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Like most oscillators, VCXOs have many specifications. APR – Absolute Pull Range – is a single specification that combines many of them to make selection easy. The term “Pull” is unique to the oscillator industry and refers to the small frequency tuning available via the voltage control pin. Pull is measured as df/fnom in ppm.
TCXO: Stability vs Accuracy
Temperature compensated crystal oscillators (TCXOs) are used for frequency reference in systems that require a frequency stability of 10 ppm or better, which typically cannot be achieved by a standard XO (crystal oscillator) or VCXO (voltage controlled crystal oscillator).
Holdover is what happens when the primary timing reference goes away for some reason and the local oscillator is left to its own devices (for a fairly well maintained summary of the topic see the Wikipedia article Holdover in synchronization applications)
The question is: How much can you trust the local oscillator and for how long?
There is a lot of information available on the internet for engineers who want to learn more about frequency control, synchronization and clocks in general.
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