and that an authentic metering pulse train at the output of inverter 30 has a mark portion intermediate the two astable periods. In operation, the rising portion of the first pulse of a train is effective to cause the flipflops 28, 29 to adopt high and low outputs respectively on their Q and Q terminals. The falling portion of the first pulse triggers the monostables. Since it is assumed that the pulse train is authentic, the next following rising portion clocks flip-flop 29 unsuccessfully, i.e. its Q output remains unchanged since its data input is by now restored to the high level with the relapse of monostable 27. On the other hand the data input of flip-flop 28 is still at a low level so that its Q output changes to a low level. The second pulse of the train is thus transmitted through NOR gate 32. The output of flip-flop 28 is maintained at the low level throughout the rest of the train thus enabling the gate 32 to pass the rest of the train. If the mark period of the incoming pulse train is longer, or shorter, than both T1 and T2 then neither, or both, bi-stables change state with the result that the pulse train is inhibited. If the frequency of the train should change then the discriminator will react accordingly.