Subminiature rubidium frequency standard: performance improvements

At the 1995 Frequency Control Symposium, performance results for the FE-5650A Rb frequency standard were presented. The FE-5650A is a 216 cc (13 cubic inch) device capable of producing any output frequency between 1 Hz and 15 MHz, with a setting resolution of 1/spl times/10/sup -14/. Since then several technical improvements have been incorporated into the FE-5650A product. These improvements primarily affect the aging and temperature performance of manufactured devices. These improvements are of particular importance in applications such as GPS receiver clocks, where performance in relatively uncontrolled temperature environments, over a time period of /spl sim/1 day are crucial. Allan variance at an averaging time of /spl sim/1 day is used as a simple measure of performance to compare and rank individual devices. Measurements with and without linear drift removal are presented, with a discussion of the practical implications of results. Statistical data from a large sample of manufactured devices are presented showing frequency aging and frequency vs. temperature performance before and after incorporation of the design improvements. Data and analysis are obtained from measurements made at FEI, as well as at NIST, in Boulder, CO. In addition, optional versions of the FE-5650A have been developed for operation over the extended military temperature range (-55 to +71/spl deg/C), and also for operation in vacuum. Performance test results are presented for these devices.