8-10
Avg. Temp. Stab.
Aging/Day
Resynch Interval*
(A/J & security)
Recalibr. Interval *
(Maintenance cost)
TCXO
1 x 10-6
1 x 10-8
10 min
10 yrs
4 hrs
80 days
OCXO
2 x 10-8
1 x 10-10
6 hours
50 years
4 days
1.5 yrs
MCXO
2 x 10-8
5 x 10-11
4 days
3 yrs
6 hours
94 yrs
RbXO
2 x 10-8
5 x 10-13
6 hours
None
needed
4 days
300 yrs
*  Calculated for an accuracy requirement of 25 milliseconds.  Many modern systems need much better. 
25
20
15
10
5
0
10
20
30
40
50
60
70
80
90
100
Days Since Calibration
Text Box: Time Error (ms)
Time Error (ms)
Aging/Day             = 5 x 10-10
Temp Stability       = 2 x 10-8
Resync Interval      = 4 days
Clock Error vs. Resynchronization Interval
   In the graph above is an example that illustrates how the time error of a clock grows with days since synchronization and syntonization (i.e., calibration).  The clock is assumed to be in a system that requires a 25 millisecond accuracy, the clock is assumed to have an average frequency offset due to temperature of 2 x 10-8, an aging rate of 5 x 10-10 per day, and negligible other error sources.  Upon calibration, the clock is assumed to have zero frequency and time error.
   The clock is assumed to be resynchronized every four days.  During the initial period, the time error is caused almost entirely by the frequency error due to temperature, i.e.,  2 x 10-8 in the example.  Aging, however, adds 5 x 10-10 to the frequency error every day, so, after 40 days, the frequency error due to aging equals that due to temperature.  After 40 days, aging is the dominant cause of the frequency error.  The time error increases more and more in each four day resynchronization interval.  Eventually, after about 100 days, the time error at the end of the the four days reaches the 25 ms limit.  At that point, either the resynchronization interval must be made shorter, or the clock must be recalibrated, i.e., its oscillator’s frequency must be readjusted to the correct value.
   The table shows other examples - for a TCXO, OCXO, MCXO and RbXO.  For each, two resynchronization and recalibration scenarios are shown that result in a 25 ms time error at the end of the recalibration and last resynchronization intervals.  Whereas a TCXO in this application needs to be resynchronized/recalibrated frequently, a Rb frequency standard or RbXO can maintain the required accuracy indefinitely.