Hi The crystal as normally cut makes a very poor thermometer compared to a thermistor.
Bob On Mar 3, 2014, at 6:46 PM, Chris Albertson <albertson.ch...@gmail.com> wrote: > The OCXO maker is forced to use a temperature sensor because he does > not have access to a frequency reference. If do have an external > frequency reference then the crystal itself makes a good thermometer. > So why not use THAT thermometer to control the heat added by the > resister. Such a system would respond to changes in ambient > temperature by adjusting the power in the resister. We don't even > have to care if the crystal's temp-co is nonlinear because we are > using a very small temperature range, so small it looks linear. > > I'll build it. Can you or anyone else subject a simple XCO > schematic? Hopefully SIMPLE. What I need is a design that can be > pulled down a few PPM so that I can raise it back with a bit of heat. > I will have to be kept at a temperer above the hottest it will ever > get inside the house, maybe 100F. > > On Mon, Mar 3, 2014 at 3:18 PM, Bob Camp <li...@rtty.us> wrote: >> Hi >> >> If you are measuring temperature in a room who's temperature does not >> change, then yes you can hold 0.000000001 C. That of course is based on the >> "room does not change temperature" and that equates to absolutely no change >> at all. >> >> The only rational way to discus temperature stability is as a response to an >> external change. It change this amount when the temperature around it >> changes that amount. Trying to compare something on the table here and the >> table there is not a very useful exercise. >> >> On an OCXO the internal temperature control is always specified with a >> defined external temperature change. The drift in the set temperature at a >> constant ambient is essentially "un-measurable" even on some pretty cheap >> ovens. >> >> Bob >> >> On Mar 3, 2014, at 9:27 AM, Jim Lux <jim...@earthlink.net> wrote: >> >>> On 3/3/14 2:18 AM, Hal Murray wrote: >>>> >>>>>> Junk crystals are good thermometers. Ballpark is 1 ppm/degree-C >>>> >>>> albertson.ch...@gmail.com said: >>>>> So does this mean I can epoxy a sandstone power resister to a junk crystal >>>>> and keep the frequency exactly perfect by varying the power in the >>>>> resister? >>>> >>>> Sure, for some values of "perfect" and such. >>>> >>>> I've occasionally thought about building something like this, just for the >>>> hell of it to see what happens and/or what I learn, and or how good I/we >>>> can >>>> get on a low budget. >>>> >>>> I think there are two problem areas. One is sensors and control >>>> algorithms. >>>> The other is board layout. >>>> >>>> Where is the sweet spot on complexity vs accuracy? I'm looking for >>>> science-fair level of goodness rather than super-expensive to get another 0 >>>> or two. >>>> >>>> What's the best low-cost way to measure temperature? Many of the obvious >>>> choices are only good to 0.1 C. That's great if you are trying to measure >>>> room temperature or or want to keep your CPU from melting, but it's >>>> probably >>>> leaving a lot on the table if you are interested in the frequency from a >>>> crystal. >>>> >>>> My straw man would be a thermistor and OP-Amp feeding into the ADC on your >>>> favorite uProc. Maybe the other side of a bridge would be adjustable. >>> >>> A number of microcontrollers have onchip temperature sensors (Freescale >>> Kinetis, for instance). If the controller were bonded to the crystal >>> housing, that might be enough coupling. >>> >>> Could you hold 0.1 or 0.001 degree? the chip has a 16 bid ADC, although I >>> wouldn't trust the bottom bit or two because of noise. But in any case 1 >>> LSB is 3.3/64k or about 50 microvolts. The temperature sensor slope is >>> 1.715 mV/C, so that's in the 0.03 C/LSB range.. On a good day, you *might* >>> be able to hold 0.1 degree, assuming there's no systematic errors. >>> >>> >>>> >>>> How much power do you need to keep things warm? I'm assuming something >>>> like >>>> a watt or 2 with something like a PWM from the uProc. >>>> >>>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > > > -- > > Chris Albertson > Redondo Beach, California > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
