[time-nuts] GPS for Nixie Clock

Chris Albertson albertson.chris at gmail.com
Fri Jul 15 19:57:06 EDT 2016

If you are going for the sawtooth correction then you also might want
to add some kind of forward correction for the delay in the tubes and
the drivers.  Your MOSFET gates the nixie tube itself have capacitance
and switch times that will delay the switch of the display and of
course the digital processing in the FPGA takes some number of
nanoseconds.   I think you might need some way to actually measure all
of these as any estimate might be your single largest source of error.
  I don't know how to measure it.  Perhaps a pair of phototransistors
one aimed at a PPS LED and one at the nixie tube.  This unknown delay
is likely larger than the sawtooth correction.  at this level you
might have to define when a digital is actually "on" as there is
likely some thermal constant and the numbers don't light up instantly.
  I'd bet the turn on time is larger than the sawtooth correction.
What is "on"?  50% brightness?

It gets hard when you start caring about tiny increments of time.   I
have a mechanical clock, about 14 inches in diameter that is slaved to
NTP.  The designer took a big short cut.  Time is kept internally at
the hundreds of microseconds level and the pulse goes off to the
stepper motor at the correct time well at least at the 100+
microsecond level but the hands don't move instantly because (1)
slight gear backlash and (2) they have mass.  I can actually SEE the
delay with my eyes.  The designer must have forgotten that a "move"
command requires some milliseconds to execute (I'm thinking about
100ms or more).  I don't care but it's fun to think the actual display
is 10,000 times less accurate then the internal timekeeping.   You
don't want this to happen to happen nixie clock

BTW I did not build my mechanical NTP clock.  I got a free broken
clock and had to fix it, cut and soldered a few traces, fixed some
cracked parts and learned how it works in the process.

Finding which PPS to use is easy, you can do that by eye.  Compare the
serial data stream to the time on your NTP sync'd computer.  A full
second off problem is easy to see.

On Fri, Jul 15, 2016 at 3:53 PM, John Swenson <johnswenson1 at comcast.net> wrote:
> Yep, that is theory. The fun part is going to be getting the right edge for
> the new PPS. Half the time it will the one before the PPS from the GPS and
> half the time it will be the one after. From the sawtooth data I should be
> able to figure out which is which to align it to the new LO.
> John S.
> On 7/15/2016 3:17 PM, Bob Camp wrote:
>> Hi
>> If you are going to go “full boat” then you probably should get the
>> sawtooth correction out of
>> the GPS and feed that into your control loop. You will need something you
>> can run out at the
>> “few hundred seconds” sort of time constant.
>> Bob
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Chris Albertson
Redondo Beach, California

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