Empathy List Archives

MC

Mike Ciholas

Thu, Aug 18, 2005 5:29 PM

Hi,

I have a challenging research project to build thousands, perhaps
millions, of devices that maintain mutual synchronization. The
devices need to be low cost (under $20 retail, $8 manufacturing),
small in size (key chain fob), and low power (operate at least 18
months on a battery). Synchronization ideally needs to be within
a second or two over a year but there is some leeway to trade
cost for performance here up to perhaps 10 seconds of variation
per year. Ideally, the device works anywhere in the world but we
may have to limit it to North America.

Crystal Modeling

First idea was to get stable 32.768KHz watch crystals, perform a
factory initial calibration, and use a temperature sensor to
correct for the crystal temp curve. This idea is the cheapest,
simplest, works everywhere, and uses the lowest power.

Initial tolerance on the crystals is +/- 20 ppm (I've not found
better in commodity parts), which equates to +/- 10 minutes a
year, clearly unacceptable. I suspect that if I did an initial
factory calibration and tracked temperature, I might improve this
to +/- 2 ppm much like Maxim did with this part:

http://pdfserv.maxim-ic.com/en/ds/DS32kHz.pdf

But even so, +/- 1 minute per year is not really good enough. I
suspect getting to a few seconds (+/- 0.1ppm) is unrealistic with
any algorithm one can come up with. The base physics is simply
not that predictable.

WWVB Receiver

A second idea is to provide some external reference and the most
logical choice is WWVB as used in several wrist watches. A
little more cost but manageable. We've dissected several wrist
watches and found they use a small ferrite antenna. The
reception performance is spotty, however. I was unable to lock
at work (lots of equipment) but did well at home (electrically
quiet). If we go to the NE tip of Maine, that's twice as far
from WWVB as we are here, so I wonder if the watch will ever pick
up the signal. The saving grace is that the device needs to get
the signal only sporadically, once a week or even once a month
would do it since we can feed that back into correcting the local
crystal.

The negatives are that such a device is limited to the US and
nearby, and it may have poor performance in many locales due to
weak signals, local interference, and the small antenna rod we
are limited to due to size (less than 1 inch). It does cost
more, maybe $1-2 more in production quantity. Right now, this
seems like the best option available to us.

There are similar time broadcasting stations in Europe and China.
We could build a unit that works in those regions, either as
different models, or as a unit with multiple receivers. Still
not global, but perhaps covering 50% of the world's population?

GPS Receiver

A more precise external reference, use a GPS receiver. This gets
us global coverage and is very precise. Uses a lot of power, so
we would only activate it very briefly and not very often (once a
week perhaps) to save battery.

Major issue here is cost. Best I can do for an OEM module is
around $25 in qty which busts the budget severely. It also has
similar problems of being used in a place with no sky visibility.
Size can be a problem in the cheaper modules. Some modules are
quite small:

http://www.u-blox.com/products/lea_la.html

Cute, huh?

GPS Time Receiver

This is fantasy land. I don't need the 100ns time reference, all
I need is something good to one second or so. In this case, it
seems possible to receive only 1 satellite, decode the digital
data, and extract the time. It would be off by the variation in
pseudo range which can't be corrected for. But I don't care
about that level of accuracy.

The question is, if you don't have to track multiple satellites
and don't need to recover the pseudo range accurately, can you
build a wickedly cheaper GPS time receiver? My expectation is no.
You probably can get down to maybe half if you are very diligent,
which still puts me out of the budget plus has a ridiculous high
NRE. Unless this already exists, anyone?

Cellular

We've done extensive work with embedded cell phone modules.
These modules are most often used for wireless remote monitoring
and transport digital data. They do get the time from the cell
system.

Again, cost is a major issue. An OEM cell module runs over $65
in qty so this idea is sunk. It would also suffer from lack of
global and local coverage.

TV Stations

TV stations broadcast a time signal that VCRs/DVRs use for clock
setting.

Again, lack of global or even regional coverage. Some TV
stations, annoyingly, broadcast the wrong time, too. Cost is
probably high, but this idea was rejected before this was
investigated.

Atomic Reference

Still research, but NIST has a small scale atomic reference:

http://www.nist.gov/public_affairs/releases/miniclock.htm

Unfortunately, not ready for commercial apps, probably will be
too expensive, and it uses too much power. The best I could do
on power is to power it up periodically and adjust the local
crystal to it which integrates long term error.

Other?

So, did I leave anything out?

It seems obvious to me that no amount of effort to make a local
crystal stable will meet the requirements. Thus we need to look
for external references. The best we can do is WWVB as it is the
only thing that can possibly meet our cost objectives. If it
works in the continental US, that would be acceptable for now.

That leaves me with two basic questions:

How well do the WWVB wrist watches work?
What merchant silicon exists for receiving WWVB?

On the first, the three watches we bought do sync up here (950
miles from WWVB). I wonder how well they work in Maine and
Florida (1900 miles from WWVB).

On the second, I've only found these leads so far:

http://www.mas-oy.com/archive/da9180.pdf
http://www.ortodoxism.ro/datasheets/Temic/mXyzuryv.pdf

These chips appear to be basic receiver circuits using an
external 60KHz crystal as a filter. At 60KHz, I was wondering
why there aren't direct digital radios? It would seem like
building in the DSP logic would be cheaper/better than the old
fashioned methods shown here and could greatly enhance the
ability to pick out weak WWVB signals. Has anyone performed such
experiments, basically digitize the antenna signal and done DSP
on it?

Thanks for all who read this far!

--
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec@ciholas.com
Evansville, IN 47713 http://www.ciholas.com

