The new PTB primary cesium clocks

Abstract

One of the two primary clocks with a vertically directed cesium beam. CS4, has been running at constant operating parameters as a frequency standard since November 1991. Its main design features are reported. The contributions to its frequency uncertainty connected with C field, cavity phase difference, and second-order Doppler effect are given. The relative frequency difference between CS4 and CS2 averaged over six months is -12*10/sup -15/. The measured frequency instability of CS4 is 8*10/sup -12// square root tau for sample times tau , 300 s >