Frequency-locked loop between a H-maser and a Cs clock

The time and frequency reference in Besancon is based on caesium clocks for absolute accuracy and long-term stability, on a H-maser for medium and long term stability, and on a liquid-He whispering gallery sapphire oscillator for best spectral purity and short-term stability. These standards are located in three different places. In order to obtain the full accuracy and stability at least in one site, we combine these references using a frequency locked loop. The traditional method consists of locking a VCO to a reference. The short-term stability of the VCO and the long term performance of the reference are therefore combined in the output signal, when the cut-off frequency of the control is properly chosen. Yet, in our case we have to lock the VCO to two references, the H-maser in the medium term and the Cs clock in the long term. The control system consists of two mixers that compare the references to a quartz oscillator (VCO), and of a recursive digital filter. The H-maser and the Cs clock are not controlled. This paper analyzes the method for choosing a suitable control algorithm, and presents the results based on simulated data.