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

Scott Stobbe scott.j.stobbe at gmail.com
Tue Sep 13 21:01:39 EDT 2016


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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