[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 <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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