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

[Mark Sims]

Here is a plot of the Thunderbolt cold starting with the "initial DAC voltage" 
setting set to the peak value of the initial spike (and not the 10.00 MHz 
setting).   The upward spike when the unit starts tracking sats (it took around 
twice as long to start tracking sats) is gone, but the DAC voltage had a new 
little glitch in the curve when it started disciplining the oscillator.  Once 
the disciplining got underway,  not much was different between the three 
different initial DAC voltage settings.

General conclusions...  the initial DAC voltage setting does not affect the 
overall disciplining, but can affect the time it takes to start tracking sats.  
 Your best initial DAC voltage setting is probably at the 10.00 MHz point 
that the auto-tune function uses.
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Re: [time-nuts] Lady Heather's Tbolt oscillator auto-tune function

[Bob Camp]

Hi

> On Sep 13, 2016, at 10:36 PM, Scott Stobbe  wrote:
> 
> 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?

There is some disagreement about the TBolt having multiple lock steps past
the initial jam set of the PPS. What is fairly clear is that it does not step 
gain 
and control settings past the initial “setup” of the PPS.

Bob


> 
> On Tuesday, 13 September 2016, Bob Camp  > 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 
>> 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  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 > 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 <
>> csteinm...@yandex.com
 >
> 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
>> 

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

[Peter Vince]

Hi Scott,

 The x-axis is shown in the dark blue text to the right of the big time
display - 60 minutes total, 5 minutes per division.

 Peter

On 14 September 2016 at 03:52, Scott Stobbe 
wrote:

> Pardon my lack of knowledge regarding Lady Heather, what is the x-axis
> scale? I assumed the text line above the plot is the various y-axis scales.
> This is good data. Thanks
>
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Re: [time-nuts] Lady Heather's Tbolt oscillator auto-tune function

[Charles Steinmetz]

Mark wrote:


Okee dokeee...  here it is.   Not much difference.   The initial step is 
smaller,  but it still spikes.  After that things are pretty much the same.   
After it cool down,  I'm doing another run with the initial voltage set to the 
peak of the spike.


Excellent!  Many thanks, Mark.  I'll be interested to see the next run, 
with INIT set to the current peak DAC voltage.  It's looking like some 
glitch may be an inherent feature of the Trimble software.


Back when I was still playing with mine, I noticed that there was often 
an ugly, long spike with an oscillatory tail coming out of holdover. 
Even if the held DAC voltage is very close to the new required DAC 
voltage, jam synch is set very tight, the allowable frequency error in 
recovery is also set very tight, and the PPS phase coming out of 
recovery is small (< 50nS), the control software sends the DAC way off 
into the wilderness for no apparent reason.  That does not seem to be 
driven by the normal discipline loop -- rather, it appears to me to be 
an error in the recovery routine.



One slight difference was with the new initial voltage setting (closer to the 
current 10. MHz operating point),  it appeared to acquire satellites a 
few seconds faster (but that could just be luck of the draw).


Yes, that's an expected outcome -- the closer you start to 10.0, 
the faster it should be able to acquire satellites.  Of course, 
acquisition has lots of other variables, including the position of the 
constellation at start-up time, weather, etc., so we expect it to be a 
statistical trend, not a firm rule.



Setting the initial voltage to something far off of optimum would be 
interesting... I seem to remember it taking several minutes to acquire.


Both of mine had way-off INIT settings when I received them (years ago, 
now), and yes, they took a few minutes longer to acquire the first two 
satellites than after I set INIT to the value that produced the 
smoothest transition.


Thanks again, Mark!

