[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.
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> 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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