Brooke (no relation) commented Hi Richard: > > It's my understanding that this optimization can be done by changing the > oscillator power level at the crystal. > > In the case of the 32768 Hz watch crystal, it must be run a very low power > and it has a very low aging rate when compared to higher frequency > crystals that are typically run at higher power levels. I think this is > related to the crystal throwing off atoms, so more power means more > acceleration and more atoms thrown off. > > Have Fun, > > Brooke Clarke, N6GCE
It has nothing to do with "throwing off atoms". A Xtal is actually a mechanical oscillator, with the quartz slab vibrating (in either its fundamental mode, or on an odd overtone); quartz is a piezo-electric material so the voltage across the pins of the xtal has a direct connection to the mechanical vibration. When an xtal oscillator starts up, the associated amplifier generates noise, which then starts the xtal vibrating, which generates signal at the right frequency and a feedback loop is set up. When you crank up the power to the mechanical resonator, the signal increases with respect to the background noise (i.e. S/N gets better) which improves the short-term stability. Going in the other direction, the mechanical resonant frequency changes with time because, as the xtal vibrates, microscopic cracks in the structure of the quartz break apart. Running at high power makes the crystal generate these microscopic faults at a faster rate; this then causes the oscillator to have poorer long-term stability. When an xtal is left vibrating (oscillating) in an undisturbed environment, the rate of cracking of the quartz decreases, and the oscillator is said to "age" to its final frequency. But if you subject that same crystal to a mechanical jolt will force some new cracks and re-start the aging "diffusion" process. Ditto turning the oscillator on & off or a thermal shock can aggravate the aging. If the metal can or glass envelope around the xtal outgasses, some of the resulting crud (a very scientific term!) from the envelope and seal will deposit onto the quartz and also cause aging. For this reason, only the cheapest crystals are housed in a metal can with a solder seal; cold welding of the can is a much better procedure; and a glass envelope is the best. Cheaper than cheap are the WW2 "FT243" xtals where the seal is just a rubber gasket or the epoxy seals used in some consumer-grade surface mount oscillators. The main reason that the 32768 Hz oscillators operate at low power is so that watches can run for years on small batteries. But even at that, the mechanical xtal resonator (which is built as a tuning fork for these low frequencies) is much better than any watch escapement ever was! 73, Tom Clark _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
