[time-nuts] Holdover recovery

[Tim Shoppa]

The chinese GPSDO's famous on E-bay are not really phase locked to begin
with, they are only frequency locked and often with a small frequency
offset. I don't think any of your questions apply to this kind of unit. See
e.g. http://www.ke5fx.com/gpscomp.htm

Units that have been spec'ed for holdover in terms of time delta, make
phase the number one priority and will gladly "oversteer" frequency to get
the phase to match back up after holdover and then settling down again,.
e.g. http://leapsecond.com/pages/z3801a-efc/

In recovery from a holdover where something went clearly wrong - the
holdover specs of the unit were greatly exceeded - they declare themselves
unlocked, and will do a phase jump and enter a "frequency lock" mode until
oscillator parameters are relearned and they again declare themselves
locked.

Tim N3QE

On Sun, Apr 3, 2016 at 11:39 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

>
> Has anybody studied what happens when a GPSDO comes out of holdover?  Has
> anybody seen any specs?  I don't think I have.
>
> I think you have a choice of quick recovery for time or frequency, but you
> can't get both.
>
> Suppose your setup has been in holdover for a while.  The frequency is
> slightly off.  The time offset of the PPS pulse will be the integral of the
> frequency offset.
>
> What happens when you come out of holdover?  If you fix the frequency, the
> PPS will stay off.
>
> Suppose the PPS has drifted by 1 ns.  If you correct that in 1 second, the
> frequency will need to be off by 1E9 during that second.
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
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