Stratum 3 and Stratum 3E oscillators have three frequency stability specifications, Free Run, Holdover and Drift as specified in the following:  

    Free Run (4.6 ppm/20 years Stratum 3 and 3E)
    Holdover (0.37 ppm/24 hours for Stratum 3 and 0.01 ppm/24 hours for Stratum 3E)
    Drift (in a lab environment: 0.04 ppm/24 hours for Stratum 3 and 0.001 ppm/24 hours for Stratum 3E)

Free Run

The Free Run requirement is easily obtainable for any precision oscillator.  However part of the system requirement is that any Stratum 3 and 3E system must be able to lock to any other stratum 3 or 3E system.   The reference signal into the system may be from an operating free running system that can be off as much as 4.6 ppm.  Further clarifying the lock requirement, a Stratum 3E system must be able to lock to a Stratum 3 system.


The Holdover requirement is defined to be overall conditions.  These conditions include aging, temperature, voltage, load and initial offset.  Initial offset is how close the unit was locked to the reference signal at the time the signal was lost; not the initial frequency accuracy of the oscillator.  The Holdover requirement can be broken into 3 major components: Initial offset, Temperature, and Drift.  Bell core suggests adding the Initial offset, Temperature and Drift to obtain the Holdover requirement.  Their suggestion does not account for voltage changes, but includes it in the Drift calculation. 


The suggested Drift value is the most difficult to satisfy but is not a requirement.  The temperature range is pseudo defined (Bell core suggest the temperature change in a lab environment of 5F).  The voltage change is included because the Drift value is to include other effects along with aging and temperature.  Since Drift is the tightest specification; aging, temperature, and voltage characteristics of the oscillator are large factors in this stability value.  Usually TCXOs will not meet this value, because they usually do not have a frequency versus temperature characteristic that is as linear as an oven oscillator.  Like the Holdover requirement, the system designer has some flexibility:

    In controlling the oscillator voltage by choice of power supply.
    Specifying the temperature change for the Drift.


The system manufacturer has some flexibility in specifying the conditions of the Holdover and Drift values.  Since Drift is not a requirement, it can be overlooked in the system specification.  A limited temperature range for the Holdover could also be specified.  The purchaser of system for end use should take into account the conditions of Holdover and Drift to determine the best value that will meet their requirements.