[time-nuts] Thunderbolt oven / non-stable operating temperature

SAIDJACK at aol.com SAIDJACK at aol.com
Wed Dec 12 06:06:32 UTC 2012

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.
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 
milli-C  TBolts.


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