Charles


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

[Scott Stobbe]

Pardon my lack of knowledge regarding Lady Heather, what is the x-axis
scale? I assumed the text line above the plot is the various y-axis scales.
This is good data. Thanks

On Tuesday, 13 September 2016, Mark Sims > wrote:

> Okee dokeee...  here it is.   Not much difference.   The initial step is
> smaller,  but it still spikes.  After that things are pretty much the
> same.   After it cool down,  I'm doing another run with the initial voltage
> set to the peak of the spike.
>
> One slight difference was with the new initial voltage setting (closer to
> the current 10. MHz operating point),  it appeared to acquire
> satellites a few seconds faster (but that could just be luck of the draw).
> Setting the initial voltage to something far off of optimum would be
> interesting... I seem to remember it taking several minutes to acquire.
>
> (Gratuitous astro feature plug...   Lady Heather can show moon
> rise/transit/set/age times in addition to the sun times)
>
> --
>
> > 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).
>
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[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 > 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 
> 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  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  >> > 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 <
> csteinm...@yandex.com
> >> >
> >>> 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 

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

[Mark Sims]

Okee dokeee...  here it is.   Not much difference.   The initial step is 
smaller,  but it still spikes.  After that things are pretty much the same.   
After it cool down,  I'm doing another run with the initial voltage set to the 
peak of the spike.

One slight difference was with the new initial voltage setting (closer to the 
current 10. MHz operating point),  it appeared to acquire satellites a 
few seconds faster (but that could just be luck of the draw).  Setting the 
initial voltage to something far off of optimum would be interesting... I seem 
to remember it taking several minutes to acquire.

(Gratuitous astro feature plug...   Lady Heather can show moon 
rise/transit/set/age times in addition to the sun times)

--

> 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).
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Re: [time-nuts] Lady Heather's Tbolt oscillator auto-tune function

[Bob Camp]

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

[Scott Stobbe]

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  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  > 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  >
> > 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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> >> ailman/listinfo/time-nuts
> >> and follow the instructions there.
> >>
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Re: [time-nuts] Lady Heather's Tbolt oscillator auto-tune function

[Bob Camp]

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

[Scott Stobbe]

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

[Lars Walenius]

Can anybody say more about the integrator term in the Tbolt? In the Arduino 
GPSDO I have, I use 1/TC/TC/damping and use damping = 2. I see Nick Sayer do 
the same but use damping 1.75. Simulations I have done in Excel seems to 
indicate that a damping in my sense of about 3 could be better to minimize 
overshoots.

Having looked at old data I have found on Time nuts seems to indicate that the 
Tbolt uses 1/TC/TC/damping/damping/2?

Lars

>Warren wrote:

>If you have a good antenna setup and a constant temperature environment:

>The way to get the lowest noise over short time periods on a TBolt is to set
the TC setting as high as you can (typically 500 to 1000+), and set the
Damping factor as fast as you can (typically 0.7 to 1).

>The reason is that the Tbolt has a simple basic PI phase lock loop
controller.
>The Proportional gain is a function of 1/TC and the Integrator gain is a
function of the damping.

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

[Charles Steinmetz]

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

[ws at Yahoo via time-nuts]

If you have a good antenna setup and a constant temperature environment:

The way to get the lowest noise over short time periods on a TBolt is to set
the TC setting as high as you can (typically 500 to 1000+), and set the
Damping factor as fast as you can (typically 0.7 to 1).

The reason is that the Tbolt has a simple basic PI phase lock loop
controller.
The Proportional gain is a function of 1/TC and the Integrator gain is a
function of the damping.
The Problem is that the GPS phase error signal is a noisy thing with short
and long term peak variations of about 10ns. 

Any GPS phase noise is directly fed thru to the Dac output scaled by the "P"
Gain.
With a 10 ns phase noise step and a 100sec TC, the "P" gain will change the
oscillator freq by 1e-10, on the very next one sec Dac update.
This is not done to keep the time correct; it is needed to correct the
Frequency in the desired time constant period.

This then causes the continuous up to 1e-10 freq steps, with the factory
default setting, the effect of which can be seen by plotting the DAC
voltage.
If the TC is made twice as long then the Dac noise step size is 1/2, etc.
If you have a good setup and a low ADEV osc at long time constants, then the
longer the TC, the better the short term Peak freq noise will be.
If you want the "Dac P-gain freq noise" peak to be under 1e-11 then the TC
must be set >1000sec. (which then needs to use the extended TC method)

Where as the Damping setting controls the integrator gain and its effect is
filtered and non linear. A change in damping from say 2.0 to 0.7 will
increase the Dac noise by up to only 25% but reduce the setting time by over
10 to one.


