[time-nuts] Tbolt temperature sensor

[SAIDJACK at aol.com]

Hi Mark,
 
from my experience even a resolution of 0.25C is several orders of  magnitude 
too low for use in holdover temperature compensation on an OCXO.
 
This would result in fairly massive EFC changes on the Thunderbolt ovens,  
especially if they are single ovens with significant thermal sensitivity.  The 
average error for a typical single oven OCXO due to this would be  ~1.25E-010, 
see below.
 
To make this work somewhat smoothly, one would need at least 0.001C or  
better 0.0001C resolution.
 
I can't get into too much detail, but our units allow for down to  ~0.000025C 
resolution by using a 24 bit ADC for the temperature sensing. Keep in  mind 
that for temperature compensation the absolute temperature is not of  interest, 
but rather the change of temperature over time. This kind of  resolution is 
not possible with I2C based absolute temp sensors, one needs  analog circuitry 
feeding high-resolution Delta-Sigma ADC's. These  circuits are not accurate on 
the absolute temperature, but they do have  very high resolution for relative 
temperature changes.
 
A resolution of 0.25C would give you only 40 individual DAC settings for a  
10C change as could be expected on a typical diurnal temperature change on a 
lab  bench. This would make the frequency jump around and really sink the ADEV  
performance of the unit during holdover.
 
To give you an example: a typical single-oven OCXO has about 1ppb per  degree 
C change. This would mean the unit could only be adjusted by 2.5E-010  steps 
with the Dallas Temp sensor as a reference. So the average error would be  
1.25E-010, which results in a massive 12.5 microseconds average drift error in  
1000s intervals due to the temperature quantization error!
 
The math looks better for double oven OCXO's, but not much.
 
I am pretty sure the Dallas chip is only used for environmental monitoring  
of the absolute temperature, and not for compensation anywhere inside the PLL  
loop.
 
Any Thunderbolt designers on this list??
 
bye,
Said
 
 
In a message dated 2/4/2009 12:45:51 Pacific Standard Time,  
holrum at hotmail.com writes:

My bet  is Dallas Semi changed their chip in a way that is not compatible 
with the  Tbolt firmware.  If Trimble changed their firmware to not do the high  
resolution read cycle,  they would not have done the first steps of the  high 
res read cycle (mask off the LSB,  subtract 0.25C)  which  actually reduces 
the temperature resolution from nine bits to eight  bits.

Trimble could have easily missed the problem if they did not plot  the 
temperature curves.  The unit reports a proper temperature,  it  just does not have 
the fine resolution of the earlier units...  which  could easily affect 
holdover  performance.