Hi, I have a challenging research project to build thousands, perhaps millions, of devices that maintain mutual synchronization. The devices need to be low cost (under $20 retail, $8 manufacturing), small in size (key chain fob), and low power (operate at least 18 months on a battery). Synchronization ideally needs to be within a second or two over a year but there is some leeway to trade cost for performance here up to perhaps 10 seconds of variation per year. Ideally, the device works anywhere in the world but we may have to limit it to North America. 1. Crystal Modeling First idea was to get stable 32.768KHz watch crystals, perform a factory initial calibration, and use a temperature sensor to correct for the crystal temp curve. This idea is the cheapest, simplest, works everywhere, and uses the lowest power. Initial tolerance on the crystals is +/- 20 ppm (I've not found better in commodity parts), which equates to +/- 10 minutes a year, clearly unacceptable. I suspect that if I did an initial factory calibration and tracked temperature, I might improve this to +/- 2 ppm much like Maxim did with this part: http://pdfserv.maxim-ic.com/en/ds/DS32kHz.pdf But even so, +/- 1 minute per year is not really good enough. I suspect getting to a few seconds (+/- 0.1ppm) is unrealistic with any algorithm one can come up with. The base physics is simply not that predictable. 2. WWVB Receiver A second idea is to provide some external reference and the most logical choice is WWVB as used in several wrist watches. A little more cost but manageable. We've dissected several wrist watches and found they use a small ferrite antenna. The reception performance is spotty, however. I was unable to lock at work (lots of equipment) but did well at home (electrically quiet). If we go to the NE tip of Maine, that's twice as far from WWVB as we are here, so I wonder if the watch will ever pick up the signal. The saving grace is that the device needs to get the signal only sporadically, once a week or even once a month would do it since we can feed that back into correcting the local crystal. The negatives are that such a device is limited to the US and nearby, and it may have poor performance in many locales due to weak signals, local interference, and the small antenna rod we are limited to due to size (less than 1 inch). It does cost more, maybe $1-2 more in production quantity. Right now, this seems like the best option available to us. There are similar time broadcasting stations in Europe and China. We could build a unit that works in those regions, either as different models, or as a unit with multiple receivers. Still not global, but perhaps covering 50% of the world's population? 3. GPS Receiver A more precise external reference, use a GPS receiver. This gets us global coverage and is very precise. Uses a lot of power, so we would only activate it very briefly and not very often (once a week perhaps) to save battery. Major issue here is cost. Best I can do for an OEM module is around $25 in qty which busts the budget severely. It also has similar problems of being used in a place with no sky visibility. Size can be a problem in the cheaper modules. Some modules are quite small: http://www.u-blox.com/products/lea_la.html Cute, huh? 4. GPS Time Receiver This is fantasy land. I don't need the 100ns time reference, all I need is something good to one second or so. In this case, it seems possible to receive only 1 satellite, decode the digital data, and extract the time. It would be off by the variation in pseudo range which can't be corrected for. But I don't care about that level of accuracy. The question is, if you don't have to track multiple satellites and don't need to recover the pseudo range accurately, can you build a wickedly cheaper GPS time receiver? My expectation is no. You probably can get down to maybe half if you are very diligent, which still puts me out of the budget plus has a ridiculous high NRE. Unless this already exists, anyone? 5. Cellular We've done extensive work with embedded cell phone modules. These modules are most often used for wireless remote monitoring and transport digital data. They do get the time from the cell system. Again, cost is a major issue. An OEM cell module runs over $65 in qty so this idea is sunk. It would also suffer from lack of global and local coverage. 6. TV Stations TV stations broadcast a time signal that VCRs/DVRs use for clock setting. Again, lack of global or even regional coverage. Some TV stations, annoyingly, broadcast the wrong time, too. Cost is probably high, but this idea was rejected before this was investigated. 7. Atomic Reference Still research, but NIST has a small scale atomic reference: http://www.nist.gov/public_affairs/releases/miniclock.htm Unfortunately, not ready for commercial apps, probably will be too expensive, and it uses too much power. The best I could do on power is to power it up periodically and adjust the local crystal to it which integrates long term error. 8. Other? So, did I leave anything out? It seems obvious to me that no amount of effort to make a local crystal stable will meet the requirements. Thus we need to look for external references. The best we can do is WWVB as it is the only thing that can possibly meet our cost objectives. If it works in the continental US, that would be acceptable for now. That leaves me with two basic questions: 1. How well do the WWVB wrist watches work? 2. What merchant silicon exists for receiving WWVB? On the first, the three watches we bought do sync up here (950 miles from WWVB). I wonder how well they work in Maine and Florida (1900 miles from WWVB). On the second, I've only found these leads so far: http://www.mas-oy.com/archive/da9180.pdf http://www.ortodoxism.ro/datasheets/Temic/mXyzuryv.pdf These chips appear to be basic receiver circuits using an external 60KHz crystal as a filter. At 60KHz, I was wondering why there aren't direct digital radios? It would seem like building in the DSP logic would be cheaper/better than the old fashioned methods shown here and could greatly enhance the ability to pick out weak WWVB signals. Has anyone performed such experiments, basically digitize the antenna signal and done DSP on it? Thanks for all who read this far! -- Mike Ciholas (812) 476-2721 x101 CIHOLAS Enterprises (812) 476-2881 fax 255 S. Garvin St, Suite B mikec@ciholas.com Evansville, IN 47713 http://www.ciholas.com

DA

David Andersen

Thu, Aug 18, 2005 6:08 PM

Mike - I've spent a fair amount of time looking in to this as part of
my Internet testbed. At the moment, I have about 25 nodes using
EndRun's CDMA time receivers ($1k-ish each), so I've been very
interested in cheaper solutions, for obvious reasons.

I assume that the devices of which you're speaking are standalone
items? Something like a sensor deployment, possibly networked?
Knowing more about how you actually plan on using these would help a
bit. For example, if they're networked within particular regions,
that gives you an easy way to synch to within milliseconds.

WWVB: Are you going to be deploying inside buildings? Buildings
with electronics and UPSes? If so, be very careful.
Most cheap WWVB watches and clocks don't work very well on the east
coast, from my experience. I poked around at a few in my old lab in
Boston, and they were a no-go.

Your analysis of GPS seems correct. You can probably build a $50 GPS
time receiver to synch to milliseconds, but not much cheaper. On the
other hand, as the 911 location requirements for cell phones expand,
this may change. But not yet.

Atomic reference: I'd say no chance in the next 5+ years.

Local stable crystal: Actually, you could make it more than stable
enough, but it would exceed your power requirements, because you'd
probably fall back to an oven controlled oscillator. There goes your
battery. But why did you try your initial experiments with 32.768Khz
watch crystals? You're much more likely to find a good, solid 10Mhz
reference with an SC cut TCXO. For instance, that maxim IC you
mentioned has +- 2ppm, which is really quite awful by instrumentation
standards. Compare to this one:

http://www.bdelectronic.com/frequency/oscillatorTCXO.html

.3ppm tempco, +- 1ppm/year. They don't show their overall allen
deviation curves, but you get the idea - it'll be within 1ppm by the
end of the year, and since that aging will probably happen over time,
I'd guess it would probably get you something like 10 seconds within
a year. Or something like:

http://www.vectron.com/products/tcxo/tc140.pdf

(... which is probably expensive, but which you can get in 0.2 ppm
accuracy vs. temperature and <2ppm/10 years).

Another option you may have just eliminated on principle: Internet
synchronization. Very easy to keep synched to within a few hundred
ms. (Internet can also be replaced by ACTS telephone, or your
favorite other technology). Very limited in where you can deploy if
you want outdoor deployment.

WWVB and its kin may well be your best bet, if you can hear them
enough places.

-Dave

On Aug 18, 2005, at 1:29 PM, Mike Ciholas wrote:

Hi,

I have a challenging research project to build thousands, perhaps
millions, of devices that maintain mutual synchronization. The
devices need to be low cost (under $20 retail, $8 manufacturing),
small in size (key chain fob), and low power (operate at least 18
months on a battery). Synchronization ideally needs to be within
a second or two over a year but there is some leeway to trade
cost for performance here up to perhaps 10 seconds of variation
per year. Ideally, the device works anywhere in the world but we
may have to limit it to North America.

Crystal Modeling

First idea was to get stable 32.768KHz watch crystals, perform a
factory initial calibration, and use a temperature sensor to
correct for the crystal temp curve. This idea is the cheapest,
simplest, works everywhere, and uses the lowest power.

