[time-nuts] Designing and building an OCXO and GPSDO

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Aug 11 19:33:28 EDT 2008


Mark Sims wrote:
> For your temperature sensor and/or ADC consider the Analog Devices AD537 voltage to frequency converter chip.  It has an onboard temperature sensor or could be interfaced to a thermistor, etc.  The on chip sensor has a fairly large thermal mass.  If you need instantaneous response to temperature use a very small thermistor.  
>
> Since the output is frequency,  any time nut worth his/her/its salt should be able to come up with a suitable counter circuit.  One system that I did had the V/F freq set around 200 Hz.  Counting a 16 MHz clock over 256 cycles of V/F waveform yielded microdegree resolution.  
> ----------------------------------------
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>   
Mark

The AD537 has too much drift and too little sensitivity to reliably 
maintain the oven temperature (at the sensor) to within a few millidegrees.
The internal dissipation of other circuitry within the chip will 
increase the sensor temperature offset from the temperature being measured.
A thermistor bridge is a better idea as it can have high stability and 
sensitivity (about 10x that of an RTD).

Bruce



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