Re: [time-nuts] GPS for Nixie Clock
I would run a test and track turn ON/OFF times against varying
intensities to get enough data to chart/graph. THEN, you can make
informed decisions about intensity level, whether you want to consider
turn off time, etc.
This could become quite voluminous... data acquisition-wise and just the
sheer amount of information.
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 7/16/2016 2:08 AM, John Swenson wrote:
Yes, I was planning on using a high speed photo diode to actually
measure the turn on time of the digits. I hadn't thought of the turn
OFF time, do I want the old digit to be turned off before the new one
lights up or for them to be overlapping? I have been thinking about
what threshold to use, 50% intensity is probably about as good as any
other. It might turn out that different digits turn on differently, so
I will have to calibrate each one separately.
John S.
On 7/15/2016 4:57 PM, Chris Albertson 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@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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How about monitoring the Nixie tube current instead of light output. I have a strong suspicion that there's a good correlation between the two.
From Tom Holmes, N8ZM
On Jul 15, 2016, at 11:40 PM, jimlux jimlux@earthlink.net wrote:
On 7/15/16 5:25 PM, Bob Camp wrote:
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.
Preionize the gas with a radioactive source. If it works for Krytrons, it can work for Nixies. You could also use a pulsed ion source that turns on slightly before the "top of the second" to irradiate and prepare the Nixie.
A true time-nut won't let such thing stand in the way of perfection.
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.
Turning an ionized gas off is always harder than turning it on. Perhaps a tailbiter type circuit or a negative pulse generator?
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On 7/15/16 6:23 PM, Chris Albertson wrote:
I did write that it's useless to have a visual display that is three
orders of magnitude better than the human perceptional system and was
corrected that such a display could be used for film based
photography. That is true. But that just adds even more problems
like making certain the display never changes while the camera shutter
is open. These old camera time loggers were hooked up to the shutter
release. I think they captured the time of day when the shutter opens
and hold it for the duration of the exposure. I have some old ones
that I can check, but I'm certain they did not change while the
shutter was open. They did not light up at all when the shutter was
closed. If they did change without regard to the camera shutter then
on order 1% of the shots would capture the increment and with a
7-segment number it would be unreadable. But this never happened.
WHen I was working in the film/tv business, I made a lot of devices (or
more properly wound up modifying existing devices) to take a camera
sync pulse so that we could be sure that the display was static while
the shutter is open, or that the xenon strobe or blinky lights would
fire at the right time.
Typically, you need an adjustable delay box with two settings that you
set empirically on set. You get a pulse for the shutter sync (it's 15
years ago, and I don't remember the details), but you need to have two
possible delays: the camera operator's viewfinder is open when the
shutter is not = that is, the optical path is essentially switched
between the view finder and the film. 555s were my friend.
This is used to great advantage on closeup or in remote control shots
where they use a laser pointer aligned the the optical axis, and only
turn it on when the viewfinder is open. You can "see" if you're
pointing the right direction, but the laser is off when the shutter is
open to the film. I imagine by now there are fancier versions that
project frame lines or corners and such.
You also sometimes need to adjust the current or multiplexing rate to a
LED display so that the brightness doesn't change with changing shutter
duration or phasing. You definitely need to sync the multiplexing with
the shutter or the display will either be partial, or will have a
strange cyclical brightness variation.
There's a whole industry of producing 24 fps TV and computer monitors so
the display "looks" ok when filmed at some rate. I used to have a bunch
of "genlockable" VGA display cards that could be put into a PC and
synced to some supplied sync signal. I wrote a fair amount of little
utility programs that would poke the registers on a video card to get
specific vertical frame rates and you'd hope the production staff had
bought monitors that could sync at 24 or 48 fps, or some other oddball
rate because they were shooting slo-mo at 120 fps or something.
I think today, with much improved automatic compositing and offline
editing, they just paint the screens green or blue on set and composite
the video information in later. Even with a camera move in the shot, the
compositing operator would mark the corners of the screen in a few
frames, and the software would do the rest.
On 7/15/16 10:04 PM, Chris Albertson wrote:
Seriously, it does not matter how long it takes to turn a nixie tube
on or off. You measure it and then compensate. Likely would need to
continuously measure and adjust the compensation. This is doable
and is the only hard part of the problem as it is new while the rest
has been done 1000 times.
It does matter, if it's not consistent. That's the idea of preionizing:
it makes the jitter in the "turnon" much lower. Otherwise your spark gap
switches aren't well synchronized enough to create the desired
converging pressure wave (at least in the most notorious application of
preionized fast switches)
We also use a radioactive source when doing HV breakdown testing: you
want to guarantee that there's a few electrons around in the gap, rather
than hoping for a cosmic ray at the right time.
On 7/16/16 12:08 AM, John Swenson wrote:
Yes, I was planning on using a high speed photo diode to actually
measure the turn on time of the digits. I hadn't thought of the turn OFF
time, do I want the old digit to be turned off before the new one lights
up or for them to be overlapping? I have been thinking about what
threshold to use, 50% intensity is probably about as good as any other.
It might turn out that different digits turn on differently, so I will
have to calibrate each one separately.
Looking at my 5326 nixie tube counter I would say that the capacitance
of the digits will be different: they're different areas and different
spacing from the grid in front. So I would expect the time constant on
the drive circuit to be (slightly) different. On the 5326, I believe
the drive circuit is lightbulbs and photoresistors (having not looked
inside in many years)
Use AC coupling to each digit to measure the ignition waveform and
detect the breakdown point like with a tunnel diode trigger. Use a
higher compliance voltage and greater negative resistance (constant
current drive?) to lower breakdown jitter.
