Modifications made to a COTS Rb standard for use under stressed operating conditions

This paper is a report on the modifications made to a commercial-of-the-shelf (COTS) rubidium standard, AccuBeat´s model AR-60A, to enable it to be operated under significantly more stressed environmental conditions. Also, an enhancement of its warm-up time has been accomplished so that, at an ambient temperature of -54°C it will achieve lock within 3.8 minutes. The design of the AR-60A unit employs surface mount technology (SMT) plastic parts, which are susceptible to moisture absorption, and which the parts vendors specify must not be operated below -40°C. Consequently, there exists the potential of damage to those parts during the manufacturing assembly soldering process. This paper discusses how international standards, such as ANSI J-Standard-20 (which deals with the handling of moisture sensitive microcircuits) and EIA JESD 26 (which is a Joint European Standard dealing with specifications for plastic encapsulated microcircuits for use in rugged applications), are being applied to address this problem. The basic unmodified AR-60A consists of just 3 modules. The unit contains a microprocessor which optimizes its performance versus external disturbances. It has a unique back-up mode which keeps the internal OCXO running with the last memorized frequency when lock is lost. In addition, a built-in synthesizer allows a very fine digital frequency control over a wide range. In order to modify the COTS AR-60A unit at minimum cost, a heater-blanket was designed to surround the plastic parts so that the AR-60A circuits would not turn on below -40°C. Additionally, the OCXO was upgraded with a low g sensitivity crystal to provide enhanced phase-noise under vibration performance