Initial tolerance on the crystals is +/- 20 ppm (I've not found
better in commodity parts), which equates to +/- 10 minutes a
year, clearly unacceptable. I suspect that if I did an initial
factory calibration and tracked temperature, I might improve this
to +/- 2 ppm much like Maxim did with this part:

http://pdfserv.maxim-ic.com/en/ds/DS32kHz.pdf

But even so, +/- 1 minute per year is not really good enough. I
suspect getting to a few seconds (+/- 0.1ppm) is unrealistic with
any algorithm one can come up with. The base physics is simply
not that predictable.

WWVB Receiver

A second idea is to provide some external reference and the most
logical choice is WWVB as used in several wrist watches. A
little more cost but manageable. We've dissected several wrist
watches and found they use a small ferrite antenna. The
reception performance is spotty, however. I was unable to lock
at work (lots of equipment) but did well at home (electrically
quiet). If we go to the NE tip of Maine, that's twice as far
from WWVB as we are here, so I wonder if the watch will ever pick
up the signal. The saving grace is that the device needs to get
the signal only sporadically, once a week or even once a month
would do it since we can feed that back into correcting the local
crystal.

The negatives are that such a device is limited to the US and
nearby, and it may have poor performance in many locales due to
weak signals, local interference, and the small antenna rod we
are limited to due to size (less than 1 inch). It does cost
more, maybe $1-2 more in production quantity. Right now, this
seems like the best option available to us.

There are similar time broadcasting stations in Europe and China.
We could build a unit that works in those regions, either as
different models, or as a unit with multiple receivers. Still
not global, but perhaps covering 50% of the world's population?

GPS Receiver

A more precise external reference, use a GPS receiver. This gets
us global coverage and is very precise. Uses a lot of power, so
we would only activate it very briefly and not very often (once a
week perhaps) to save battery.

Major issue here is cost. Best I can do for an OEM module is
around $25 in qty which busts the budget severely. It also has
similar problems of being used in a place with no sky visibility.
Size can be a problem in the cheaper modules. Some modules are
quite small:

http://www.u-blox.com/products/lea_la.html

Cute, huh?

GPS Time Receiver

This is fantasy land. I don't need the 100ns time reference, all
I need is something good to one second or so. In this case, it
seems possible to receive only 1 satellite, decode the digital
data, and extract the time. It would be off by the variation in
pseudo range which can't be corrected for. But I don't care
about that level of accuracy.

The question is, if you don't have to track multiple satellites
and don't need to recover the pseudo range accurately, can you
build a wickedly cheaper GPS time receiver? My expectation is no.
You probably can get down to maybe half if you are very diligent,
which still puts me out of the budget plus has a ridiculous high
NRE. Unless this already exists, anyone?

Cellular

We've done extensive work with embedded cell phone modules.
These modules are most often used for wireless remote monitoring
and transport digital data. They do get the time from the cell
system.

Again, cost is a major issue. An OEM cell module runs over $65
in qty so this idea is sunk. It would also suffer from lack of
global and local coverage.

TV Stations

TV stations broadcast a time signal that VCRs/DVRs use for clock
setting.

Again, lack of global or even regional coverage. Some TV
stations, annoyingly, broadcast the wrong time, too. Cost is
probably high, but this idea was rejected before this was
investigated.

Atomic Reference

Still research, but NIST has a small scale atomic reference:

http://www.nist.gov/public_affairs/releases/miniclock.htm

Unfortunately, not ready for commercial apps, probably will be
too expensive, and it uses too much power. The best I could do
on power is to power it up periodically and adjust the local
crystal to it which integrates long term error.

Other?

So, did I leave anything out?

It seems obvious to me that no amount of effort to make a local
crystal stable will meet the requirements. Thus we need to look
for external references. The best we can do is WWVB as it is the
only thing that can possibly meet our cost objectives. If it
works in the continental US, that would be acceptable for now.

That leaves me with two basic questions:

How well do the WWVB wrist watches work?
What merchant silicon exists for receiving WWVB?

On the first, the three watches we bought do sync up here (950
miles from WWVB). I wonder how well they work in Maine and
Florida (1900 miles from WWVB).

On the second, I've only found these leads so far:

http://www.mas-oy.com/archive/da9180.pdf
http://www.ortodoxism.ro/datasheets/Temic/mXyzuryv.pdf

These chips appear to be basic receiver circuits using an
external 60KHz crystal as a filter. At 60KHz, I was wondering
why there aren't direct digital radios? It would seem like
building in the DSP logic would be cheaper/better than the old
fashioned methods shown here and could greatly enhance the
ability to pick out weak WWVB signals. Has anyone performed such
experiments, basically digitize the antenna signal and done DSP
on it?

Thanks for all who read this far!

--
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec@ciholas.com
Evansville, IN 47713 http://www.ciholas.com

