CS2: The PTB's new primary clock

The design principles of the Physikalisch-Technische Bundesanstalt's (PTB's) new primary clock, CS2, are reported. Experimental results concerning C field homogeneity, RF power dependence, cavity phase difference, and its gradient are given. The theoretical frequency instability due to shot and thermal noise, σ(2, 1 s) = 2.7 × 10
⁻¹²
, is compatible with the data so far obtained. The present uncertainty of CS2 is estimated to be 22 × 10
⁻¹⁵
. Frequency intercomparisons of the PTB's two primary clocks, CSI, and CS2, averaged over one year yield a fractional difference of (3 ± 4) × 10
^−l5
.