[time-nuts] Thunderbolt oven / non-stable operating temperature
SAIDJACK at aol.com
SAIDJACK at aol.com
Wed Dec 12 06:06:32 UTC 2012
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.
In a message dated 12/11/2012 19:41:58 Pacific Standard Time,
warrensjmail-one at yahoo.com 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
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