[time-nuts] GPS for Nixie Clock

[Bob Camp]

Hi

You can do a pretty good job with a high speed photo diode. They are not cheap, but
you can get fast ones if your Visa card is up to it. 

The next layer will be that at the relatively low strike voltages normally used, Nixie’s don’t 
light up consistently. You either need to compensate for temperature and ambient light / then
calibrate each segment or sense each one as it turns on. Either way … it’s a major learning 
experience just to get it into the microseconds range. You can get to nanoseconds, but that
may or may not be possible with conventional Nixie’s. 

Once you have them turned on, you go back through something similar when you turn them
off. It takes a bit of time for all the little gas molecules to go back to rest state. The data I have seen
on that sort of thing suggests a “many microseconds” to millisecond decay process depending 
on the gas and how it was driven. 

Bob

> On Jul 15, 2016, at 7:57 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 
> 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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