>Bert posted:
>For our work frequency has to be better than 1xE-11 one second.
>Unless someone can help us with settings we will not revisit Tbolt.

The attached shows that a modified Tbolt can do 1 sec freq error of about
1e-11 peak over most of this 2hr run. The 1 sec ADEV of this run was about
1e-12.
Sounds like best to go back to your previous system, because you are not
going to get there by randomly changing the settings.


ws



>> A damping setting in a Tbolt of greater than 2 is not a good idea.

>It is if one is only concerned about frequency stability and accuracy 
>and wants the best performance that can be obtained at tau < about 10 
>seconds.

>> And a TC setting of >1000 is not a good idea unless using the 'extended
>>TC' method.
>I did not suggest setting the TC >1000. 

>Best regards,
>Charles

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

[Mark Sims]

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.

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

[Charles Steinmetz]

Warren wrote:


A damping setting in a Tbolt of greater than 2 is not a good idea.


It is if one is only concerned about frequency stability and accuracy 
and wants the best performance that can be obtained at tau < about 10 
seconds.



And a TC setting of >1000 is not a good idea unless using the 'extended TC'
mode.


I did not suggest setting the TC >1000.  My suggesetion was to set the 
"recovery" mode "max offset frequency" (the furthest the Tbolt will 
allow the oscillator to go off frequency in order to correct the PPS 
phase) in that range.  That only affects recovery (speeds it up), it 
doesn't do anything during normal operation.  It may not need to be 
>1000, 100 may be plenty.  I do not recall exactly where mine ended up.


Best regards,

Charles


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

[Bob Camp]

Hi

> On Sep 11, 2016, at 11:04 AM, Charles Steinmetz  wrote:
> 
> I wrote:
> 
>>> Actually, for best results from a cold start, it is best to set  the 
>>> initial DAC voltage to whatever voltage produces 10.0 MHz *when the 
>>> oven is cold*.
> 
> Bob replied:
> 
>> …… errr … that’s pretty far off :) DAC at 10 or 20 minutes is probably the 
>> target.
> 
> No, DAC for 10.0 MHz with a stone cold oven is best for cold starts.  
> This setting is still best even if the oven is partly warm, because the loop 
> is moving the DAC in the same direction it needs to go -- it can "catch up" 
> gracefully without racing in the wrong direction in full saturation to meet 
> the crystal as it warms up, then banging back and forth from saturation in 
> one direction to saturation in the other direction before finally leveling 
> out.  It is the damped oscillatory approach to the capture zone, and 
> recovering from saturation (not just once, but multiple times), that slows 
> things down.
> 
> By starting the DAC to produce 10.0 MHz from the cold oven, all that 
> is avoided and the loop just tracks the warming crystal (or, if it can't 
> quite keep up, at least it is moving in the same direction and can catch up 
> gracefully without overshoot -- still much faster than starting from the DAC 
> voltage that produces 10.0 MHz with a fully warm oven).
> 
> Trust me, I tried all of this experimentally and know whereof I speak.
> 

To me, “stone cold oven” = OCXO has been off power for > 24 hours and the 
entire internal structure is at ambient. That might be 25C, it might be 
something else. You applied power about 2 seconds ago and you have an output. 
The oven is now beginning to warm up to some temperature in the 80C to 120C 
range. 

It’s an SC cut crystal. When the oven is at 25C (or worse, even colder) the 
frequency is off by > 30 ppm. The tuning range of the DAC is *maybe* a couple 
of ppm. Effectively, with an un-heated oven, there is no DAC setting. The DAC 
is railed and you still are not on 10 MHz exactly. As the oven warms up, the 
frequency is changing at ppm’s / minute sort of rates. You are not going to be 
able to follow that with the TBolt loop. As a practical matter, you have to let 
the oven warm up a bit before there even is a DAC voltage. You have to let it 
warm up a bit past the "EFC in range” point to be able to begin the lock 
process. That is likely to be some number of minutes (5, 10, 15) past the point 
that you apply power to a cold OCXO. 

My guess is that we have different interpretations of the term “stone cold 
oven”. 

