Hi Ulrich, good details on how to set the time constant for best GPSDO performance etc! Some issues you did not mention are but that are essential to get a good GPSDO are: * Aging compensation * Temperature compensation * fault recovery, such as mechanical shock to crystal, power surge, acceleration, thermal shock, atomic particle impact (pop noise) etc. => Aging is mostly constant over periods of several days, except after first cold power-on. I just received a plot from an OCXO factory showing 1E-07 aging the first day, asymptotically slowing down to <1E-010 after 10 days! The oscillator is rated at 1E-010 per day (which is quite good) but clearly does not meet this spec in the first couple of days. Aging will show up as a constant offset as the GPS and the DAC is constantly "chasing" the crystal. The slope of the change in DAC voltage over time can be measured (but only after it has been temperature corrected!!) and then the software can compensate for aging so the TIC/GPS/Loop filter doesen't have to do it. This helps tremendously for maintaining stability during hold-over as well. I have seen some cheaper OCXO's age more than 1E-013 per second after being on for weeks and that's not a bad spec either! => Temperature compensation is helpful even on a near perfect oscillator (Rb or even Cesium!) because as long as the unit is being corrected by a voltage from an external DAC, the DAC, and Dac reference will have temperature dependencies that can be corrected. The problem becomes measuring temperature changes on the order of 0.001 Degrees Celsius resolution... Temperature compensation should be done for Aging prediction/compensation to work well. Otherwise one would not know if the drift is caused by aging or temp changes. Electronic temp compensation works great to make a good OCXO out of a mediocre one, but the temperature sensor's thermal response must match the OCXO's (and DAC/Reference) response as well as possible, otherwise the unit will be over/under-compensated. => Some non-linear processing can be done to speed up fault recovery. The problem then is: is it a GPS induced error, or an OCXO induced problem? Fuzzy logic helps to get the OCXO locked quickly when a big jump happens by for example momentarily changing the loop time constant for fast locking, then slowly going back to the intercept point on the ADEV curve you mentioned. bye, Said
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