Manufacturing issues for a high performance crystal oscillator

Abstract

Manufacturing issues affecting a high performance balanced-bridge controlled crystal oscillator are described. This paper covers design-for-manufacturability decisions relating to that design, as well as equipment and processes that were created specifically for the purpose of manufacturing this oscillator Special precision plating techniques are used to hold calibration tolerance to below 1 part in 10/sup 6/. The crystal parameters are then carefully measured over temperature in an oil bath. The measured crystal data is put through an algorithm to determine necessary manufacturing parameters. A laser-operated bar code engraving system marks the individual crystals. Statistics on large numbers of crystals are recorded in a database and used to adjust the manufacturing process as necessary to maintain extremely tight angle-of-cut tolerances to hold turnover temperatures within a relatively narrow range. The next step is to install the crystal in a "bridge" PC board. Finished bridge boards, with crystals, are then plugged into oscillator boards that are installed in oven assemblies. The exact turnover temperature is determined and errors such as thermistor tolerance are calibrated out. This information is added to the database that is used later to store the oven set point data in nonvolatile RAM on the controller board that holds the puck assembly. Tolerancing issues of the extremely high thermal gain oven are described. Great attention must be paid to the fabrication of the flexible circuit heaters for the oven mass. Another critical parameter is the ratio of heat applied to the top and bottom of the oven mass vs. the rim. We elected to choose a single value used for all ovens, which limited the thermal gain achieved in production to the neighborhood of about 100,000. A technique is described that we could have used, if necessary, to individually set the thermal gains of the ovens to approach 1,000,000.