Hi If you have a *really bad* crystal, it will follow a very nice log aging cure. A good crystal is much less predictable. The reason is fairly simple, the bad crystal has a single dominant cause for it's aging. It missed a step somewhere along the line and it's got a problem. A precision part is going to be run through a process that results in no single effect being dominant.
Bob On Jun 24, 2010, at 2:40 PM, Don Cross wrote: > > Hi, my name is Don. I have been lurking on this list for a while, so here is > my first post. > I am a hobbyist who has just built my second home-made quartz digital clock. > Both are based on a microcontroller that counts timer interrupts and uses > software tricks to allow me to tune the clock rate based on comparison with > atomic time via NTP. The second clock uses a temperature sensor and a heater > (a grid of resistors) enclosed with the microcontroller board in a glass jar > to regulate its own temperature. > Of course, I am noticing a drift in the clock rate over time due to crystal > aging. The frequency of the crystal is gradually increasing over time. For > example, on June 2, the frequency was 15.99927052 MHz. As of yesterday it is > hovering around 15.99927796 MHz. > I have read through several online resources, including the very interesting > one that was posted here recently: > http://www.am1.us/Papers/U11625%20VIG-TUTORIAL.PDF > On page 4-7 of that document there is a slide titled "Typical Aging > Behaviors" where it shows that long-term crystal aging can be represented as > a sum of logarithmic functions. I was wondering if this is just an > approximation, or if there is a theoretical reason why logarithms would > describe such phenomena? Looking at the causes of aging, they seem to do > with changes in the bonding with the electrodes, deposition or oxidation of > the components inside the crystal enclosure, etc. > I am thinking about trying to measure the aging process over the coming > months, and then try to model and even predict future aging. If I can get > that to work, perhaps I can even incorporate the formula for predicted aging > right into my software. Any insights on this would be much appreciated. > FYI, here is a link to what I did on my first clock. I have not yet finished > the web page for the second clock, but I will get around to it, eventually. > http://cosinekitty.com/digitalclock/ > I do have the firmware for my second clock online, along with some crude > schematics, if anyone is interested: > http://cosinekitty.com/thermalclock/thermal_clock_firmware.cpphttp://cosinekitty.com/thermalclock/main_board_schematic.pnghttp://cosinekitty.com/thermalclock/heater_switch.png > - Don > _________________________________________________________________ > Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. > http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:WL:en-US:WM_HMP:042010_1 > _______________________________________________ > 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.
