Synthesizing frequency from a convenient lab reference to a stable interrogation frequency, or clock transition, in various atomic standards can, by itself, be a formidable task. Clearly the best short- and long-term stability can be obtained only if the frequency synthesis does not introduce noise that masks the atomic noise limit. In the best standards, the signal used for atomic interrogation must have exceptionally clean spectral purity. This means low broadband phase noise and extremely low long-term random walk of phase (RWPM) characteristics. To reduce phase noise over the widest possible range of averaging times, NIST's Phase Noise Group has adopted a successful strategy of designing synthesizers that are limited by the very low short-term phase fluctuations inherent in regenerative and digital dividers and that use a series of optimized phase-locked loops. RWPM linked to the environment and predominantly due to a myriad of phase drifts in other designs that use narrowband filters, multipliers, etc., has been substantially reduced. A new temperature-compensation scheme has resulted in unprecedentedly low synthesis phase variations, summarized by the figure below showing the phase of a synthesized Cs interrogation signal and (100 MHz) lab reference output vs. environmental temperature. Correlation of phase deviation to temperature has been suppressed to below the flicker of phase limit. Tests indicate an internal fractional frequency stability of 1.5 x 10-15 at 10 s and 1 x 10-18 at 1 day with an estimated temperature coefficient of 0.1 ps to 0.5 ps/K for 5, 10, and 100 MHz relative to the Cs transition at 9.192 GHz .
A new generation of synthesizers includes digital control of the oscillators so that no mechanical tuning is needed over a 25-year lifetime of normal aging; power from single 24 ±4 VDC power supply; RS-422 interface for output frequency and phase control and monitoring functions; simultaneous outputs of 9.192 GHz for Cs, 6.834 GHz for Rb, 1.42 GHz for H-maser, 40.5 GHz for Hg+, 10 GHz for femtosecond pulse-repetition-rate generation, etc. The NIST synthesizer can also be phase-locked to an external reference of 5, 10, or 100 MHz or a microwave cryogenic oscillator. The Group's synthesizer is the mainstream technology for many laboratories engaged in fountain and slow-beam atomic standards. Devices have been delivered to NASA-JPL, NPL-England, Observatoire de Neuchatel-Switzerland, CRL-Japan, and NPL-India, with additional deliveries pending.
 A. Sen Gupta, J. F. Garcia Nava, C. Nelson, D. A Howe, and F. L. Walls, "Microwave Synthesisers for Atomic Frequency Standards," Proc. 6th Symposium on Frequency Standards and Metrology, St. Andrews, Scotland, Sept. 9 - 14, 2001.
Low-phase-noise synthesizers play a critical role in the performance of primary frequency standards, so the Phase Noise Group has been working to improve the reliability and performance of these devices to support rapid advances in these standards. Craig Nelson and Dave Howe, along with guest researchers Francisco Garcia and Archita Hati, have recently made substantial improvements that are important not only for fountain standards but also for other frequency standards such as the laser-cooled space clock for the PARCS mission. Of particular importance to this latter project is the complete re-engineering of the system to operate on the lower DC voltages that are used on the space station. However, most important has been the reduction in sensitivity to temperature variations. The synthesizers now exhibit a temperature coefficient on the order of 0.1 ps/K. This is an important parameter, because variations in phase have an impact on phase changes that occur during dead time in standards.
NIST leadership in this area is highlighted by the fact that most new standards in the world are or will be using the NIST synthesizer. In fact, there are very few programs that are building their own synthesizers. NIST has now completed and delivered 10 synthesizers to other programs, primarily other primary standards laboratories, and four more are under construction. Because the operational mode and requirements of the various standards are different, some custom design changes have been made for almost every one of these synthesizers.