Re: [time-nuts] GPS for Nixie Clock
As I mentioned in a previous post, this Nixie retrofit is not about
"good enough", it is a learning experience for me to understand the ins
and outs of GPS based time, so it is going to do all kinds of things
that are not NEEDED but that is the fun of the project.
I'm also looking into designing my own patch antennas to get the best
sensitivity in the restricted confines of the block of wood, yet
something else to learn about.
That's what this is all about.
I yes I probably will do an FPGA controlled PLL for the fun of it (I
have done that before).
John S.
On 7/15/2016 9:54 AM, Bob Camp wrote:
Hi
On Jul 15, 2016, at 9:44 AM, David J Taylor david-taylor@blueyonder.co.uk wrote:
Hi
Ok, I guess we need to go into this again:
All of the output signals generated by one of these cheap GPS modules
come from the internal TCXO on the module. All the signals.
None of the TCXO’s on any of these modules are tuned to match the GPS.
None of them, zero, not any.
All of the output signals from all of these modules are matched up to GPS
by guessing which clock edge to use.
The result for all of these modules and all of their output signals is a signal
with a lot of jitter.
All GPS based systems are limited by the noise of the GPS signal. It is
really dirty at short time intervals. The shorter the interval, the more noise
it has. Any signal that directly tracks GPS will be very dirty.
The only way to clean up GPS to make it useful as a frequency source is
with a very narrowband loop.
If you are implementing a < 0.01 Hz wide loop, it is no harder to do at 1 Hz
than 10 KHz or 100 MHz. In many respects it is easier to do at 1 Hz.
If the objective is a time display that is read with a human eye, anything
under 1 ms is not of much use. Your eye can’t detect it. Getting to 1 ns
is a different task than getting to 1 ms. A Hydrogen Maser flywheel is
not needed as part of a basic wall clock design.
Lots of variables, but also lots of basic facts.
Bob
That seems a somewhat negative assessment, Bob. For time purposes (even to within a microsecond), the PPS output from the ublox etc. modules is more than good enough (for a Nixie clock and many more needs). Couple the PPS with the time from the serial output in your micro and that's completely adequate. TCXO not even needed.
Yes, frequency is a different matter.
…. we had headed off into the “virtues” of PLL’s locking to 10 KHz ….
=====
To get a time resolution of 10 ms (yes 10X 1 ms), you don’t really need the pps. The timing of
the serial string is probably “good enough”. That assumes you don’t have all sorts of other
messages turned on as well. In the case of a wall clock, it’s not clear why anything other
than a basic timing message would be enabled. A sub $10 module likely will do everything
you need to do.
Bob
Cheers,
David
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv
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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
On Jul 15, 2016, at 5:25 PM, John Swenson johnswenson1@comcast.net wrote:
As I mentioned in a previous post, this Nixie retrofit is not about "good enough", it is a learning experience for me to understand the ins and outs of GPS based time, so it is going to do all kinds of things that are not NEEDED but that is the fun of the project.
I'm also looking into designing my own patch antennas to get the best sensitivity in the restricted confines of the block of wood, yet something else to learn about.
That's what this is all about.
I yes I probably will do an FPGA controlled PLL for the fun of it (I have done that before).
John S.
On 7/15/2016 9:54 AM, Bob Camp wrote:
Hi
On Jul 15, 2016, at 9:44 AM, David J Taylor david-taylor@blueyonder.co.uk wrote:
Hi
Ok, I guess we need to go into this again:
All of the output signals generated by one of these cheap GPS modules
come from the internal TCXO on the module. All the signals.
None of the TCXO’s on any of these modules are tuned to match the GPS.
None of them, zero, not any.
All of the output signals from all of these modules are matched up to GPS
by guessing which clock edge to use.
The result for all of these modules and all of their output signals is a signal
with a lot of jitter.
All GPS based systems are limited by the noise of the GPS signal. It is
really dirty at short time intervals. The shorter the interval, the more noise
it has. Any signal that directly tracks GPS will be very dirty.
The only way to clean up GPS to make it useful as a frequency source is
with a very narrowband loop.
If you are implementing a < 0.01 Hz wide loop, it is no harder to do at 1 Hz
than 10 KHz or 100 MHz. In many respects it is easier to do at 1 Hz.
If the objective is a time display that is read with a human eye, anything
under 1 ms is not of much use. Your eye can’t detect it. Getting to 1 ns
is a different task than getting to 1 ms. A Hydrogen Maser flywheel is
not needed as part of a basic wall clock design.
Lots of variables, but also lots of basic facts.
Bob
That seems a somewhat negative assessment, Bob. For time purposes (even to within a microsecond), the PPS output from the ublox etc. modules is more than good enough (for a Nixie clock and many more needs). Couple the PPS with the time from the serial output in your micro and that's completely adequate. TCXO not even needed.
