We studied how to control the frequencies of those simple crystals used in typical telecommunication transmitter. According to the common standards, the uncertainty of carrier frequency should be less than 0.3 ppm. That of simple XO is only 100 ppm. Two approaches have been studied here. The first one is the German transmitter DCF 77. The second one is the utilization of the NTP-time servers. We studied phase distribution and calculated Allan variances, both for raw data and cleaned data. Three-standard-deviation-criterion was applied to detect outliers. The results point out, that usually DFC 77 enables sufficient phase locking in a couple of seconds. The main problem is the atmospheric noise during thunderstorms in the late summer seasons. The NTP-time servers suffer from 100 mus level time jumps. Typically the 10 min integration time is needed to achieve the 0.3 ppm accuracy. Therefore, some kind of temperature compensation of those XOs is needed to fulfill the requirements.
|Joint Conference of the IEEE International Frequency Control and the European Frequency and Time Forum
|2007 IEEE International Frequency Control Symposium and Exposition
|29/05/07 → 1/06/07