The onboard clock crystal of Sync I/O is factory calibrated to a number between -9999 to +9999. To check a SYNC I/O's factory calibration, see the sticker on the bottom of the unit. With normal usage, SYNC I/O should never require recalibrating. Each unit is factory calibrated to within +/- 5 ppm (parts per million).You may want to recalibrate SYNC I/O in the following situations:- If greater than 5 ppm accuracy is required- If the unit needs to be matched to a unique (nonstandard) frequency- To precisely compensate for component aging- To restore the original factory settingNote: Oscillator recalibration does not occur during firmware updating or when resetting SYNC I/O to factory defaults.