[time-nuts] Lady Heather's Tbolt oscillator auto-tune function

[Scott Stobbe]

Hi bob, thank you for the polite response regarding rise time, indeed I
would fully agree.

The rise time I was referring to was the DAC efc value in the plot mark had
previously attached. He just included a second plot as well. It would
appear the tbolt is doing something else aggressively before going into
a PLL, perhaps coarsely phase steering the last +-50 ns, but then runs out
microseconds in error?

On Tuesday, 13 September 2016, Bob Camp <kb8tq at n1k.org
<javascript:_e(%7B%7D,'cvml','kb8tq at n1k.org');>> wrote:

> Hi
>
> The rise time of the edge is not a good measure of the accuracy of the
> timing It
> simply is a way to look at how fast your gate can ramp a signal.
>
> If you do a long term comparison of the frequency vs time and the time
> error vs time
> you will see that a tight (small) damping keeps the time close at the
> expense of
> jerking the frequency around a lot. A loose (large) damping does not
> change the
> frequency much, but the time wanders quite a bit.
>
> Simply put: There is no free lunch.
>
> Bob
>
> > On Sep 13, 2016, at 9:01 PM, Scott Stobbe <scott.j.stobbe at gmail.com>
> wrote:
> >
> > Bob, that is an excellent proof by contradiction. The reason I asked is
> on
> > the plot Mark shared that first rising edge is pretty sharp for a system
> > with a 500 s time constant.
> >
> > On Tuesday, 13 September 2016, Bob Camp <kb8tq at n1k.org> wrote:
> >
> >> Hi
> >>
> >> The pps sync is done by resetting the counter that generates the PPS.
> At a
> >> 1 ppm frequency
> >> offset, it could take 500,000 seconds to steer it in with the OCXO. It
> >> unlikely people would wait
> >> for over a week for the PPS to line up ….
> >>
> >> Bob
> >>
> >>> On Sep 13, 2016, at 5:58 PM, Scott Stobbe <scott.j.stobbe at gmail.com
> >> <javascript:;>> wrote:
> >>>
> >>> Interesting discussion about startup. At startup the phase error of the
> >>> synthesized PPS is +- 0.5 s. Is this coarsely set to the nearest ocxo
> >> cycle
> >>> once gps time is established (would make sense to do it this way), or
> is
> >>> the half second recovered steering the ocxo?
> >>>
> >>> On Tuesday, 13 September 2016, Charles Steinmetz <
> csteinmetz at yandex.com
> >> <javascript:;>>
> >>> wrote:
> >>>
> >>>> Mark wrote:
> >>>>
> >>>> I just ran a tbolt (which has been off for a couple of months) and
> >> logged
> >>>>> the state for a couple of hours...  and then remembered something
> >> about the
> >>>>> initial DAC value setting that I had figured out long ago... it has
> >> little
> >>>>> to nothing to do with oscillator disciplining.    The tbolt drives
> the
> >> GPS
> >>>>> from the 10 MHz ocxo.  If the ocxo is too far off freq it can't track
> >>>>> satellites.   The initial dac setting is used to speed up acquisition
> >> of
> >>>>> satellites and not to speed up the OCXO disciplining loop lock.
> >>>>>
> >>>>
> >>>> Well...  by doing the one, it also does the other.
> >>>>
> >>>> As soon as a satellite is acquired (after a couple of minutes), the
> DAC
> >>>>> voltage jumps and the disciplining starts.  A few seconds later when
> >> more
> >>>>> sats are tracked, it gets underway in earnest (and by then the OCXO
> is
> >> warm
> >>>>> enought to be within 0.1 Hz).  After 1 hour the box temperature has
> >>>>> stabilized and the freq is within a couple of milli Hz.  After two
> >> hours
> >>>>> the oscillator has settled down to the point where the DAC curve goes
> >> into
> >>>>> "wandering around" instead of following  a smooth decay compensating
> >> for
> >>>>> the oscillator warm-up.  The attached image show the first hour of
> the
> >>>>> process.
> >>>>>
> >>>>
> >>>> If you look carefully at the first 3-4 minutes, you'll see it does
> >> exactly
> >>>> what I described.  The DAC reference is 0.510v, and the scale is
> >>>> 5000uV/division (=5mV/division).  According to the paramaters, the
> >> initial
> >>>> DAC voltage (INIT) = 0.499v.  I assume this was previously stored as
> the
> >>>> DAC value after the Tbolt had fully stabilized, some time in the past.
> >>>>
> >>>> Sure enough, the DAC voltage starts at just about 0.499v (it looks
> like
> >>>> 0.494v on the graph), and when the second satellite is acquired it
> jumps
> >>>> very quickly to 0.529v -- an overshoot of some 55% -- before settling
> >> back
> >>>> to ~0.518v, at which time it appears to be on frequency within 1e-8 or
> >> so.
> >>>> From that point disciplining continues as the crystal warms up.
> >>>>
> >>>> If one accepted my suggestion, the initial DAC voltage would be set to
> >>>> ~0.518v for this oscillator.  In that case, it should be within a few
> >>>> millivolts of the voltage required when the second satellite is
> acquired
> >>>> and the huge step with its 55% overshoot should be avoided.
> >>>>
> >>>> I would be very interested to see the result of another dead cold
> start
> >> of
> >>>> this same Tbolt, with INIT set to 0.518v.  Of course, the time at
> which
> >> the
> >>>> second satellite is acquired (hence, the temperature of the crystal
> when
> >>>> discipline begins, and thus, the exact DAC voltage required for a
> >> stepless
> >>>> transition, will be a bit different from one start to the next, so it
> >> won't
> >>>> be perfect.  But it will be a hell of a lot better than starting from
> >>>> 0.499v).
> >>>>
> >>>> Now -- does what happens during the first five minutes really make any
> >>>> difference, given that no time-nut is going to do serious work with a
> >> GPSDO
> >>>> for at least several hours after a cold start?  No, probably not.  But
> >> we
> >>>> are time-nuts, after all, aren't we?
> >>>>
> >>>> Best regards,
> >>>>
> >>>> Charles
> >>>>
> >>>>
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