[time-nuts] GPSDO recovery from holdover

Said Jackson saidjack at aol.com
Sat Dec 1 17:42:58 UTC 2012

Hi Hal,

We selected 10 steps upon customer request. The counter resolution is 16.667ns on the FF-IIA boards, so we end up pretty close to UTC.

"Slowly" is determined by the PLL parameter serv:phaseco. By default we add very little frequency error, so drifting say 100ns can take a couple of hours if the ocxo is stable ( not warming up anymore). The settings vary per oscillator type, and are more aggressive for single oven units than SC cut DOCXO units. The user can "crank up" the gain and thus reduce the adjustment time, by increasing that parameter, all the way until the loop becomes unstable.

We made it so slow for two reasons: one we wanted the best frequency accuracy, and drifting the phase requires adding a frequency error, so we want to keep that error as small as possible. 

Two: we indicate this phase offset via pushed or polled software command with 0.1ns resolution, so the customer can adjust for it if desired by delay line or via software calculation. Or the customer can simply set the gain higher for faster phase corrections..


Sent from my iPad

On Nov 30, 2012, at 8:36 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> saidjack at aol.com said:
>> New JLT GPSDOs step back in 10 Steps over 10 seconds if more than 250ns off,
>> then adjust the last ns slowly.
> Thanks/interesting.
> How/why did you pick 10?
>> If within 250ns, they just slowly slew back.
> How fast is "slowly"?
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