Bob


> Best regards,
> 
> Charles
> 
> 
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Re: [time-nuts] Lady Heather's Tbolt oscillator auto-tune function

[Charles Steinmetz]

I wrote:


Actually, for best results from a cold start, it is best to set  the initial 
DAC voltage to whatever voltage produces 10.0 MHz *when the oven is 
cold*.


Bob replied:


…… errr … that’s pretty far off :) DAC at 10 or 20 minutes is probably the 
target.


No, DAC for 10.0 MHz with a stone cold oven is best for cold 
starts.  This setting is still best even if the oven is partly warm, 
because the loop is moving the DAC in the same direction it needs to go 
-- it can "catch up" gracefully without racing in the wrong direction in 
full saturation to meet the crystal as it warms up, then banging back 
and forth from saturation in one direction to saturation in the other 
direction before finally leveling out.  It is the damped oscillatory 
approach to the capture zone, and recovering from saturation (not just 
once, but multiple times), that slows things down.


By starting the DAC to produce 10.0 MHz from the cold oven, all 
that is avoided and the loop just tracks the warming crystal (or, if it 
can't quite keep up, at least it is moving in the same direction and can 
catch up gracefully without overshoot -- still much faster than starting 
from the DAC voltage that produces 10.0 MHz with a fully warm oven).


Trust me, I tried all of this experimentally and know whereof I speak.

Best regards,

Charles


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

[ws at Yahoo via time-nuts]

A cold oven freq is off by hundred's by Hz, Much much greater than the range
of the Dac
The TBolt does not start tuning until it's oven has warmed up.
Setting anything in the Tbolt biased on cold start turn-on is counter
productive. It takes days or weeks for it settle down.
At initial turn-on and when returning from a lost signal drop-out if the
phase is off, the Tbolts enters a fast Dac slue tune mode that sets the
phase and then the Freq to zero cross before entering its normal mode. 
Because of these, it does not mater in the least what the Dac is set to at a
cold start.
A damping setting in a Tbolt of greater than 2 is not a good idea.
And a TC setting of >1000 is not a good idea unless using the 'extended TC'
mode.
. . .



ws
 
*  

>> Actually, for best results from a cold start, it is best to set the
initial DAC voltage to whatever voltage produces 10.0 MHz *when the
oven is cold*.

Bob replied:

> .. errr . that's pretty far off :) DAC at 10 or 20 minutes is probably the
target.

No, DAC for 10.0 MHz with a stone cold oven is best for cold 
starts.  This setting is still best even if the oven is partly warm, 
because the loop is moving the DAC in the same direction it needs to go 
-- it can "catch up" gracefully without racing in the wrong direction in 
full saturation to meet the crystal as it warms up, then banging back 
and forth from saturation in one direction to saturation in the other 
direction before finally leveling out.  It is the damped oscillatory 
approach to the capture zone, and recovering from saturation (not just 
once, but multiple times), that slows things down.

By starting the DAC to produce 10.0 MHz from the cold oven, all 
that is avoided and the loop just tracks the warming crystal (or, if it 
can't quite keep up, at least it is moving in the same direction and can 
catch up gracefully without overshoot -- still much faster than starting 
from the DAC voltage that produces 10.0 MHz with a fully warm oven).

Trust me, I tried all of this experimentally and know whereof I speak.

Best regards,

Charles




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

[Bob Camp]

Hi

> On Sep 11, 2016, at 3:35 AM, Charles Steinmetz  wrote:
> 
> Mark wrote:
> 
>> Here's a little info on Lady Heather's oscillator autotune function for the 
>> Thunderbolt GPSDO
> 
> Thanks, Mark, that is very helpful.
> 
> Accordingly, for people interested in best frequency accuracy and stability, 
> I suggest (1) running autotune, then (2) manually setting damping to around 
> 10, and (3) setting the holdover recovery parameters manually as I described 
> in my last post (jam synch at 65-75nS and allow a large frequency error in 
> recovery mode).  The value autotune sets for loop time constant (500 seconds) 
> is a good starting point for Tbolts with the Trimble p/n 37265 OCXO.  A 
> particular Tbolt may respond to further optimization by tweaking the TC to 
> match the individual OCXO (by trial and error -- Plot ADEV, adjust TC, plot 
> ADEV, adjust TC, etc.), but the improvement will most likely be subtle.  You 
> should also review the elevation mask settings and adjust if necessary.
> 
>> Since the unit should be locked and stable, the current DAC setting is where 
>> the oscillator is at 10.000 MHz and will be set in EEPROM as the initial 
>> DAC setting. The tbolt uses this value to speed up locking the oscillator 
>> when powering up.
> 
> Actually, for best results from a cold start, it is best to set  the initial 
> DAC voltage to whatever voltage produces 10.0 MHz *when the oven is 
> cold*.

