[time-nuts] Tbolt temperature sensor

[SAIDJACK at aol.com]

Hello Tom,
 
over the last week we've been doing a test on a double oven Fury  GPSDO in 
response to our discussion.
 
The first part of the test was running the unit on the lab bench without  
modifications. A couple of days ago, we removed the OCXO Supply regulator  
feedback sense resistor, and put the sense point right at the VCC pin of the  OCXO, 
therefore Kelvin-sensing the supply voltage behind the current sense  
resistor, and thus having an extremely well regulated voltage directly on the  OCXO, 
without any VCC current-drop related changes.
 
This was to see if there is any effect on the thermal stability caused by  
the voltage drop across the Power supply sense resistor, traces, filters, etc.  
This drop is over 50mV worst case on an un-modified unit.
 
You can see the before and after effects on the plot here:
 
   _http://www.jackson-labs.com/images/gpsstat.htm_ 
(http://www.jackson-labs.com/images/gpsstat.htm) 
 
You can see the point where we switched the sense resistor as a  
discontinuity in the EFC voltage.
 
The measured tempco of the unit before and after did not change, it is  about 
6.0 in both cases (which is a very good number on this particular  unit).
 
The unit's performance as measured by GPSCon before and after the  
modification is also identical, it measures an average of 0.05ns, and 2ns  standard 
deviation. The worst-case deviation is +9ns, and -8ns.
 
Thus on this particular unit there is no measurable effect from the voltage  
drop between the power supply regulator and the OCXO power supply pin, and in  
both cases the measured tempco is around 6, which is very good, and means  
there is very little thermally-induced EFC voltage change required.
 
Keeping in mind that our double oven OCXOs have about 0.12A to 0.15A  current 
range (a 30mA+ delta), and the single oven OCXO's have only about 4mA  
current change (about 10x less), then we can deduce that the 10x smaller current  
change in our single-oven OCXO's will have even less VCC/Ground current-loop  
induced error.
 
bye,
Said
 
 
 
 
In a message dated 2/4/2009 17:51:17 Pacific Standard Time,  
tvb at LeapSecond.com writes:

> To  make this work somewhat smoothly, one would need at least 0.001C or   
> better 0.0001C resolution.

Hi Said,

Can you provide  some real data, to help me believe this claim?

When you get below 1C or  0.1C resolution it starts to matter
from which direction the temperature  gradient is headed, or
from which angle air is flowing. Or which side is  up. Or what
the humidity is, etc.

This, because temperature  gradients break any steady state
model you have based on a point source  (e.g., temp sensor)
or average temperature measurement (e.g., oven  current).

Then there is the matter of the rate at which temperature  changes;
slow temp changes and rapid temp changes affect an OCXO
quite  differently. This, due to different thermal time constants of
all the metal  and insulation materials in and around the OCXO
or GPSDO.


>  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!

Are you assuming a  design where one takes a Dallas reading
each second and stuffs it into the  EFC every second? No one
would actually do that. Instead if one averages  the temperature
sensor over 10, 100 or more seconds you avoid large EFC  steps.
Everything else in a GPSDO is all about slow averaging;  there's
no reason temperature adaptation cannot be treated the  same.

I suppose I should add a random temperature cycle  hold-over
test to the abuse I inflict on each GPSDO here. Are you  saying
the Fury would do much better than others in this  regard?

Again, given this is time-nuts, some real data would be  nice.

/tvb


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