A precision frequency-meter of range 0 to 2000 Mc./sec.
Louis Essen · Proceedings of the Physical Society · 1940
The paper describes a new method and equipment for the measurement of frequencies in the range 0 to 2000 Mc./sec. Frequencies less than 30 Mc./sec. are compared directly with a signal obtained from oscillators controlled by the standard, and the accuracy of the measurement is limited only by that of the standard, which is ±2 parts in 10 8 if the primary quartz clock at the National Physical Laboratory is used. Frequencies above 30 Mc./sec. are compared with harmonics of an uncontrolled oscillator of which the frequency is less than 30 Mc./sec. and is measured by the direct method. The accuracy is then governed by the stability of this oscillator during the measurement and is of the order of 1 part in 10 6 . The method involves a simple extension of the principles of frequency multiplication and division. From the standard frequency f , controlled oscillations are produced at frequencies ( a / b ). f , where a has the whole-number values between 8 and 11 and b has values between 9 and 22. A scale of frequencies n ( a / b ). f is thus obtained, where n is the order of harmonic, and the various possible combinations of n, a and b give frequencies on this scale at an average spacing of 2.4 kc./sec. The apparatus is very simple in construction and operation. It does not, however, permit of direct frequency reading on a decade scale, but gives the values of n, a and b , the actual value of frequency then being read from tables. The paper includes notes on the accuracy of harmonic methods of frequency measurement and on the division of high frequencies.