time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

Mike - I've spent a fair amount of time looking in to this as part of my Internet testbed. At the moment, I have about 25 nodes using EndRun's CDMA time receivers ($1k-ish each), so I've been very interested in cheaper solutions, for obvious reasons. I assume that the devices of which you're speaking are standalone items? Something like a sensor deployment, possibly networked? Knowing more about how you actually plan on using these would help a bit. For example, if they're networked within particular regions, that gives you an easy way to synch to within milliseconds. WWVB: Are you going to be deploying inside buildings? Buildings with electronics and UPSes? If so, be very careful. Most cheap WWVB watches and clocks don't work very well on the east coast, from my experience. I poked around at a few in my old lab in Boston, and they were a no-go. Your analysis of GPS seems correct. You can probably build a $50 GPS time receiver to synch to milliseconds, but not much cheaper. On the other hand, as the 911 location requirements for cell phones expand, this may change. But not yet. Atomic reference: I'd say no chance in the next 5+ years. Local stable crystal: Actually, you could make it more than stable enough, but it would exceed your power requirements, because you'd probably fall back to an oven controlled oscillator. There goes your battery. But why did you try your initial experiments with 32.768Khz watch crystals? You're much more likely to find a good, solid 10Mhz reference with an SC cut TCXO. For instance, that maxim IC you mentioned has +- 2ppm, which is really quite awful by instrumentation standards. Compare to this one: http://www.bdelectronic.com/frequency/oscillatorTCXO.html .3ppm tempco, +- 1ppm/year. They don't show their overall allen deviation curves, but you get the idea - it'll be within 1ppm by the end of the year, and since that aging will probably happen over time, I'd guess it would probably get you something like 10 seconds within a year. Or something like: http://www.vectron.com/products/tcxo/tc140.pdf (... which is probably expensive, but which you can get in 0.2 ppm accuracy vs. temperature and <2ppm/10 years). Another option you may have just eliminated on principle: Internet synchronization. Very easy to keep synched to within a few hundred ms. (Internet can also be replaced by ACTS telephone, or your favorite other technology). Very limited in where you can deploy if you want outdoor deployment. WWVB and its kin may well be your best bet, _if_ you can hear them enough places. -Dave On Aug 18, 2005, at 1:29 PM, Mike Ciholas wrote: > > Hi, > > I have a challenging research project to build thousands, perhaps > millions, of devices that maintain mutual synchronization. The > devices need to be low cost (under $20 retail, $8 manufacturing), > small in size (key chain fob), and low power (operate at least 18 > months on a battery). Synchronization ideally needs to be within > a second or two over a year but there is some leeway to trade > cost for performance here up to perhaps 10 seconds of variation > per year. Ideally, the device works anywhere in the world but we > may have to limit it to North America. > > 1. Crystal Modeling > > First idea was to get stable 32.768KHz watch crystals, perform a > factory initial calibration, and use a temperature sensor to > correct for the crystal temp curve. This idea is the cheapest, > simplest, works everywhere, and uses the lowest power. > > Initial tolerance on the crystals is +/- 20 ppm (I've not found > better in commodity parts), which equates to +/- 10 minutes a > year, clearly unacceptable. I suspect that if I did an initial > factory calibration and tracked temperature, I might improve this > to +/- 2 ppm much like Maxim did with this part: > > http://pdfserv.maxim-ic.com/en/ds/DS32kHz.pdf > > But even so, +/- 1 minute per year is not really good enough. I > suspect getting to a few seconds (+/- 0.1ppm) is unrealistic with > any algorithm one can come up with. The base physics is simply > not that predictable. > > 2. WWVB Receiver > > A second idea is to provide some external reference and the most > logical choice is WWVB as used in several wrist watches. A > little more cost but manageable. We've dissected several wrist > watches and found they use a small ferrite antenna. The > reception performance is spotty, however. I was unable to lock > at work (lots of equipment) but did well at home (electrically > quiet). If we go to the NE tip of Maine, that's twice as far > from WWVB as we are here, so I wonder if the watch will ever pick > up the signal. The saving grace is that the device needs to get > the signal only sporadically, once a week or even once a month > would do it since we can feed that back into correcting the local > crystal. > > The negatives are that such a device is limited to the US and > nearby, and it may have poor performance in many locales due to > weak signals, local interference, and the small antenna rod we > are limited to due to size (less than 1 inch). It does cost > more, maybe $1-2 more in production quantity. Right now, this > seems like the best option available to us. > > There are similar time broadcasting stations in Europe and China. > We could build a unit that works in those regions, either as > different models, or as a unit with multiple receivers. Still > not global, but perhaps covering 50% of the world's population? > > 3. GPS Receiver > > A more precise external reference, use a GPS receiver. This gets > us global coverage and is very precise. Uses a lot of power, so > we would only activate it very briefly and not very often (once a > week perhaps) to save battery. > > Major issue here is cost. Best I can do for an OEM module is > around $25 in qty which busts the budget severely. It also has > similar problems of being used in a place with no sky visibility. > Size can be a problem in the cheaper modules. Some modules are > quite small: > > http://www.u-blox.com/products/lea_la.html > > Cute, huh? > > 4. GPS Time Receiver > > This is fantasy land. I don't need the 100ns time reference, all > I need is something good to one second or so. In this case, it > seems possible to receive only 1 satellite, decode the digital > data, and extract the time. It would be off by the variation in > pseudo range which can't be corrected for. But I don't care > about that level of accuracy. > > The question is, if you don't have to track multiple satellites > and don't need to recover the pseudo range accurately, can you > build a wickedly cheaper GPS time receiver? My expectation is no. > You probably can get down to maybe half if you are very diligent, > which still puts me out of the budget plus has a ridiculous high > NRE. Unless this already exists, anyone? > > 5. Cellular > > We've done extensive work with embedded cell phone modules. > These modules are most often used for wireless remote monitoring > and transport digital data. They do get the time from the cell > system. > > Again, cost is a major issue. An OEM cell module runs over $65 > in qty so this idea is sunk. It would also suffer from lack of > global and local coverage. > > 6. TV Stations > > TV stations broadcast a time signal that VCRs/DVRs use for clock > setting. > > Again, lack of global or even regional coverage. Some TV > stations, annoyingly, broadcast the wrong time, too. Cost is > probably high, but this idea was rejected before this was > investigated. > > 7. Atomic Reference > > Still research, but NIST has a small scale atomic reference: > > http://www.nist.gov/public_affairs/releases/miniclock.htm > > Unfortunately, not ready for commercial apps, probably will be > too expensive, and it uses too much power. The best I could do > on power is to power it up periodically and adjust the local > crystal to it which integrates long term error. > > 8. Other? > > So, did I leave anything out? > > It seems obvious to me that no amount of effort to make a local > crystal stable will meet the requirements. Thus we need to look > for external references. The best we can do is WWVB as it is the > only thing that can possibly meet our cost objectives. If it > works in the continental US, that would be acceptable for now. > > That leaves me with two basic questions: > > 1. How well do the WWVB wrist watches work? > > 2. What merchant silicon exists for receiving WWVB? > > On the first, the three watches we bought do sync up here (950 > miles from WWVB). I wonder how well they work in Maine and > Florida (1900 miles from WWVB). > > On the second, I've only found these leads so far: > > http://www.mas-oy.com/archive/da9180.pdf > http://www.ortodoxism.ro/datasheets/Temic/mXyzuryv.pdf > > These chips appear to be basic receiver circuits using an > external 60KHz crystal as a filter. At 60KHz, I was wondering > why there aren't direct digital radios? It would seem like > building in the DSP logic would be cheaper/better than the old > fashioned methods shown here and could greatly enhance the > ability to pick out weak WWVB signals. Has anyone performed such > experiments, basically digitize the antenna signal and done DSP > on it? > > Thanks for all who read this far! > > -- > Mike Ciholas (812) 476-2721 x101 > CIHOLAS Enterprises (812) 476-2881 fax > 255 S. Garvin St, Suite B mikec@ciholas.com > Evansville, IN 47713 http://www.ciholas.com > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > >

DF

David Forbes

Thu, Aug 18, 2005 6:18 PM

David Andersen wrote:

Local stable crystal: Actually, you could make it more than stable
enough, but it would exceed your power requirements, because you'd
probably fall back to an oven controlled oscillator. There goes your
battery. But why did you try your initial experiments with 32.768Khz
watch crystals? You're much more likely to find a good, solid 10Mhz
reference with an SC cut TCXO. For instance, that maxim IC you
mentioned has +- 2ppm, which is really quite awful by instrumentation
standards. Compare to this one:

http://www.bdelectronic.com/frequency/oscillatorTCXO.html

.3ppm tempco, +- 1ppm/year. They don't show their overall allen
deviation curves, but you get the idea - it'll be within 1ppm by the
end of the year, and since that aging will probably happen over time,
I'd guess it would probably get you something like 10 seconds within a
year. Or something like:

http://www.vectron.com/products/tcxo/tc140.pdf

(... which is probably expensive, but which you can get in 0.2 ppm
accuracy vs. temperature and <2ppm/10 years).

