Page 29
Technical Notes
——————————————————————————————————————
This section contains technical details about the operation of MicroSet, and explains the
limits and benefits of different settings.
Seconds Per Beat or Beats Per Hour
Why would you select one mode or the other? It depends on personal preference. Most
people are used to Beats Per Hour. Others may prefer Seconds Per Beat.
Limits in the oscillator calibration
MicroSet is tuned to a GPS reference signal when we assemble it to get the greatest
accuracy from the crystal timebase. When MicroSets are assembled, the oscillator
components are hand selected to arrive at a frequency that’s within one or two parts per
million of the correct rate. The trim value entered in the MicroSet Configuration Mode
can adjust the timing calculations to correct any remaining error of the crystal. When we
ship a MicroSet, it’s accurate to one part per million at room temperature. This is better
than one second per week.
Thermal stability
Quartz oscillators like the one in MicroSet are susceptible to temperature changes. If
you measure a watch when MicroSet is at 100 degrees the rate will appear different
than if you measure it at 50 degrees. A temperature change like this is likely to have a
bigger effect on the watch itself, but we would still like to have some idea about how
MicroSet changes with temperature. We have found that, on average (and quite linearly)
MicroSet will read an increase of 0.65 parts per million per degree Fahrenheit. In other
words, if the temperature of the timer were to change 5 degrees, you could expect the
value of your readings to change by about 3 parts per million. This is the equivalent of
2 seconds per week in a watch.
However, it takes a couple of hours for temperature changes to penetrate to the crystal.
We have found that, in typical operation, the rate changes by just 2 or 3 parts per million
for a room temperature change of over 20 degrees. These changes are probably too
small to matter in most watch work. If you wish better stability than this from MicroSet,
keep the temperature constant or buy the optional TCXO timebase.
