Poul-Henning Kamp wrote: > You got me wondering: How is the TOT determined ? Is it per unit > or is it per batch ? How precisely does an OCXO hold the temperature > on the TOT ? For crystals used in precision OCXO the TOT is measured and recorded per unit. There are two main methods: passive and active In the passive method the crystal is inserted in a test fixture and its frequency and other parameters are measured using a network analyzer (state of the art). The crystals with its test fixture is in a precision temperature chamber, whose temperature is varied in fine steps in the vicinity of the expected TOT. The TOT is then calculated by curve fitting of the f(T) data by using a 2nd or 3rd order curve fitting algorithm, depending on the temperature span. Accuracy and repeatability are in the range of a (few) tenth of a degC. In the active method the crystal is operating in its oven, whose temperature is varied over a certain interval. Evaluation of the TOT by similar curve fitting as above. Accuracy and reproducibility is about 1/100 degC or slightly better. The temperature accuracy and stability of an oven dpends strongly on its construction, and there are several orders of magnitude difference betewen a simple low-cost DIL14 size OCXO and a high-end OCXO with sub-ppb stability. THe electronic part of the temperature control is not the difficult task, the main issue is to have a UNIFORM temperature over all frequency determining components, which is accurate and stable - and uniformity needs physical size. Regarding stability over time, the most critical components are the temperature sensors, mostly thermistors.
Regards Bernd DK1AG _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.