Application critical parameters for rubidium standards

The purpose of this paper is to provide guidance to the novice in the field as well as a reference for the experienced. It helps identify issues for discussion with vendors and customers. Rubidium standards find two general applications, as frequency sources and as timekeepers. When used as frequency generators, spectral purity, long-term and short-term stability are key parameters. Spectral purity encompasses phase noise, spurs, harmonics and sub-harmonics. Clock applications mainly focus on long-term and medium-term stability and accuracy. In either application, the environment and its impact on the performance of the oscillator must be carefully considered. Not only environmental extremes but also the rate of change must be taken into account. Error budgets, based on the oscillator's specific environmental sensitivities allow performance prediction under actual operating conditions. Key parameters to consider are temperature, pressure, humidity, vibration, shock, magnetic field etc. The oscillator may show a transient and a permanent response to these stimuli. The performance also depends significantly on the operating time of the standard. A major benefit to the operator of a rubidium standard is its self-check feature, which indicates that its internal crystal oscillator is locked to the rubidium hyperfine transition. With the help of microprocessors and integrated sensors, the oscillator performance can be modeled. This is especially true if an external reference is available for a significant portion of the time. It is then possible to auto-correlate the measurements and achieve significant performance improvements even if the external reference is lost. This is of special interest in CDMA (Code Division Multiple Access) wireless telephone systems where a GPS receiver is needed to provide UTC timing. In such an application, the predictability of the standard ultimately limits its performance. Other important considerations in most applications are the MTBF and the time of maintenance free operation. Some pending maintenance actions can be predicted and reported so that the user can schedule the service event. This paper discusses these aspects in some detail and provides help in specifying/understanding critical parameters for frequency generators as well as for clocks.