Discussion of the uncertainty budget and of long term comparison of PTB´s primary frequency standards CS1, CS2 and CSF1

PTB continuously operates two primary clocks, the more than 30 year old CS1 and the CS2. The clocks´ standard uncertainties (1/spl sigma/) to realize the SI second were estimated as u/sub B/(CS1)=7/spl middot/10/sup -15/ and u/sub B/(CS2)=12/spl middot/10/sup -15/. Since 2000, the caesium atomic fountain CSF1 has been operated quasi continuously during 14 intervals, each of at least 15 days duration, in the so-called routine operation mode for which an uncertainty u/sub B/(CSF1) of typically 1/spl middot/10/sup -15/ was specified. During these intervals the CS2 and the CSF1 agreed well within the uncertainty u/sub B/(CS2). On the contrary, the CS1 frequency deviates slightly more from CSF1 than u/sub B/(CS1). The /spl sigma//sub E/ of the CS2 comparison data can be explained as white frequency noise of the CS2 (calculated from the CS2 signal parameters) combined with an extra contribution of less than 1/spl middot/10/sup -15/, whereas an extra noise contribution of 3.7/spl middot/10/sup -15/ is needed to explain the /spl sigma//sub E/ of the CS1 comparison data. In view of this, the individual contributions to the clocks´ uncertainty budget are analyzed.