[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sun Dec 17 08:51:24 EST 2006
SAIDJACK at aol.com wrote:
> Hi Bruce,
> would you have pointers to good temperature sensing circuits with sub
> millidegree resolution?
> time-nuts mailing list
> time-nuts at febo.com
The traditional method uses an AC transformer bridge with a platinum
resistance sensor and a stable low temperature coefficient reference
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
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.
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