GB1506711A - Frequency responsive circuits - Google Patents

Abstract

1506711 Frequency responsive circuit SOLARTRON ELECTRONIC GROUP Ltd 21 March 1975 [23 March 1974] 13007/74 Heading H3T The passage of a signal from a frequency domain transducer to an output 40 of frequency responsive circuit 10 is inhibited whenever the signal at input 12 lies without a range bounded by predetermined limits, the range embracing the normal range of frequencies produced by the transducer, the output signal at terminal 40 being derived from a monostable circuit 22. The duration of the unstable state of monostable 22 is just below the minimum period of signals produced by the transducer 14. Pulses (b) Fig. 3 (not shown) derived by clipping the transducer output waveform (a) trigger bistable 30 in a known frequency sensitive I.C. switch circuit 26, one bi-stable output Q being directly connected to comparator and averaging counter 32, the Q bi-stable output passing via cascaded monostables with timing circuits 34, 38; 36, 39, whose respective unstable states have durations t and t 2 , where t 1 = 1/f 1 , t 1 + t 2 = 1/f 2 where f 1 and f 2 are the upper and lower limit values, counter/comparator 32 providing an output signal only if the Q pulse, averaged over a number of pulses, terminates during the existence of the unstable state of monostable 36, in which case the transducer output frequency lies between the limits f 1 and f 2 . The counter/comparator output renders a F.E.T., connected in series and the input, or output of monostable 18 conducting to allow passage of the transducer signal. Alternatively a shunt F.E.T. may short the input of the monostable when blocking is required. The input signal clipping is performed by a pair of back to back diodes (51,52), Fig. 2 (not shown) in the input circuit of an op. amp. with feedback. When the signal is blocked, circuit 42, or an inverter (not shown) coupled to connection 28 provides an alarm signal, indicating that a total mass flow being produced by integrating the mass flow derived in circuit 42, receiving the signal from output 40 representing gas or liquid density, and a flow signal from turbine volume flowmeter 46 is invalid. The known transducer comprises a hollow cylinder in the liquid or gas, excited in a hoop mode resonance whose frequency is detected, and noise on cable 16 tends either to place the frequency of the signal at input 12 outside the transmitted range, or, for interference pulses, merely tends to change the timing, but not the number, of pulses from monostable 22; (c, Fig. 3).