GB727434A - Improvements relating to frequency measuring instruments - Google Patents

Abstract

727,434. Frequency discrimination. CROMPTON PARKINSON, Ltd. Nov. 24, 1953 [Dec. 4, 1952], No. 30821/52. Class 40 (5). A frequency-measuring bridge comprises tuned circuit arms 1, 2 and half-wave rectifiers 5, 6 shunting resistor arms 3, 4. The rectifiers 5, 6 are similarly poled and resistors 3, 4 may be equal in value with the bridge fed at 12 from the supply mains and 1, 2 resonant above and below and having equal impedance at the normal frequency of the supply mains 12 so that under these conditions the bridge is balanced and centre-zero microammeter 17 remains undeflected. If the frequency 12 deviates from normal the bridge is unbalanced and the impedances of 1, 2 vary oppositely and resistors 3, 4 conduct during the same half-cycles and microammeter 17 averages half-cycle pulses and thus indicates the sense and magnitude of the deviation in the supply frequency 12. During the half-waves when rectifiers 5, 6 conduct, resistors 3, 4 are short-circuited and bridge output is always zero. The bridge comprises a balancing potentiometer 11 and is paralleled with a stabilizing load resistor 16 and fed through high-frequency attenuator-inductor 15 and this may be adjustable to ensure pure sinewaves at a desired voltage across the bridge. To compensate for supply voltage variations the microammeter 17 is shunted by reverse-parallel half-wave rectifiers 20, 21. In a modification the bridge is balanced at off-normal frequency so that frequency change in one sense only is indicated. Also an arrangement may be used in which the resistor arms are not equal so that at bridge balance the impedances of the tuned circuits are not equal.