-Dave

Dave,

The requirement that you seem to have missed is the 18 month battery
lifetime. A 10 MHz oscillator is a couple milliapmeres, so it won't do
the trick. The watch crystal needs only about 10 microamperes to
oscillate.

Mike,

The 32K crystal may be usable, but you'd have to put some effort into
the design to get the temp compensation tuned to the particular
crystal, and you'd have to grade the crystals for tempco in the mfg
stage. That might be doable in quantity, if you come up with the right
sort of computerized test fixture in an oven.

I have built a few nixie tube wristwatches using the cheap 32KHz
crystals, so I have direct experience in this matter. (Has anyone else
on this list built an electronic wristwatch?) Getting the crystal
adjusted to 1ppm is not too hard. You'd have to temperature compensate
it to get to 0.1 ppm, and that would be limited to perhaps 10C-30C
temperature range.

It's a lot easier to compensate the crystal if it's worn on the wrist
rather than sitting in a car, since a person's wrist is essentially an
oven. The real world has ridiculous temperature extremes - don't even
think about stabilizing a crystal used outdoors unless it's thermally
connected to a human.

You should be able to evaluate the feasibility of using a compensated
crystal based on the above.

David Andersen wrote: > > Local stable crystal: Actually, you could make it more than stable > enough, but it would exceed your power requirements, because you'd > probably fall back to an oven controlled oscillator. There goes your > battery. But why did you try your initial experiments with 32.768Khz > watch crystals? You're much more likely to find a good, solid 10Mhz > reference with an SC cut TCXO. For instance, that maxim IC you > mentioned has +- 2ppm, which is really quite awful by instrumentation > standards. Compare to this one: > > http://www.bdelectronic.com/frequency/oscillatorTCXO.html > > .3ppm tempco, +- 1ppm/year. They don't show their overall allen > deviation curves, but you get the idea - it'll be within 1ppm by the > end of the year, and since that aging will probably happen over time, > I'd guess it would probably get you something like 10 seconds within a > year. Or something like: > > http://www.vectron.com/products/tcxo/tc140.pdf > > (... which is probably expensive, but which you can get in 0.2 ppm > accuracy vs. temperature and <2ppm/10 years). > > -Dave Dave, The requirement that you seem to have missed is the 18 month battery lifetime. A 10 MHz oscillator is a couple milliapmeres, so it won't do the trick. The watch crystal needs only about 10 microamperes to oscillate. Mike, The 32K crystal may be usable, but you'd have to put some effort into the design to get the temp compensation tuned to the particular crystal, and you'd have to grade the crystals for tempco in the mfg stage. That might be doable in quantity, if you come up with the right sort of computerized test fixture in an oven. I have built a few nixie tube wristwatches using the cheap 32KHz crystals, so I have direct experience in this matter. (Has anyone else on this list built an electronic wristwatch?) Getting the crystal adjusted to 1ppm is not too hard. You'd have to temperature compensate it to get to 0.1 ppm, and that would be limited to perhaps 10C-30C temperature range. It's a lot easier to compensate the crystal if it's worn on the wrist rather than sitting in a car, since a person's wrist is essentially an oven. The real world has ridiculous temperature extremes - don't even think about stabilizing a crystal used outdoors unless it's thermally connected to a human. You should be able to evaluate the feasibility of using a compensated crystal based on the above.

JD

John Day

Thu, Aug 18, 2005 7:04 PM

A couple of points here.

Yes, a 10MHz oscillator will severely blast the budget - power wise. But
you should remember that 10MHz is also not actually a good frequency for
temperature coefficient of crystals - except for SOME SC cut types.
Generally speaking the zero tempco rollover frequency for many SC's and
most AT's is between 4.5 and 5.5MHz. You can actually get very good
tempco's by running two oscillators either 500kHz or 1MHz apart and mixing
them. Mounted in a thermal mass but without an oven they will oven give
better than TCXO and moderate oven type performance.

Real time temperature compensation is actually quite common. In several
radio applications I have designed systems using two techniques.

1.. Have the manufacturer supply AT crystals with carefully controlled cut
angles - for a given cut angle the tempco will not vary much. Then use a
simple compensation table based on angle.

2.. Learn the crystal characteristics. In a couple of chamber runs measure
the characteristics of each crystal. You probably only need to do 20
temperatures to cover -10 to +60C. Work out the slope and using some nice
interpolation algorithm work out a correction table. Then voltage control
the Xtal osc to keep the frequency on target.

#2 works well enough that a pretty standard crystal will deliver TCXO or
better performance because very few TCXO's use active compensation.

But really, can we fit all of this ins key fob? Nope! I suspect in the best
research tradition the OP needs to look at WHY his application demands
synchronization at this sort of level. In one project where my client
initially asked for timing accuracy of this order, we found that we could
collect data with timestamps and temperatures, and when we looked at the
logger and the data we could measure the offset for a particular logger and
then back correct the timestamps on the data. In that case we used a
correction system like #1 above, but we had something like 150x50x10mm to
work in, not a key fob!

John

At 02:18 PM 8/18/2005, you wrote:

David Andersen wrote:

Local stable crystal: Actually, you could make it more than stable
enough, but it would exceed your power requirements, because you'd
probably fall back to an oven controlled oscillator. There goes your
battery. But why did you try your initial experiments with 32.768Khz
watch crystals? You're much more likely to find a good, solid 10Mhz
reference with an SC cut TCXO. For instance, that maxim IC you
mentioned has +- 2ppm, which is really quite awful by instrumentation
standards. Compare to this one:
http://www.bdelectronic.com/frequency/oscillatorTCXO.html
.3ppm tempco, +- 1ppm/year. They don't show their overall allen
deviation curves, but you get the idea - it'll be within 1ppm by the
end of the year, and since that aging will probably happen over time,
I'd guess it would probably get you something like 10 seconds within a
year. Or something like:
http://www.vectron.com/products/tcxo/tc140.pdf
(... which is probably expensive, but which you can get in 0.2 ppm
accuracy vs. temperature and <2ppm/10 years).
-Dave

Dave,

The requirement that you seem to have missed is the 18 month battery
lifetime. A 10 MHz oscillator is a couple milliapmeres, so it won't do the
trick. The watch crystal needs only about 10 microamperes to oscillate.

Mike,

The 32K crystal may be usable, but you'd have to put some effort into the
design to get the temp compensation tuned to the particular crystal, and
you'd have to grade the crystals for tempco in the mfg stage. That might
be doable in quantity, if you come up with the right sort of computerized
test fixture in an oven.

I have built a few nixie tube wristwatches using the cheap 32KHz crystals,
so I have direct experience in this matter. (Has anyone else on this list
built an electronic wristwatch?) Getting the crystal adjusted to 1ppm is
not too hard. You'd have to temperature compensate it to get to 0.1 ppm,
and that would be limited to perhaps 10C-30C temperature range.

It's a lot easier to compensate the crystal if it's worn on the wrist
rather than sitting in a car, since a person's wrist is essentially an
oven. The real world has ridiculous temperature extremes - don't even
think about stabilizing a crystal used outdoors unless it's thermally
connected to a human.