…… errr … that’s pretty far off :) DAC at 10 or 20 minutes is probably the 
target. 

>  Setting it to the voltage that produces 10.0 MHz when the oven is 
> warm guarantees that it is set wrong when the oven is cold, so on a cold 
> start the loop immediately goes into saturation to slew the DAC to the 
> voltage that does produce 10.0 MHz.  This slows down locking by quite 
> a bit.  Setting the initial DAC voltage so that the frequency is 10.0 
> MHz with a cold oven allows the loop to slowly adjust the DAC as the oven 
> warms up, rather than racing off at full speed to meet the warming crystal.  
> This speeds up locking from a cold start very substantially.
> 
> It is not clear to me how one could automate this process -- I found the 
> correct DAC settings by trial and error.  Ideally, this would be determined 
> every time the Tbolt does a cold start and the new value would take into 
> account any crystal drift since it was last set.  But LH has no way to tell 
> that any given start is a cold start (AFAIK), and it would want to determine 
> the correct cold-oven DAC voltage very quickly after power-up from a known 
> cold start (say, within 10 seconds).  It would be great if Tbolts had an oven 
> on/off command, but to my knowledge they don't.  LH could possibly give the 
> user instructions ("OK, now power down the Tbolt for at least 30 minutes.  
> When the Tbolt is fully cold, click the RESUME button below and then re-apply 
> power to the Tbolt...”).

…. It’s a little worse than that :)

Temperature does indeed get into the act, what was the temperature when the DAC 
setting was recorded, what is it now, what is the frequency vs temperature on 
that assembly? One could do a temperature run (or series of runs) and save the 
data. There are OCXO’s done that way. 

Exactly how long has the OCXO been off? There is a difference between 3 hours 
and 5 hours. Similarly there is a difference between one day and a week.

What temperature (and humidity and …) has the assembly been at while off? 
Temperature will impact the sealed OCXO. The parts on the board most certainly 
are sensitive to humidity. 

Did something weird happen? Was the unit rotated? (2g tip does matter). Was it 
dropped? Was it shaken around? There are a number of weird things that *could* 
happen. Some OCXO’s get acceleration compensated to take care of this. 

Is it the DAC at 5, 10, 15 or 60 minutes? The OCXO is moving pretty fast right 
after it turns on. For instance, the FE 405’s compensate for this in firmware. 

That’s a short list, one could easily double it in length. 

=

The *simple* answer is to use software. Rather than having the TBolt in one 
mode all the time, change it’s parameters when it first turns on. Give it a 
shorter TC and get it stabilized. Then feed it the “real” TC and let it go from 
there. I have absolutely no idea how a TBolt responds when you do this. I do 
have a lot of data on how other vendors parts respond when this takes place. In 
general it works pretty well.

Bob


> 
> Best regards,
> 
> Charles
> 
> 
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Re: [time-nuts] Lady Heather's Tbolt oscillator auto-tune function

[Charles Steinmetz]

Mark wrote:


Here's a little info on Lady Heather's oscillator autotune function for the 
Thunderbolt GPSDO


Thanks, Mark, that is very helpful.

Accordingly, for people interested in best frequency accuracy and 
stability, I suggest (1) running autotune, then (2) manually setting 
damping to around 10, and (3) setting the holdover recovery parameters 
manually as I described in my last post (jam synch at 65-75nS and allow 
a large frequency error in recovery mode).  The value autotune sets for 
loop time constant (500 seconds) is a good starting point for Tbolts 
with the Trimble p/n 37265 OCXO.  A particular Tbolt may respond to 
further optimization by tweaking the TC to match the individual OCXO (by 
trial and error -- Plot ADEV, adjust TC, plot ADEV, adjust TC, etc.), 
but the improvement will most likely be subtle.  You should also review 
the elevation mask settings and adjust if necessary.



