An apparatus for recording information indicative of the logarithmic decrement or the logarithmic decrement and the frequency of damped harmonic oscillations

摘要

886,227. Frequency measurements. OLSSON, C. O. April 22, 1958, No. 12808/58. Class 37. [Also in Group XL (b)] In apparatus for recording information indicative of the logarithmic decrement and frequency of a damped oscillation, two signals each representative of the oscillation but mutually phase displaced by 90 degrees are applied respectively to two sets of deflection plates of a cathode-ray tube and measurements are performed when the C.R.T. spot, which moves in a spiral of decreasing radius as the amplitude of the signal decreases, is in an annular zone concentric with the centre of the tube screen. The damped oscillatory signal is applied via amplifier A to one set of C.R.T. deflection plates and phase-shifted 90 degrees in a network B before application to the other set of plates. A mask D, formed with a plurality of radial slots of equal length which extend outwards from a fixed distance from the mask centre, is placed between the tube screen and photo-cell E. When the spot is visible through one of the slots a pulsed signal is generated in the cell. This pulse is shaped in a trigger circuit F and recorded in a damping counter G. The logarithmic decrement is inversely proportional to the total count recorded. A predetermined one of the generated pulses opens a gate H via which the output of an oscillator J of known frequency is applied to a period counter K, a predetermined number of generated pulses later the gate is closed. From the predetermined number of generated pulses and the period count the frequency of the damped oscillation may be determined.