Bruce Griffiths wrote: > If you are serious forget the fancy digital or semiconductor > temperature sensors they aren't good enough.
I was intending to use the slow Dallas chips as a calibration reference (out-of-box they're usually quite accurate) and for testing. Is there any particular reason the analog-output (Microchip TC1047A) sensors are no good? > For the best performance, unless you use a bridge oscillator circuit > of some type, you will need to control the temperature of all the > oscillator components as well. I did have a "plan B" -- a hollow metal box with a metal sheet soldered inside at the half-way point. The crystal and oscillator circuitry would be mounted in the bottom half, and the temperature control in the top half. The temperature sensor is a three-pin SOT23 (about the size of a grain of rice) and the ground is a single pin on one side. I was thinking about mounting the sensor directly on the copper, using a small piece of Kapton tape to stop the sensor's Vout or Vcc shorting against the grounded copper sheet. That would leave two hollow air-filled cavities for the control circuitry and hold the temperature of that reasonably close to that of that of the crystal (minus a few degrees). > An analog bridge using an RTD or an NTC thermistor can have much > better stability. That sounds about right.. I was going to use a Pt100 or Pt1000 RTD, but couldn't find any decent information on them other than the resistance being 100R or 1kR at 25C -- even the manufacturer's datasheets were somewhat thin on information. > If you use an appropriate high resolution sigma delta ADC it can > reverse the bridge excitation polarity as part of the measurement > sequence and give you most of the benefits of an AC bridge with fewer > devices and lower cost. IIRC the A/D on the PIC is a 10-bit successive-approximation type with a built-in sample-and-hold (though other types have 12-bit converters). That's a measurement range of 1024 counts, which with the 4.096V reference provides a resolution of 4mV, or 1/2.5 of a degree C per count. 4V is actually the minimum reference voltage the A/D can accept. Sensor output is ((degrees_c * 10) + 500) mV. > I would build a room temperature version first for debugging. I'm planning to do that anyway. I've got a few 10MHz room-temp crystals of a similar spec to the oven crystals that I can use, and I can probably use the same parts in the prototype oven for testing. > To minimise the phase noise contributed by the varicap the EFC range > should be as small as is practical. That's the part that's going to need "a bit" of experimentation I think :) > A very low noise power supply is also required for good performance. > A modified version (uses 2 transistors and larger capacitors) of > Wenzels active supply filter can be used to reduce the power supply > noise by 30-40dB for frequencies above 1Hz or so. > http://www.wenzel.com/documents/finesse.html > > I can provide circuit schematics if you are interested. That would be great, if it's not too much trouble. Thanks, Phil. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
