[time-nuts] First success with very simple, very low cost GPSDO

[Tom Miller]

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
>
>