Since the unit should be locked and stable, the current DAC setting is where 
the oscillator is at 10.000 MHz and will be set in EEPROM as the initial 
DAC setting. The tbolt uses this value to speed up locking the oscillator when 
powering up.


Actually, for best results from a cold start, it is best to set  the 
initial DAC voltage to whatever voltage produces 10.0 MHz *when 
the oven is cold*.  Setting it to the voltage that produces 10.0 
MHz when the oven is warm guarantees that it is set wrong when the oven 
is cold, so on a cold start the loop immediately goes into saturation to 
slew the DAC to the voltage that does produce 10.0 MHz.  This 
slows down locking by quite a bit.  Setting the initial DAC voltage so 
that the frequency is 10.0 MHz with a cold oven allows the loop 
to slowly adjust the DAC as the oven warms up, rather than racing off at 
full speed to meet the warming crystal.  This speeds up locking from a 
cold start very substantially.


It is not clear to me how one could automate this process -- I found the 
correct DAC settings by trial and error.  Ideally, this would be 
determined every time the Tbolt does a cold start and the new value 
would take into account any crystal drift since it was last set.  But LH 
has no way to tell that any given start is a cold start (AFAIK), and it 
would want to determine the correct cold-oven DAC voltage very quickly 
after power-up from a known cold start (say, within 10 seconds).  It 
would be great if Tbolts had an oven on/off command, but to my knowledge 
they don't.  LH could possibly give the user instructions ("OK, now 
power down the Tbolt for at least 30 minutes.  When the Tbolt is fully 
cold, click the RESUME button below and then re-apply power to the 
Tbolt...").


Best regards,

Charles


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

[Mark Sims]

Here's a little info on Lady Heather's oscillator autotune function for the 
Thunderbolt GPSDO:

The autotune function tries to optimize the settings for the oscillator 
disciplining parameters, antenna signal mask angle, and the signal level 
amplitude mask beyond what the default setting (which are more for telecom 
timing applications) are.

To use the auto-tune function the receiver should be warmed up and stable.  
Manually set the antenna mask angle to a low value (say 0-5 degrees) with the 
FE keyboard command and set the signal level mask to a low value (1 AMU,  20-30 
dBc) with the FL command.  Clear the signal level log with the CS command. Let 
the receiver collect signal level data for several hours (overnight is good).  
The data collected is used to determine where your signal levels begin to drop 
vs the satellite elevation angles.  

Since the unit should be locked and stable, the current DAC setting is where 
the oscillator is at 10.000 MHz and will be set in EEPROM as the initial 
DAC setting. The tbolt uses this value to speed up locking the oscillator when 
powering up.

Then issue the autotune command ().  This will put the DAC into manual 
control mode and step the DAC control voltage 5 mV high and then low and 
measure how the oscillator frequency changes.  This takes a few minutes to 
complete.  This is used to determine the oscillator voltage gain.   The 
calculated value seems to be quite accurate.  Tbolts set up for external 
oscillators can be used to determine unknown 10 MHz oscillator EFC 
characteristics.

Next,  Lady Heather sets the loop time constant to 500 seconds and the damping 
to 1.0   These value seem to be  good conservative general purpose values for a 
typical unit and should not cause any loop stability issues.

Finally, Lady Heather sets the antenna mask angle setting to where the signal 
level starts to drop off rapidly and the signal level mask value to 30 dBc (or 
1 AMU unit).   After the auto-tune completes, the antenna mask angle setting 
might need to be manually set to a lower value...  it tends to find that the 
signal level falls off at a higher angle than one might expect.  You can check 
the signal level vs elevation plot with the SAE keyboard command (or ZE in the 
next release will show the plot zoomed to full screen).   High antenna mask 
angles cause more satellite constellation changes which is not good.  Low 
antenna mask angles subjects the receiver to multi-path effects which is also 
not good.  You are damned if you do and damned if you don't...








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