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

[Dana Whitlow]

Hal,

The older (and probably the newer models, too) Trimble Thunderbolt GPSDOs
have
a user-adjustable time constant accessible via the serial port using a
program like
"Tboltmon.exe" (from Trimble).  I suspect that  "Lady Heather"  may also do
this.  I
am fortunate in owning a still functioning PC with an actual hardware RS232
port
and a usable O/S (Win XP), so running Tboltmon is a trivial exercise for me.

In addition to time constant, tboltmon also lets one examine (and set,
where appropriate)
a large number of other items, and it's reasonably intuitive to use.  I've
never had to resort
to a manual. (if there even is one) to do what I needed.

My own experience is that a time constant around 50 sec works best in my
environment,
providing the best compromise between filtering PPS jitter from out of the
unit's GPS
receiver and tamping down OCXO wanderings due to my home HVAC system's
cycling.
Setting a far longer time constant value (say, much longer than 500 sec)
tends to lead
to "funny business" so I just don't go there.

I have convinced myself that the PPS output from my Tbolt is derived from
the produced
10 MHz output, because if I trigger an o'scope from the PPS output, the 10
MHz sinewave
shows very little time jitter, perhaps 1 or 2 nsec.  So, I'm pretty happy
with the T'bolt, with
two (minor) exceptions:

>  Its RF sensitivity seems rather poor compared to that of "modern"
receivers (my unit
    was apparently made in the early 2000s).

>  In tboltmon, the signal "strength" indications are displayed in units
called 'AMU',
    for which I've been unable to find a definition.

Dana


On Wed, Mar 10, 2021 at 6:53 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?
>
> I would have guessed that a GPSDO would average over many GPS pulses thus
> reducing the noise.
>
> 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?
>
>
> --
> 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.
>