Hello Tom, the GPS noise dominates for typical double oven OCXO's where the tempcos are very small (say below 5E-012 per degree C). On single oven units, the tempcos are typically 50 to 200 times larger, and thus the required EFC change over temperature is also that much larger. If I am not mistaken Thunderbolts have double oven units, and Mini-T's have single oven units? That EFC change can only be done through either prediction (sensing temperature and applying a correction factor) or through generating a phase error that feeds through the loop system. Crystal aging also typically requires a constant phase error which will in turn create a constant change in EFC voltage to correct for aging until active aging compensation can measure and apply this change in EFC. The magnitude of this phase error typically depends on the loop gain and the rate of change of the crystal frequency over time. In summary, we see fairly significant improvements in single oven systems with active compensation even during GPS reception, and we don't see much improvement in double oven units for temperature, but similar improvements for double oven units on aging. Now double oven units typically have SC-cut crystals, and single oven units typically have AT-cut crystals where the AT cuts typically have larger aging and retrace than SC cut crystals, so that can skew the performance in favor of double oven units as well. Bye, Said In a message dated 12/11/2012 19:41:58 Pacific Standard Time, [email protected] writes:
I agree this is true in theory (where epsilon != zero), but it's hard for me to believe true in practice. I would guess the error term is totally dominated by short-term GPS noise, and anything else, like tempco or frequency drift, is secondary. That's why it makes sense to apply these 2nd or 3rd order corrections during hold-over mode (where there is no GPS noise) but not for normal operation. > So what does this mean for the average Nut's Tbolt? Mostly nothing. > The only time the temperature sensor has any effect is during holdover. Thanks for stating both these facts so clearly. I cringe every time I hear someone replacing a 1C DS1620 sensor in their TBolt. The TAPR TBolts I tested years ago worked equally well regardless if they were 1C TBolts or ten milli-C TBolts. /tvb _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
