Connection to the Network
37
• Locking: The oscillator has finished the initialization
sequence and it starts tracking the external reference fre-
quency and phase. The objective of this state is to mini-
mize the phase and frequency difference between the
local oscillator and the reference. During the Locking
period the oscillator frequency and phase may not be
accurate or they may change quickly. For this reason no
test should be run in Locking state. The Locking status
lasts for around 20 minutes in Rubidium units and 5 min-
utes in OCXO units.
• Fine locking: The unit is still trying to minimize the phase
difference between the internal oscillator and the reference
but no sudden phase changes are expected in this state.
When the unit achieves the Fine locking status, it becomes
ready for all tests not requiring the maximum accuracy
degree. The Fine locking state lasts for a few seconds in
OCXO units and about four hours in Rubidium units.
• Locked: The unit cannot reduce the frequency and phase
difference between the reference clock and the local oscil-
lator any more and it just tracks the output to keep the dif-
ferences to the minimum. In this situation, the test error is
minimum and therefore the unit is ready for any measure-
ment.
•Holdover: The unit has been previously locked to an exter-
nal time or phase reference but now the reference input is
not being used. The unit keeps an accurate reference fre-
quency and phase for a limited period of time. When the
unit is operating in holdover mode it is still ready for testing
even if no external clock reference is available.
• Holdover time out: The unit has been working in holdover
for longer than programmed in the Holdover duration field.
The local oscillator frequency and phase are not reliable
when working the holdover is timed out.
• Free running: The local oscillator is not disciplined and it is
operating on its natural oscillation frequency. How accu-
rate this frequency is depends on which oscillator is being
used (TCXO, OCXO or Rubidium) and how well the oscil-
lator is calibrated. The phase and time supplied by a free
running oscillator are not reliable.
Table 2.9: Oscillator Settings and Status
Result Description
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