Thermal effects in high performance frequency synthesizers

Very high performance frequency standards using cold atoms should reach a frequency stability σ_y(τ) of the order of 10^-16 or better. Such a frequency stability implies hard requirements in the whole standard, specially for the phase stability in the synthesis of the microwave probe signal. These problems have already been considered in previous papers but only a few microwave synthesizers fulfil the requirements in laboratory conditions. For space applications such as PHARAO, the phase coherence between the user output and the microwave probe signal must be better than 1 ps/°C. This performance is possible by means of a proper RF architecture and a few additional simple ideas. The theoretical approach, experimental results in the range 0.04 to 1 ps/°C and a possible design that should provide less than 1 ps/°C for a square or sine 10 MHz user output are presented