THE NEW PTB PRIMARY CESIUM CLOCKS

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. T he contributions to its frequency uncertainty connected with C field, cavity phase difference, and second-order Doppler effect are given. Th e relative frequency difference between CS4 and CS2 averaged over six months is -12 . 10(-15). The measured frequency instability of CS4 is 8 . 10(-12)/square-root tau for sample times tau, 300 s less-than-or-e qual-to tau less-than-or-equal-to 20000 s.