GB637112A - Device for measuring the time constants of oscillating systems - Google Patents

Abstract

637,112. Time -constant measurements. LABORATOIRES RADIOELECTRIQUES. April 28, 1947, No. 11308. Convention date, Nov. 23, 1944. [Class 37] A device for measuring the time constant of oscillating systems comprises means for generating a physical quantity which is a known function of the amplitude of the free oscillations of the system and means for integrating this physical quantity between two instants at which the amplitude presents two determinate values. In the arrangement shown in Fig. 1 for measuring the time constant of a piezoelectric crystal, the integrated physical quantity is the current obtained by rectifying the free oscillations of the oscillating system after any necessary amplification. The piezo - electric. crystal 1 of which the time constant is to be measured is inserted into the grid circuit of an oscillating valve 2, an adjustable resistance 3 being connected in series with the crystal to permit measurement of the time constant of a circuit including a crystal and a given resistance in series. A switch 4 permits the crystal 1 and resistance 3 to be short-circuited to convert the sustained oscillations of the system into free oscillations. An E.M.F. proportional to the current through the resistance 3 is applied by means of a potentiometer 5 to the grid of a Class A amplifier valve 6, the anode of which is connected through condensers 7, 8 to the grid of a Class C amplifier valve 9 and to the anode of a diode detector 10. The output from the valve 9 is passed to amplifier valves 12, 14. The cathode of the valve 14 is connected to earth through a resistance 15 and a condenser 16 in parallel and also to one terminal of an integrating instrument 17 whose other terminal is connected to the cathode of the diode detector 10. The integrating instrument 17 may be a fluxmeter, a condenser or other integrating device. When the switch 4 is open the valve 2 operates as an oscillator and the potentiometer 5 is adjusted so as to obtain on the anode of the valve 6 an alternating voltage having a predetermined amplitude V1, Fig. 3, higher than the voltage Ve at which the integrator is to begin to operate. The. switch 4 is then closed, thus suppressing the self-oscillating system and leaving the piezo-electric crystal 1 in a state of free oscillation. The amplitude takes up a value V2 and decreases according to an exponential law. So long as the voltage exceeds the throwing-in voltage Ve by a few tenths of a volt the valve 12 becomes saturated and the valve 14 passes a strong current giving a high voltage drop across the resistance 15, the alternating component being eliminated by the condenser 16. The cathode of the diode 10 is thus brought to a potential higher than the voltage applied to its anode so that no rectification occurs. When the voltage falls to Ve the valve 14 passes a negligible current and the diode 10 operates normally, so that rectified current flows through the integrator 17. As soon as the voltage falls to Vd, corresponding to the small voltage drop which has established itself across the resistance 15, the detector 10 ceases to operate and no more current flows through the integrator 17. Instead of being a component of a self-oscillating system the crystal may be excited by a separate oscillator, which may be synchronized by the crystal. The potential difference Vd which stops detection may alternatively be obtained by connecting the anode of the diode 10 to a point at negative potential with respect to earth. The oscillations supplied by the oscillating system under investigation may first be changed in frequency and the detector and integrator supplied with oscillations at the changed frequency.