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.