You should be able to evaluate the feasibility of using a compensated
crystal based on the above.

time-nuts mailing list
time-nuts@febo.com
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

A couple of points here. Yes, a 10MHz oscillator will severely blast the budget - power wise. But you should remember that 10MHz is also not actually a good frequency for temperature coefficient of crystals - except for SOME SC cut types. Generally speaking the zero tempco rollover frequency for many SC's and most AT's is between 4.5 and 5.5MHz. You can actually get very good tempco's by running two oscillators either 500kHz or 1MHz apart and mixing them. Mounted in a thermal mass but without an oven they will oven give better than TCXO and moderate oven type performance. Real time temperature compensation is actually quite common. In several radio applications I have designed systems using two techniques. 1.. Have the manufacturer supply AT crystals with carefully controlled cut angles - for a given cut angle the tempco will not vary much. Then use a simple compensation table based on angle. 2.. Learn the crystal characteristics. In a couple of chamber runs measure the characteristics of each crystal. You probably only need to do 20 temperatures to cover -10 to +60C. Work out the slope and using some nice interpolation algorithm work out a correction table. Then voltage control the Xtal osc to keep the frequency on target. #2 works well enough that a pretty standard crystal will deliver TCXO or better performance because very few TCXO's use active compensation. But really, can we fit all of this ins key fob? Nope! I suspect in the best research tradition the OP needs to look at WHY his application demands synchronization at this sort of level. In one project where my client initially asked for timing accuracy of this order, we found that we could collect data with timestamps and temperatures, and when we looked at the logger and the data we could measure the offset for a particular logger and then back correct the timestamps on the data. In that case we used a correction system like #1 above, but we had something like 150x50x10mm to work in, not a key fob! John At 02:18 PM 8/18/2005, you wrote: >David Andersen wrote: >>Local stable crystal: Actually, you could make it more than stable >>enough, but it would exceed your power requirements, because you'd >>probably fall back to an oven controlled oscillator. There goes your >>battery. But why did you try your initial experiments with 32.768Khz >>watch crystals? You're much more likely to find a good, solid 10Mhz >>reference with an SC cut TCXO. For instance, that maxim IC you >>mentioned has +- 2ppm, which is really quite awful by instrumentation >>standards. Compare to this one: >> http://www.bdelectronic.com/frequency/oscillatorTCXO.html >>.3ppm tempco, +- 1ppm/year. They don't show their overall allen >>deviation curves, but you get the idea - it'll be within 1ppm by the >>end of the year, and since that aging will probably happen over time, >>I'd guess it would probably get you something like 10 seconds within a >>year. Or something like: >> http://www.vectron.com/products/tcxo/tc140.pdf >>(... which is probably expensive, but which you can get in 0.2 ppm >>accuracy vs. temperature and <2ppm/10 years). >> -Dave > >Dave, > >The requirement that you seem to have missed is the 18 month battery >lifetime. A 10 MHz oscillator is a couple milliapmeres, so it won't do the >trick. The watch crystal needs only about 10 microamperes to oscillate. > >Mike, > >The 32K crystal may be usable, but you'd have to put some effort into the >design to get the temp compensation tuned to the particular crystal, and >you'd have to grade the crystals for tempco in the mfg stage. That might >be doable in quantity, if you come up with the right sort of computerized >test fixture in an oven. > >I have built a few nixie tube wristwatches using the cheap 32KHz crystals, >so I have direct experience in this matter. (Has anyone else on this list >built an electronic wristwatch?) Getting the crystal adjusted to 1ppm is >not too hard. You'd have to temperature compensate it to get to 0.1 ppm, >and that would be limited to perhaps 10C-30C temperature range. > >It's a lot easier to compensate the crystal if it's worn on the wrist >rather than sitting in a car, since a person's wrist is essentially an >oven. The real world has ridiculous temperature extremes - don't even >think about stabilizing a crystal used outdoors unless it's thermally >connected to a human. > >You should be able to evaluate the feasibility of using a compensated >crystal based on the above. > > > > >_______________________________________________ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >

MC

Mike Ciholas

Thu, Aug 18, 2005 7:34 PM

On Thu, 18 Aug 2005, David Andersen wrote:

I assume that the devices of which you're speaking are
standalone items? Something like a sensor deployment, possibly
networked? Knowing more about how you actually plan on using
these would help a bit.

Yes, it would. The devices are part of a social experiment and
are meant to be carried by humans. The devices, at preprogrammed
times, signal the human with an audio signal. We want all the
devices to create this signal "simultaneously", which means
something on the order of a few seconds variation among all the
units anywhere in the world. There is no network, no display, no
user interface, zip. A key fob would be ideal and low cost is
critical.

Most cheap WWVB watches and clocks don't work very well on the
east coast, from my experience. I poked around at a few in my
old lab in Boston, and they were a no-go.

Yes, this is my worry. Switching power supplies in the
vicinity seem to be the determining factor in how well these
systems work.

Local stable crystal: Actually, you could make it more than
stable enough, but it would exceed your power requirements,
because you'd probably fall back to an oven controlled
oscillator. There goes your battery.

It is important to note that we don't need an accurate time
reference, we need s stable one. That means we're happy to
correct, in software, a crystal that is running 10ppm slow if
it holds that error throughout it's life. We can build
adjustments into the software, so no need to pull, tweak, or PLL
to some arbitrary frequency.

But why did you try your initial experiments with 32.768Khz
watch crystals?

Power. A CR1620 battery is 75mAH and a CR2032 is 225 mAH. For
18 months, this is 6uA or 17uA average current draw. The only
thing that can meet that is a low KHz crystal.

You're much more likely to find a good, solid 10Mhz reference
with an SC cut TCXO. For instance, that maxim IC you mentioned
has +- 2ppm, which is really quite awful by instrumentation
standards. Compare to this one:

http://www.bdelectronic.com/frequency/oscillatorTCXO.html

.3ppm tempco, +- 1ppm/year.

One idea is to keep the TCXO off most of the time, turn it on and
let it stabilize, then compare it to the low power crystal. Make
an adjustment based on that. But, I suspect the constant on/off
will affect the TCXO poorly, and you miss most of the low power
crystal variations (temperature, shock, etc). So this doesn't
seem possible. I'd also guess these TCXOs are >$10 in qty.

Lastly, +/- 1ppm is +/- 30 seconds per year which is still not
really good enough.

Another option you may have just eliminated on principle:
Internet synchronization.

Technically, this works really well. The concept we had for this
was a USB key fob. Plug it in to an Internet connected computer
and capture the time. The device could also have a super cap on
board for power, charge it from the USB slot, and you don't need
a battery. I was thinking this would be an ideal concept with all
sorts of flexibility. The sync operation would not need to be
done too often, perhaps once every 90 days, so limited access is
okay as long as it occurs often enough.

However, there are issues. Namely that the USB device would need
software on the computer to make the connection and that the
experiment wants to be as accessible to people as possible, even
those which have no access to a computer.

