Techniques of frequency measurement in the centimetre-wave region

Abstract

During the past five years the methods of frequency measurement, based on the quartz-crystal standard at the National Physical Laboratory, have been greatly extended both in frequency coverage and in accuracy. It is now possible to measure any frequency up to 50000 Mc/s with a precision of one part in 108.The standard method of frequency measurement in the centimetre-wave region, up to 1947, consisted of the use of a two- or three-stage heterodyne wavemeter calibrated against an internal crystal-controlled oscillator, which in turn was compared with the 100-kc/s standard.Since 1947, the development of the frequency standard has proceeded along two separate lines, first by the construction of a frequency synthesizer, deriving its output directly from the 100-kc/s quartz standard and having the advantages of ease in operation and continuous frequency coverage and secondly by a system of frequency analysis in which crystal-controlled oscillations derived from a simplified multiplier chain are successively subtracted from the unknown frequency leaving a small portion in the a.f. range to be measured by any convenient means according to the accuracy required. This system is of more limited application than the synthesizer but has a higher inherent accuracy owing to the reduction in the number of processes involved.With both the methods now in use, the production of the standard frequency in the centimetre-wave region depends upon the use of a crystal diode as a harmonic generator for continuing the frequency-multiplication process beyond the point where thermionic valves are readily available for this purpose.The properties of a number of types of crystal diode used in this way have accordingly been investigated and the optimum requirements of input power and bias voltage determined. It is found that, under the right conditions, most types of silicon diode and several types of germanium diode are suitable as harmonic generators, the germanium types in general requiring less input power than the silicon to produce the same amount of harmonic power. Some measurements have been made on the power which can be obtained over a wide range of harmonics including a few lower harmonics when the input power is limited. An empirical law is suggested which enables the power available in any given harmonic to be predicted with a useful degree of approximation.