Patent GB-625428-A (GB)
Title: Improvements in and relating to precision frequency meters
IPC: G01R23/02
Filed: 1946-06-06
Abstract: 625,428. Frequency measuring. BRITISH THOMSON-HOUSTON CO., Ltd. June 6, 1946, No. 17204. Convention date, June 9, 1945. [Class 37] [Also in Group XL (c)] Frequency measuring apparatus comprises a standard oscillator and an electronic switching circuit controlled by the oscillator and by the frequency to be measured to produce square pulses of constant amplitude and duration at the unknown frequency, so that the average current of the square pulses is proportional to the unknown frequency. The operation of the invention is explained with reference to Fig. 1, in which the motor 9 is continuously operated by the standard frequency, and the lower unknown frequency is applied to relay 14 to disengage latch 13 once each cycle and allow cam 12 to be rotated by motor 9 through the friction clutch 11; thus, cam 12 rotates once for each cycle of the unknown frequency to connect contact 3 to resistance 6 for a constant period; resistance 6 is thus traversed by constant duration, constant amplitude pulses at the unknown frequency, and the average potential developed thereacross is measured by a potentiometer 7 and is proportional to the unknown frequency. In the practical embodiment of Fig. 4, a tuning fork 32 controls an oscillator 34 to produce a relatively high constant frequency sinusoidal oscillations 016, Fig. 5, which are'applied to a trigger-pair 36 to produce square pulses 017, Fig. 5, which pass through condenser 43 to become alternately positive and negative pulses which are applied to the outer cathodes of valves 41, 42 constituting an electronic switching device. The unknown frequency 045, Fig. 5, is applied at 45, Fig. 4, to an amplifier 19 which feeds a trigger-pair 48 to produce square pulses 020, Fig. 5, at the unknown frequency which are applied through a condenser and a rectifier (left half of valve 49) to produce positive impulses, 049, Fig. 5, which trip the right half of valve 49 to render it conducting, which in turn renders the left half of valve 44 non-conducting. The valves 49, 44 thus provide a positive pulse 044, Fig. 5, whose commencement is determined by pulse 049, and this pulse 044 is applied to the outer grids of valves 41, 42. As a result, the pulses from the trigger pair 36 produce a pulse S42, Fig. 5, on the anode of the inner half of valve 42 which is fed back over condenser SO to valve 44 (right-hand half) to provide a positive pulse over condenser 51 which is applied to the left half of valve 44 to terminate the pulse 044 and thus prevent the further production of pulses S42 in the anode circuit of valve 42. Thus, valves 41, 42 under the control of trigger pair 36 and valves 49, 44 produce a pulse S42 whose duration is one period of oscillator 34 and which occurs once each cycle of the unknown frequency 045. This pulse, and its inverse S52, Fig. 5, are applied to valves 53, 52 respectively to render them alternately conducting and allow current to pass from the valve 54 through the resistance 6 for the periods S42 only, so that the average potential can be measured by comparison with a potentiometer 7 through an indicator 8. The valve 54 is coupled to valve 55 to maintain a constant current through valve 54. The potentiometer may be self-balancing, Fig. 3 (not shown). The range may be adjusted by switch 46 tapped on to appropriate portions of resistance 6; in its final position, the switch 46 connects standard oscillations from oscillator 34 to amplifier 19 and takes the output from resistances 61, 62 for calibration. Resistance 15 is used for zero adjustment, and an adjustable generator 67 is employed to prevent hunting of indicator 8. The unknown frequency may be developed from a tachometer-for measuring speed.