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.