Yes, frequency is a different matter.
…. we had headed off into the “virtues” of PLL’s locking to 10 KHz ….
=====
To get a time resolution of 10 ms (yes 10X 1 ms), you don’t really need the pps. The timing of
the serial string is probably “good enough”. That assumes you don’t have all sorts of other
messages turned on as well. In the case of a wall clock, it’s not clear why anything other
than a basic timing message would be enabled. A sub $10 module likely will do everything
you need to do.
Bob
Cheers,
David
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-taylor@blueyonder.co.uk
Twitter: @gm8arv
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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and follow the instructions there.
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
HI
The only practical way to do a “filter” is with a flywheel oscillator. You will need at least a TCXO to hold the ADEV
somewhere reasonable while the system it doing its thing. Something VCO based will not do the job.
Bob
On Jul 15, 2016, at 6:53 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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and follow the instructions there.
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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Chris Albertson
Redondo Beach, California
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@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@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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Chris Albertson
Redondo Beach, California
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and follow the instructions there.
It does not matter how slowly a nixie tube and it's driver operate,
you can still get them to light up exactly one time. I've been
playing with robots lately and if you think tubes take time to light
up, you should try moving a few kilograms around with a battery
powered motor. But even the motors can be made to move on time. The
key in both cases is feedback. The motor have shaft encoders, your
"microsecond level nixie tube" will need photo sensors. Then in both
cases you apply a forward correction so as to start the process going
early enough that it finishes on time. A servo controller algorithm
adjusts the correction based on measured error. Of course I could
just estimate everything and run open loop but you don't get to
microseconds that way.
I said "photosensor" for feedback but maybe something else will work,
perhaps yu measure current on the common anode or do both. I think
you may be inventing new engineering as I doubt anyone else has tried
to get a nixie tube to change at timing accuracy under a few
milliseconds.
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.
On Fri, Jul 15, 2016 at 5:25 PM, Bob Camp kb8tq@n1k.org 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.
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@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@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
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Chris Albertson
Redondo Beach, California
I, for one, will be following your progress...
I think it would be cool as heck having an ultra-accurate clock with a
Nixie display... It'd be cool to make it flexible enough to output the
time sync to other equipment...
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 7/15/2016 4:25 PM, John Swenson wrote:
As I mentioned in a previous post, this Nixie retrofit is not about
"good enough", it is a learning experience for me to understand the
ins and outs of GPS based time, so it is going to do all kinds of
things that are not NEEDED but that is the fun of the project.
I'm also looking into designing my own patch antennas to get the best
sensitivity in the restricted confines of the block of wood, yet
something else to learn about.
That's what this is all about.
I yes I probably will do an FPGA controlled PLL for the fun of it (I
have done that before).
John S.
Hi
As this is going, it’s not a clock at all. It’s a GPSDO with a Nixie display on it
and now with IRIG timing output.
Do we put an Rb in it or go straight to a Cesium?
Bob
On Jul 15, 2016, at 9:38 PM, Clay Autery cautery@montac.com wrote:
I, for one, will be following your progress...
I think it would be cool as heck having an ultra-accurate clock with a
Nixie display... It'd be cool to make it flexible enough to output the
time sync to other equipment...
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 7/15/2016 4:25 PM, John Swenson wrote:
As I mentioned in a previous post, this Nixie retrofit is not about
"good enough", it is a learning experience for me to understand the
ins and outs of GPS based time, so it is going to do all kinds of
things that are not NEEDED but that is the fun of the project.
I'm also looking into designing my own patch antennas to get the best
sensitivity in the restricted confines of the block of wood, yet
something else to learn about.
That's what this is all about.
I yes I probably will do an FPGA controlled PLL for the fun of it (I
have done that before).
John S.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
I think it would be cool as heck having an ultra-accurate clock with a Nixie display...
http://leapsecond.com/pages/atomic-nixie/
/tvb
----- Original Message -----
From: "Clay Autery" cautery@montac.com
To: time-nuts@febo.com
Sent: Friday, July 15, 2016 6:38 PM
Subject: Re: [time-nuts] GPS for Nixie Clock
I, for one, will be following your progress...
I think it would be cool as heck having an ultra-accurate clock with a
Nixie display... It'd be cool to make it flexible enough to output the
time sync to other equipment...
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 7/15/2016 4:25 PM, John Swenson wrote:
As I mentioned in a previous post, this Nixie retrofit is not about
"good enough", it is a learning experience for me to understand the
ins and outs of GPS based time, so it is going to do all kinds of
things that are not NEEDED but that is the fun of the project.
I'm also looking into designing my own patch antennas to get the best
sensitivity in the restricted confines of the block of wood, yet
something else to learn about.
That's what this is all about.
I yes I probably will do an FPGA controlled PLL for the fun of it (I
have done that before).
John S.