Patent GB-637111-A (GB)
Title: Device for measuring the time constants of oscillating systems
IPC: G01R29/22
Filed: 1947-04-28
Abstract: 637,111. Time - constant measurements. LABORATOIRES RADIOELECTRIQUES. April 28, 1947, No. 11307. Convention date, Nov. 23, 1944. [Class 37] The time constant of a mechanical or electrical oscillating system, such as a piezoelectric crystal, is measured by making the system oscillate freely and integrating a physical quantity which is a known function of time between two instants for which the amplitude of the free oscillations takes two determinate values in order to measure the time between these two instants. As shown in Fig. 1, the piezo-electric crystal 1, of which the time constant is to be measured, is connected to the grid circuit of an oscillating valve 2. The crystal may be short-circuited by a switch 3 in order to substitute conditions of free oscillations for those of sustained oscillations. The voltage across the terminals of the crystal is applied to the input terminals of two tripping devices 4, 5 whose output terminals are connected to the integrating instrument 6. The devices 4, 5 are arranged to trip at two different values of the amplitude of the free oscillation of the crystal 1 and current flows through the integrator 6 only during the interval. The tripping devices 4, 5 may be constructed as shown in Fig. 2, in which the crystal 1 is connected through a potentiometer 7 to the grid of a Class A amplifier valve 8. This is followed by amplifier valves 10, 12, 14, the cathode of the last of which is connected to earth through a resistance 15 and condenser 16 in parallel and also to one terminal of the integrator 6. The other terminal is connected to an identical circuit. The bias and anode voltages of the valves 10, 12 are so chosen that when the amplitude of the alternating voltage on the anode of the valve 8 is below that at which time measurement is to begin no current flows in the valve 12. The amplification factor of the valve 10 and the characteristics of the valve 12 are so chosen that the valve 12 becomes saturated when this amplitude is exceeded by a few tenths of a volt. The bias on the valve 14 is adjusted so that when there is no output voltage on the valve 12, the valve 14 passes no current. When the crystal begins to oscillate freely, and while the amplitude is above that at which time measurement is to begin, the amplitude of the voltage variations on the grid of the valve 10 causes saturation of the valve 12 and therefore large voltage variations on the grid of the valve 14. A strong anode current flows and causes a high voltage drop across the resistance 15, the condenser 16 eliminating the alternating components. As soon as the amplitude falls to the specified value the valve 14 passes no current, the voltage drop across the resistance 15 falls to zero, and the integrator 6 allows current to pass. When the amplitude falls below that at which the time measurement is to end, the voltage applied to the integrator by the second tripping device becomes zero and the integrator no longer passes current. In a modification, Fig. 3, a pentode 17 is inserted between the integrator 6 and one of the tripping devices 4. The grid voltages are so chosen that the pentode is blocked before the first tripping device operates and also after the second tripping device has operated. In the arrangement shown in Fig. 4 the integrator 6 is connected between the anodes of two diodes 23, 24 whose cathodes are connected to the output terminals of the tripping devices 4, 5. The anodes are connected to the same source of supply 25 through resistances 26, 27 of such values as to form two arms of a balanced bridge, the other two arms comprising the diodes 23, 24 and the resistances between the outputs of the devices 4, 5 and earth. Specification 637,112 is referred to.