Microwave Synthesis for the PTB Cesium Fountain Clocks

Microwave synthesizers that are used to generate microwave signals for the CSF1 and CSF2 fountain clocks at the Physikalisch-Technische Bundesanstalt (PTB) are described. The synthesizers are based on a divider chain and a digital frequency synthesis and have been designed for use with an optically stabilized microwave oscillator. They have been designed in a modular fashion to assure long-term operation and low down times. The results of an in-depth examination of spectral purity, phase noise, and long-term phase stability are presented. Also, the implications of measured parameters on the fountain timing are assessed. Two methods of suppressing the frequency-shifting effect of microwave leakage fields are implemented, with one method in each synthesizer. The first method uses an interferometric switch, and the second uses a phase-coherent and phase-preserving frequency detuning scheme; their contribution to the systematic uncertainty of the fountains is assessed. The overall systematic uncertainty contribution associated with the fountain electronics is below 1 × 10
^-17
for both fountain clocks.