[time-nuts] First success with very simple, very low cost GPSDO
Tom Miller
tmiller11147 at verizon.net
Thu Apr 10 20:53:20 UTC 2014
Don't you also need to wait for the GPS at first power up?
----- Original Message -----
From: "Charles Steinmetz" <csteinmetz at yandex.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
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
>
>
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