[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Dec 17 17:26:47 EST 2006


SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> would you have pointers to good temperature sensing circuits with sub  
> millidegree resolution?
>  
> thanks,
> Said
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>   
Said

This was originally sent at 3am local time but doesn't seem to have made it.

The traditional method uses an AC transformer bridge with a platinum 
resistance sensor and a stable low temperature coefficient reference 
resistor.
However this is probably too bulky and expensive for your application.

As I understand it, you don't really need to measure the temperature but 
some temperature dependent quantity that is a monotonic function of 
temperature with a resolution and and stability equivalent to better 
than 0.001K.

Measuring the ratio of the resistance of a platinum resistor with to a 
stable reference resistor with a low temperature coefficient will 
suffice as long as they both have the same temperature. A reference 
resistor with a small non zero tempco will not affect the monotonicity 
however the sensitivity will be affected slightly.

It is not even necessary to use a platinum resistance sensor, a copper 
one will work just as well, and may even be convenient as a length of 
enamelled copper wire can be would around an object (metal container?) 
whose temperature one wishes to sense. Indeed the NBS (now NIST) used 
copper resistance thermometers to sense the temperature of its 10pF 
fused silica standard capacitors.

One could replace the variable variable ratio cascaded tapped 
transformers used in the the AC bridge by a pair of multiplying DACs 
with an AC reference, however the DACs would need to be 20 bit DACs that 
are monotonic to better than 1 lsb. The DACs being adjusted to null the 
output at the junction of the 2 resistors.

Another possibility is to use a 24 bit sigma delta ADC to measure the 
ratio of the voltages across the RTD and the reference resistor. Off 
course it would be prudent to measure this ratio also when the current 
through the 2 resistors is reversed in direction so that residual 
offsets due to thermoelectric and other causes cancel out. Since the 
ratio of the voltages is being measured  the current flowing through the 
2 resistors need only have good short term stability as must the ADC 
gain. As long as it is monotonic the ADC need not have an integral 
nonlinearity as small as 1 lsb. An LTC2412 or similar ADC should 
suffice. The trick is to reverse the current flowing through the series 
connected RTD and reference resistor without affecting its absolute 
value. A current source feeding the pair of series connected resistors 
via a suitably connected set of switches will suffice. Details will 
follow later its 3am here.

An alternative technique is to use a quartz crystal cut with a high 
tempco in an oscillator and just measure the oscillator frequency using 
the OCXO as a timebase.
The HP2804 quartz crystal thermometer used to do this.

Bruce




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