[time-nuts] World's most precise.... wall clock

[Magnus Danielson]

Hi,

On 2021-03-10 14:23, Bob kb8tq wrote:
> Hi
>
>> On Mar 10, 2021, at 6:04 AM, Hal Murray <hmurray at megapathdsl.net> wrote:
>>
>>
>> kb8tq at n1k.org said:
>>> The gotcha here is that if you want accurate *time*, you are better off using
>>> the sawtooth corrected output from a (good) GPS module rather than a GPSDO. 
>> Why is that?
> The controller gets in the way. If you want good frequency stability you have long
> time constants. The local reference is free to (and does) wander inside that time
> constant. 

Indeed. It is different to optimize for ADEV or TDEV performance.

However, even when phase/time is interesting, a frequency optimization
has benefits when it comes to hold-over performance, i.e. how stable it
will be after loosing tracking. You very quickly come to a situation
where frequency error dominates the phase error, and only later does
drift and environment start to kick in.

>
>> I would have guessed that a GPSDO would average over many GPS pulses thus 
>> reducing the noise.
> It does and it does. However that does not help the accuracy of your 1 second
> time tick. A good GPS module *with* sawtooth. can get you down to a fraction
> of a nanosecond (ADEV) on your 1 PPS. There is no need to average that noise
> down any further. 
Actually, you go below that if you average long enough, but the sawtooth
for sure helps. There is enough of noise and systematics to make things
move around. It's still not very good thought, so you end up with a
fuzzy border about there. As always, it depends. :)
>
>> Is it something like GPSDOs are normally designed for good frequency rather 
>> than good time, so when they find the time is off, they use a small frequency 
>> offset for a long time to correct rather than a big frequency offset for a 
>> short time?
>>
>> Are there any GPSDOs designed for good time?  Or any with parameters that can 
>> be tweaked to provide good time?
> There are a few. They steer the 1 PPS so it follows the PPS output of the module. 
> If all you want is time, just use the module PPS and save on the electric bill. 

Indeed, unless you need hold-over capability, that's when an OCXO or
Telecom Rubidium may kick in and be relevant. As time-nuts, we do it
regardless, because it is fun, but that is a different account. :)

Cheers,
Magnus