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

[Charlie]

Bob-

As a rank amateur e astronomer, I am a  lurker. I am amazed at what I have
learned here. I know that there are differences between the meaning of
precision and accuracy, but please correct my understanding if I am
imprecise.

I have a need for precise time, as all sorts of calculations are dependent
on precise geocentric position, and of course time to convert to other times
e.g. sidereal, utc, etc., as related to the motion control of a large
telescope.

I have an old hp z3805a; seems to be really precise, agreeing with my
location (surveyed). Other gps's that I have seem to wander more.

My question is thus: It seems that procuring a more precise PPS/time output
unit is quite a bit more costly than what I have; even more costly is a unit
that has both more precise PPS/time output, and a really stable 10 Mhz
output ( I might add that I am a Ham, where 1 uhz  error is detrimental).

Assuming I can afford an upgrade, would  getting a more precise PPS/time
unit then and feed that data into separate OCXO? Getting both seems out of
my league.

Seems that  could the best of both worlds.

All advise welcome; it's how we learn....

Thank you,

Charlie
N6CFH












-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of Bob
kb8tq
Sent: Wednesday, March 10, 2021 5:24 AM
To: Discussion of precise time and frequency measurement
<time-nuts at lists.febo.com>
Cc: hmurray at megapathdsl.net
Subject: Re: [time-nuts] World's most precise.... wall clock

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.
> 
> 
> 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.


_______________________________________________
time-nuts mailing list -- time-nuts at lists.febo.com
To unsubscribe, go to
http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
and follow the instructions there.