Circuits employed in the N.P.L. caesium standard

A description is given of the electronic circuits used for the excitation of the caesium resonance and the measurement of the resonant frequency, attention being drawn to the precautions which have to be taken in order to obtain an accuracy of ± 1 part in 1010. The exciting source is derived by the multiplication of the frequency 5.0069 Mc/s of a quartz oscillator. The oscillator is then set to give the resonant frequency exactly. The measurements are made at the high frequency of approximately 9 Gc/s so that an accuracy of 1 c/s is obtained in one second using a counter in the final stage. An alternative and rather simpler system is also described, and some comments are made on the problem of frequency synthesis.