Javier,

If you are aim to do hold-over as you switch between two sources, you are looking at reasonably short times, then just keep a fixed voltage to the oscillator suffice.

Even if you need a little longer times, say 10-20 s, it suffice.

Temperature changes and oscillator drift may be the main problems for longer times.

Cheers,
Magnus

On 02/06/2015 10:21 AM, Javier Serrano wrote:
Dear all,

We would like to start working on holdover performance for White
Rabbit [1]. This is a new domain for us. Our main use case is a WR
switch losing its reference because someone disconnects a fiber. We
can have redundancy, but it will take some time for a switch to change
over to another reference. During this time, the oscillator in that
switch will be free-running. We want to minimize the phase drift
during that interval, which we think should be a couple of seconds
maximum. We have never worked on holdover, and I am wondering if we
can do something smarter than the obvious feeding of some constant
voltage to the VCXO, based on averaging during the locked state. Does
anybody know of any good references on holdover?

Thanks!

Javier

[1] http://www.ohwr.org/projects/white-rabbit/wiki
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