A possible compromise was suggested: build a GPS time module with
USB ports on it that serves the role of the time provider. This
would replace or augment the PC. This complicated the experiment
as well but provided a way for unconnected or distant people to
sync up. It would only work well if people using the device are
"grouped" which may not be compatible with the experiment's
goals.

WWVB and its kin may well be your best bet, if you can hear
them enough places.

Yes, so far that is the leading candidate. Maybe if we all
chipped in a $0.01 each ($3M total), they can get the power up to
1MW? That would probably get world wide coverage (and cook
nearby hot dogs... :-).

--
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec@ciholas.com
Evansville, IN 47713 http://www.ciholas.com

On Thu, 18 Aug 2005, David Andersen wrote: > I assume that the devices of which you're speaking are > standalone items? Something like a sensor deployment, possibly > networked? Knowing more about how you actually plan on using > these would help a bit. Yes, it would. The devices are part of a social experiment and are meant to be carried by humans. The devices, at preprogrammed times, signal the human with an audio signal. We want all the devices to create this signal "simultaneously", which means something on the order of a few seconds variation among all the units anywhere in the world. There is no network, no display, no user interface, zip. A key fob would be ideal and low cost is critical. > Most cheap WWVB watches and clocks don't work very well on the > east coast, from my experience. I poked around at a few in my > old lab in Boston, and they were a no-go. Yes, this is my worry. Switching power supplies in the vicinity seem to be the determining factor in how well these systems work. > Local stable crystal: Actually, you could make it more than > stable enough, but it would exceed your power requirements, > because you'd probably fall back to an oven controlled > oscillator. There goes your battery. It is important to note that we don't need an *accurate* time reference, we need s *stable* one. That means we're happy to correct, in software, a crystal that is running 10ppm slow *if* it holds that error throughout it's life. We can build adjustments into the software, so no need to pull, tweak, or PLL to some arbitrary frequency. > But why did you try your initial experiments with 32.768Khz > watch crystals? Power. A CR1620 battery is 75mAH and a CR2032 is 225 mAH. For 18 months, this is 6uA or 17uA average current draw. The only thing that can meet that is a low KHz crystal. > You're much more likely to find a good, solid 10Mhz reference > with an SC cut TCXO. For instance, that maxim IC you mentioned > has +- 2ppm, which is really quite awful by instrumentation > standards. Compare to this one: > > http://www.bdelectronic.com/frequency/oscillatorTCXO.html > > .3ppm tempco, +- 1ppm/year. One idea is to keep the TCXO off most of the time, turn it on and let it stabilize, then compare it to the low power crystal. Make an adjustment based on that. But, I suspect the constant on/off will affect the TCXO poorly, and you miss most of the low power crystal variations (temperature, shock, etc). So this doesn't seem possible. I'd also guess these TCXOs are >$10 in qty. Lastly, +/- 1ppm is +/- 30 seconds per year which is still not really good enough. > Another option you may have just eliminated on principle: > Internet synchronization. Technically, this works really well. The concept we had for this was a USB key fob. Plug it in to an Internet connected computer and capture the time. The device could also have a super cap on board for power, charge it from the USB slot, and you don't need a battery. I was thinking this would be an ideal concept with all sorts of flexibility. The sync operation would not need to be done too often, perhaps once every 90 days, so limited access is okay as long as it occurs often enough. However, there are issues. Namely that the USB device would need software on the computer to make the connection and that the experiment wants to be as accessible to people as possible, even those which have no access to a computer. A possible compromise was suggested: build a GPS time module with USB ports on it that serves the role of the time provider. This would replace or augment the PC. This complicated the experiment as well but provided a way for unconnected or distant people to sync up. It would only work well if people using the device are "grouped" which may not be compatible with the experiment's goals. > WWVB and its kin may well be your best bet, _if_ you can hear > them enough places. Yes, so far that is the leading candidate. Maybe if we all chipped in a $0.01 each ($3M total), they can get the power up to 1MW? That would probably get world wide coverage (and cook nearby hot dogs... :-). -- Mike Ciholas (812) 476-2721 x101 CIHOLAS Enterprises (812) 476-2881 fax 255 S. Garvin St, Suite B mikec@ciholas.com Evansville, IN 47713 http://www.ciholas.com

JM

John Miles

Thu, Aug 18, 2005 7:40 PM

Are the subjects all within major metro areas? I wonder if the TV stations
are sending usable time codes in their blanking intervals these days.

-- john, KE5FX

Are the subjects all within major metro areas? I wonder if the TV stations are sending usable time codes in their blanking intervals these days. -- john, KE5FX

MC

Mike Ciholas

Thu, Aug 18, 2005 7:45 PM

On Thu, 18 Aug 2005, David Forbes wrote:

The 32K crystal may be usable, but you'd have to put some
effort into the design to get the temp compensation tuned to
the particular crystal, and you'd have to grade the crystals
for tempco in the mfg stage. That might be doable in quantity,
if you come up with the right sort of computerized test fixture
in an oven.

All of the temp compensation can be in software and "after the
fact". I don't need the crystal "tweaked", I just need to know
what numerical corrections I need to apply to the counter. Thus
it becomes zero electronics (besides some temp sensor) and only
software.

I have built a few nixie tube wristwatches using the cheap
32KHz crystals, so I have direct experience in this matter.
(Has anyone else on this list built an electronic wristwatch?)
Getting the crystal adjusted to 1ppm is not too hard. You'd
have to temperature compensate it to get to 0.1 ppm, and that
would be limited to perhaps 10C-30C temperature range.

You give me more hope than I had previously. I understand how to
capture the initial tolerance (operate the device at the factory
and record the variation in internal memory). I know how to
correct the temperature curve (the parabolic deviation away from
25C). But I am left with aging as a concern. Most of these
crystals claim aging to be +/- 3ppm the first year. That's +/-
1.5 minutes for a year which is unacceptable. Maybe crystals
from one batch all age the same, maybe they age based on
shock/vibration (which will vary from unit to unit). I just
don't know.

Here is an example datasheet:

http://www.ecsxtal.com/pdf/ecs-3x8.pdf

It's a lot easier to compensate the crystal if it's worn on the
wrist rather than sitting in a car, since a person's wrist is
essentially an oven. The real world has ridiculous temperature
extremes - don't even think about stabilizing a crystal used
outdoors unless it's thermally connected to a human.

Just like someone who leaves their watch in the car on a sunny
day, I can't be sure the device will be in a temperature stable
environment. The best I can do is try to model the temperature
effect and correct for it. Hence the temp curve in the
datasheet. Tracking it to within +/- 0.1ppm seems tough.

You should be able to evaluate the feasibility of using a
compensated crystal based on the above.

Yes, I think it is going to be possible to achieve +/- 1.0ppm. I
think +/- 0.5ppm can be done with some hard work. I don't think
+/- 0.1ppm (+/- 3 seconds/year) is realistic.

