GB1089372A - Improvements in or relating to frequency measuring apparatus - Google Patents

Abstract

1,089,372. Frequency detectors. GENERAL ELECTRIC CO. Ltd. April 14, 1965 [April 17, 1964], No. 16000/64. Heading H3A. A method of comparing two nominally equal frequencies comprises producing a form of sawtooth wave of one frequency and sampling it at the other frequency to produce a constant amplitude sawtooth wave at a multiple of the beat frequency, the slope of this sawtooth wave being measured by a differentiating circuit feeding an output meter which indicates the beat frequency. In Fig. 3, which can measure differences of 10<SP>-4</SP> cycles/sec., the input frequency #1 is converted to a sawtooth wave of " acme thread " form having an initial slope 20 equal to a multiple n (e.g. 10) times that of the equivalent symmetrical wave. The other input #2 is multiplied in unit 31 by the same multiple n and converted by unit 32 into two anti-phase pulse trains of frequency n#2. These two pulse trains open respective gates 23 and 24 to sample the initial slope 20 at ntimes the beat frequency. to produce waveforms a or b according to whether the frequency difference is positive or negative. These wavefors are differentiated by circuits 25 and 26, one of which feeds the output meter 27 according to the state of contacts 28 and 29 of relay 38. Gates 33 in the path of pulses from unit 32 are opened by bi-stable circuits 35 set by pulses from the pulse former 21 and re-set by the outputs of the gates via delay elements 37. Detectors 36 change over relay 38 when necessary so that as one set of pulses from unit 32 drifts away from the slope 20, the other set of pulses is made operative via relay 38 and its contacts 28 and 29. A check circuit for the beat period comprises a 1 cycle/second generator feeding a counter 42. via gate 43 controlled by a bi-stable circuit 44 triggered by a further contact 45 of relay 38. Thus alternate beat periods are counted, giving the number of seconds per beat cycle.