Don't you also need to wait for the GPS at first power up?
----- Original Message -----
From: "Charles Steinmetz" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Sent: Thursday, April 10, 2014 3:31 PM
Subject: Re: [time-nuts] First success with very simple, very low cost GPSDO
I should have said warm start, not cold. I was referring to the code, not
the oscillator. So tell me, the OCXO is warm, there's no previous EFC
information to draw upon, and the oscillator is off-frequency by more than
can be measured with, let's say eight timer bits. What do those early
measurements tell me, and which direction from midway should the EFC be
adjusted?
That's why I suggested a timer. Certainly, the delay chosen for a cold
start would be excessive for a warm start, but I'm assuming that the
GPSDOs we're discussing are not used in life-and-death circumstances where
every second of unavailability is critical. Whenever you power up -- warm
or cold -- you wait (probably ~ 5 minutes) for availability.
Using the PPS to discipline the oscillator during warmup may seem like a
good idea. However: (i) it will not be disciplined to useful time-nuts
standards both because it is drifting and because the frequency is being
set by the noisy, jittery GPS PPS signal. But much worse, (ii) if the
loop is fast enough to track the oscillator as it warms up, it is almost
certainly too fast to give best performance at low to medium tau when the
oscillator is warm, because the noisy, jittery PPS will be contributing to
stability at tau where the nice, quiet OCXO should be in charge.
Precision takes time. Time nuts can't afford to be impatient.
Best regards,
Charles
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
