Frequency Measurement in Electrical Communication

Abstract

The need for increased accuracy in the measurement of any frequency between one and several million cycles per second is first considered. A brief discussion of general types of frequency standards leads to the reasons for choosing a continuously operating generator of alternating current. The generator developed is, in effect, a regenerative system comprising a 100-cycle tuning fork, of low decrement, maintained in vibration by a vacuum tube amplifier. The absolute value of the frequency of the alternating current delivered by this generator is determined by counting the number of cycles executed in an accurately known time. A small synchronous motor is used for making this count. Recording apparatus for comparing the rate of the fork with various time indicators is described. Data are given showing the effect upon the frequency of such external conditions as temperature, potential of power supply, circuit constants of the amplifier and vacuum tubes. This information is obtained either by comparisons with a constant frequency or by determinations of the absolute value of the frequency. The ratio of the rate of the fork to the rate of a carefully maintained clock has been found to be constant to within six parts in 1,000,000 over considerable periods of time. For measuring the frequency of any alternating current used in electrical communication in terms of the known frequency there has been developed a decade arrangement of harmonic producers and a special modulator-rectifier circuit for combining known harmonics of the base and comparing other frequencies with them.