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

[Charlie]

Bob, Tom and others-

It is 1 uHz. My error, not putting in PPM or PPB /24 hours, typical transmission/reception time frame for (very) vlf.


Thanks all,

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 8:05 AM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Subject: Re: [time-nuts] World's most precise.... wall clock

Hi

> On Mar 10, 2021, at 9:39 AM, Charlie <charlie at drhabekost.com> wrote:
> 
> 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.

I suspect that is a function of how the 3805 presents the data. 

> 
> 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).

Sub ns *jitter* is doing well at 1 second with GPS. Accuracy is different 
than jitter. Since the GPS clock is not a direct expression of UTC from 
BIH ( nothing is … sorry about that …) there is some back tracking to get
*very* accurate time. 

> 
> 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.

If you have the $300 to $2000 for a multi band GNSS timing receiver, it will 
indeed help a bit. How much will depend a lot on the state of the ionosphere 
and the correction process. Troposphere also gets into things. I don’t know of
any receiver that directly estimates Tropo delay. 

> 
> Seems that  could the best of both worlds.

Best would team the fancy receiver with a fancy standard. An OCXO is better
than a TCXO. Most Rb's beats the OCXO long term. A Cs will beat them both
if you run out long enough. You then get into things like GNSS disciplined 
Passive Hydrogen Masers. Properly done they should perform quite well. A
disciplined Active Maser would / could beat a Passive Maser …..

The better the “flywheel” the better the result, at least for frequency / stability. 
It will count off seconds quite nicely. Just how far off from “right” those seconds
are is a bit unclear. ( = you still are not accurate)

For accuracy you need a path back to BIH and the “official” definition of UTC.
That’s true even with the brand new fresh from the factory disciplined Active
Maser that you sold the house to buy ….. There are lots of nasty little delays
that creep into the mix …. All of them need to be taken care of to below your
target error level. If you are after < 10 ns accuracy, this could get pretty exciting. 


Bob

> 
> 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.
>> 
>> 
>> 
>> 
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