Patent GB-1020649-A (GB)
Title: Telemetering of analog and digital information
IPC: G08C15/00
Filed: 1964-08-10
Abstract: 1,020,649. Electric selective signalling. LEEDS & NORTHRUP CO. Aug. 10, 1964 [Aug. 29, 1963], No. 32521/64. Heading G4H. [Also in Divisions H3 and H4] A diplex system comprises means for producing a carrier signal whose frequency is swung between two values at a repetition frequency corresponding to the analogue value of a first variable, and means for changing one or other of the frequency values in accordance with changes in the digital value of a second variable. As shown in Fig. 2, an analogue signal from a transducer 13 varies the frequency of an oscillator (not shown) which effects a corresponding vibration of contact 12 between contacts 12U, 12L, closure of contacts 12, 12U or 12, 12L producing a carrier of frequency F + #F or F - #F, respectively, from the transmitter 14. The digital keyer 16 includes switches 17U, 17L controlled by a cam 19 which has a neutral position in which both switches 17U, 17L are closed and further positions in which one or other switch is opened. When both switches are closed the transmitter frequency is swung between F + #F and F - #F about a mean frequency F but if one or other of the switches 17U, 17L is opened the frequency swing will be between F and F + #F or between F and F - #F at a repetition frequency determined by the analogue signal. The shaft 20 of the cam could be coupled to a device measuring the output of an electrical generator, the open position of switch 17U corresponding to an upper limit, the open position of switch 17L corresponding to a lower limit, and the closed position of both switches representing the condition when neither limit is exceeded. The analogue quantity could be the actual value generated. Alternatively, the digital signals could be control signals. At the receiver 26 a discriminator converts the frequency deviations into corresponding voltage deviations and 51 a direct voltage of magnitude determined by the repetition frequency of the signal is produced at 29. Zener diodes 51 limit the peak-to-peak voltage applied to converter 29. The positive halfwaves of the demodulated signal correspond with deviations in one sense from the mean carrier frequency F and the negative half-waves correspond with deviations in the other sense, ER these voltages - being supplied to digital 2 detectors 30U, 30L respectively. Transistor relay circuit. Each detector 30U, 30L may comprise a pair of transistors 35, 36, Fig. 3, connected to provide a high gain and input impedance, which in the absence of a periodic half-wave signal are fully conducting so that the relay 9 or alternatively the lamps 38 are operated. When the periodic half-wave signal is present either alone or as a component of a full-wave signal each positive-going rise charges capacitor 44 via diode 45 and for each subsequent negative-going fall the capacitor 44 discharges via the base emitter paths of transistors 35, 36 so that the transistors are cut off and the relay or lamps de-energized. A diode 56 acts as a protective device for the transistors, a capacitor 48 acts as an integrator to prevent relay chattering during application of the periodic signal, and a capacitor 49 prevents self-oscillation.