Abstract :
Frequencies up to about 50 000 Mc/s can be measured in terms of a standard frequency by means of the apparatus described. The unknown frequency is compared, by the heterodyne process, with that of a harmonic of an auxiliary oscillator, the frequency of which is adjustable about 3 000 Mc/s and measured by means of a frequency-meter previously described. The method is capable of an accuracy of the order of 1 part in 106, but the present sources of frequencies of the order of 104 Mc/s are not stable to this degree even with the highest practicable stability of operating voltages. For example, several low-power oscillators with frequencies in the region of 25 000 Mc/s were found to vary in frequency by as much as 1 part in 104 during periods of 15 min, even under very stable conditions of power supply and temperature. This instability of the unknown frequency imposes a practical limitation on the narrowness of the band-width of the components used for the detection of the heterodyne frequency. In the present design the accuracy is limited thereby to about 5 parts in 105. The apparatus has been applied to an investigation of an absorption wavemeter of the resonant cavity type, for a similar region of frequency, oscillating in the H11n mode. It was found that although the first half-wavelength may differ by as much as 5% from the value calculated from simple wave-guide theory, the subsequent half-wavelengths agree with this value within the accuracy of measurement. The frequency of resonance may be calculated from the measured value of wavelength with an accuracy of 2 parts in 104, but the same wavemeter can be calibrated in frequency and thereafter used as a single-reading instrument with an accuracy of 3 parts in 105.
