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

[Bob kb8tq]

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. 

> 
> 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. 

> 
> 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. 

Bob

> 
> 
> -- 
> These are my opinions.  I hate spam.
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> 
> 
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