On Sat, 16 Jul 2016 00:08:45 -0700, you wrote:
Yes, I was planning on using a high speed photo diode to actually
measure the turn on time of the digits. I hadn't thought of the turn OFF
time, do I want the old digit to be turned off before the new one lights
up or for them to be overlapping? I have been thinking about what
threshold to use, 50% intensity is probably about as good as any other.
It might turn out that different digits turn on differently, so I will
have to calibrate each one separately.
John S.
Hi
Since we have moved into synchronizing this stuff at the nanosecond level
(maybe we are even lower than that by now ..), simply getting a wide band
enough signal off of a Nixe socket is going to be “interesting”. An array of picosecond
photo diodes on each tube may be the only way to go. How many channels
this all will take depends a bit on how many digits past the second the display
will show. Is it 9 digits past the second?
Since you will only know the ignition point after it has happened, the system
only works to a certain degree. Trigger point is dependent on the light level.
You will need to collect real time data to keep things consistent.
Bob
On Jul 16, 2016, at 10:49 AM, David davidwhess@gmail.com wrote:
Use AC coupling to each digit to measure the ignition waveform and
detect the breakdown point like with a tunnel diode trigger. Use a
higher compliance voltage and greater negative resistance (constant
current drive?) to lower breakdown jitter.
On Sat, 16 Jul 2016 00:08:45 -0700, you wrote:
Yes, I was planning on using a high speed photo diode to actually
measure the turn on time of the digits. I hadn't thought of the turn OFF
time, do I want the old digit to be turned off before the new one lights
up or for them to be overlapping? I have been thinking about what
threshold to use, 50% intensity is probably about as good as any other.
It might turn out that different digits turn on differently, so I will
have to calibrate each one separately.
John S.
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The point of measuring the actual ignition point is to predictably
remove delay by driving the element earlier. CRT grid structures
support transition times in the 5 to 20 nanosecond range; the smaller
distances involved with a nixie tube should support faster operation.
Something which just occurred to me is that ultraviolet can be used to
provide ionization to the gas without radioactivity. Flame detector
tubes work like this so bathe the tubes in a small amount of UV. I do
not know how transparent the nixie tube envelope to UV is though.
On Sat, 16 Jul 2016 11:04:22 -0400, you wrote:
Hi
Since we have moved into synchronizing this stuff at the nanosecond level
(maybe we are even lower than that by now ..), simply getting a wide band
enough signal off of a Nixe socket is going to be interesting. An array of picosecond
photo diodes on each tube may be the only way to go. How many channels
this all will take depends a bit on how many digits past the second the display
will show. Is it 9 digits past the second?
Since you will only know the ignition point after it has happened, the system
only works to a certain degree. Trigger point is dependent on the light level.
You will need to collect real time data to keep things consistent.
Bob
On Jul 16, 2016, at 10:49 AM, David davidwhess@gmail.com wrote:
Use AC coupling to each digit to measure the ignition waveform and
detect the breakdown point like with a tunnel diode trigger. Use a
higher compliance voltage and greater negative resistance (constant
current drive?) to lower breakdown jitter.
Hi
The gotcha is that it’s the sum of the radiation arriving in the vicinity of the gas.
Supplying a bit can flood small variations, but they still are present. You are trying
to get what is essentially a neon bulb to trigger accurately to a very tight budget.
There is a lot of prior art on the pitfalls. Since you do not have a structure that was
custom designed as a high frequency transmission line, there are a lot of lumps and
bumps along the way…
The only real point is that going from millisecond to microseconds is a leap for these
gizmos. Going from microseconds to nanoseconds (or picoseconds) is an even bigger leap.
There is a way to do it, if your budget it big enough. It’s going to be a major effort all by
it’s self. Being able to forward predict the process (which is the goal) at these levels has
a lot of variables in it. Feedback does not help if you have second to second jitter that
is all over the place….(gas tubes are used as noise generators for a reason ….).
Bob
On Jul 16, 2016, at 11:36 AM, David davidwhess@gmail.com wrote:
The point of measuring the actual ignition point is to predictably
remove delay by driving the element earlier. CRT grid structures
support transition times in the 5 to 20 nanosecond range; the smaller
distances involved with a nixie tube should support faster operation.
Something which just occurred to me is that ultraviolet can be used to
provide ionization to the gas without radioactivity. Flame detector
tubes work like this so bathe the tubes in a small amount of UV. I do
not know how transparent the nixie tube envelope to UV is though.
On Sat, 16 Jul 2016 11:04:22 -0400, you wrote:
Hi
Since we have moved into synchronizing this stuff at the nanosecond level
(maybe we are even lower than that by now ..), simply getting a wide band
enough signal off of a Nixe socket is going to be interesting. An array of picosecond
photo diodes on each tube may be the only way to go. How many channels
this all will take depends a bit on how many digits past the second the display
will show. Is it 9 digits past the second?
Since you will only know the ignition point after it has happened, the system
only works to a certain degree. Trigger point is dependent on the light level.
You will need to collect real time data to keep things consistent.
Bob
On Jul 16, 2016, at 10:49 AM, David davidwhess@gmail.com wrote:
Use AC coupling to each digit to measure the ignition waveform and
detect the breakdown point like with a tunnel diode trigger. Use a
higher compliance voltage and greater negative resistance (constant
current drive?) to lower breakdown jitter.
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