--
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec@ciholas.com
Evansville, IN 47713 http://www.ciholas.com

On Thu, 18 Aug 2005, David Forbes wrote: > The 32K crystal may be usable, but you'd have to put some > effort into the design to get the temp compensation tuned to > the particular crystal, and you'd have to grade the crystals > for tempco in the mfg stage. That might be doable in quantity, > if you come up with the right sort of computerized test fixture > in an oven. All of the temp compensation can be in software and "after the fact". I don't need the crystal "tweaked", I just need to know what numerical corrections I need to apply to the counter. Thus it becomes zero electronics (besides some temp sensor) and only software. > I have built a few nixie tube wristwatches using the cheap > 32KHz crystals, so I have direct experience in this matter. > (Has anyone else on this list built an electronic wristwatch?) > Getting the crystal adjusted to 1ppm is not too hard. You'd > have to temperature compensate it to get to 0.1 ppm, and that > would be limited to perhaps 10C-30C temperature range. You give me more hope than I had previously. I understand how to capture the initial tolerance (operate the device at the factory and record the variation in internal memory). I know how to correct the temperature curve (the parabolic deviation away from 25C). But I am left with aging as a concern. Most of these crystals claim aging to be +/- 3ppm the first year. That's +/- 1.5 minutes for a year which is unacceptable. Maybe crystals from one batch all age the same, maybe they age based on shock/vibration (which will vary from unit to unit). I just don't know. Here is an example datasheet: http://www.ecsxtal.com/pdf/ecs-3x8.pdf > It's a lot easier to compensate the crystal if it's worn on the > wrist rather than sitting in a car, since a person's wrist is > essentially an oven. The real world has ridiculous temperature > extremes - don't even think about stabilizing a crystal used > outdoors unless it's thermally connected to a human. Just like someone who leaves their watch in the car on a sunny day, I can't be sure the device will be in a temperature stable environment. The best I can do is try to model the temperature effect and correct for it. Hence the temp curve in the datasheet. Tracking it to within +/- 0.1ppm seems tough. > You should be able to evaluate the feasibility of using a > compensated crystal based on the above. Yes, I think it is going to be possible to achieve +/- 1.0ppm. I think +/- 0.5ppm can be done with some hard work. I don't think +/- 0.1ppm (+/- 3 seconds/year) is realistic. -- Mike Ciholas (812) 476-2721 x101 CIHOLAS Enterprises (812) 476-2881 fax 255 S. Garvin St, Suite B mikec@ciholas.com Evansville, IN 47713 http://www.ciholas.com

CH

Chuck Harris

Thu, Aug 18, 2005 7:57 PM

Mike Ciholas wrote:

Hi,

I have a challenging research project to build thousands, perhaps
millions, of devices that maintain mutual synchronization. The
devices need to be low cost (under $20 retail, $8 manufacturing),
small in size (key chain fob), and low power (operate at least 18
months on a battery). Synchronization ideally needs to be within
a second or two over a year but there is some leeway to trade
cost for performance here up to perhaps 10 seconds of variation
per year. Ideally, the device works anywhere in the world but we
may have to limit it to North America.

Crystal Modeling

Phase lock the crystal to the 50/60 Hz powerline? The signal seems
to be ubiquitous.

-Chuck

Mike Ciholas wrote: > Hi, > > I have a challenging research project to build thousands, perhaps > millions, of devices that maintain mutual synchronization. The > devices need to be low cost (under $20 retail, $8 manufacturing), > small in size (key chain fob), and low power (operate at least 18 > months on a battery). Synchronization ideally needs to be within > a second or two over a year but there is some leeway to trade > cost for performance here up to perhaps 10 seconds of variation > per year. Ideally, the device works anywhere in the world but we > may have to limit it to North America. > > 1. Crystal Modeling Phase lock the crystal to the 50/60 Hz powerline? The signal seems to be ubiquitous. -Chuck

MC

Mike Ciholas

Thu, Aug 18, 2005 8:13 PM

On Thu, 18 Aug 2005, John Day wrote:

You can actually get very good tempco's by running two
oscillators either 500kHz or 1MHz apart and mixing them.
Mounted in a thermal mass but without an oven they will oven
give better than TCXO and moderate oven type performance.

Curious idea. I guess the assumption is that the two crystals
experience the same environment and that has the same effect on
them.

I wonder if you did the same thing with two 32.768KHz crystals?
You would learn their initial calibration at the factory, then
track the time so that the difference is maintained. I'd have to
sit down and do the math to see if this makes sense as this would
be only a few ppm typically. The assumption here is that the two
crystals, being of the same type in the same environment, would
age and change at the same rate. An experiment can be performed
to check this.

Quick example: One crystal is 1ppm faster than the other. Then
the two counters will "slip" 1 count every 30.5 seconds. You
count the "slips" to record time, the slips per unit time being
measured at the factory. My intuition says there is something
mathematically wrong here, the slip rate probably won't be stable
to <1ppm.

I guess this would generalize to more than 2 crystals. We could
put a dozen in there and average the results to reduce the error.
Seems like it would reach diminishing returns fairly quickly,
though.

Random question: Any good books on the finer details of crystals?
I'm looking for something that goes into all the gory details of
how they vary, age, drift, etc.

Real time temperature compensation is actually quite common. In
several radio applications I have designed systems using two
techniques.

1.. Have the manufacturer supply AT crystals with carefully
controlled cut angles - for a given cut angle the tempco will
not vary much. Then use a simple compensation table based on
angle.

2.. Learn the crystal characteristics. In a couple of chamber
runs measure the characteristics of each crystal. You probably
only need to do 20 temperatures to cover -10 to +60C. Work out
the slope and using some nice interpolation algorithm work out
a correction table. Then voltage control the Xtal osc to keep
the frequency on target.

I can get a start from the chart in the datasheet on the crystal:

http://www.ecsxtal.com/pdf/ecs-3x8.pdf

I don't actually need to adjust the crystal itself, all I need to
do is accumulate the error mathematically and "fix" the answer
past the counter output. So no need for additional circuits.

#2 works well enough that a pretty standard crystal will
deliver TCXO or better performance because very few TCXO's use
active compensation.

Are we well under +/- 1ppm? If we can get down to +/- 0.1 or 0.2
ppm, I think that would do it, but 1ppm isn't enough.

But really, can we fit all of this ins key fob? Nope!

I was thinking a tiny microcontroller, a small watch crystal, and
a thermistor would easily fit into a key fob. The question is
whether the embedded algorithm in the device can be good enough
to keep it on time.

Still, the WWVB approach seems like the best bet so far but the
crystal only approach would be so nice.

--
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec@ciholas.com
Evansville, IN 47713 http://www.ciholas.com

On Thu, 18 Aug 2005, John Day wrote: > You can actually get very good tempco's by running two > oscillators either 500kHz or 1MHz apart and mixing them. > Mounted in a thermal mass but without an oven they will oven > give better than TCXO and moderate oven type performance. Curious idea. I guess the assumption is that the two crystals experience the same environment and that has the same effect on them. I wonder if you did the same thing with two 32.768KHz crystals? You would learn their initial calibration at the factory, then track the time so that the difference is maintained. I'd have to sit down and do the math to see if this makes sense as this would be only a few ppm typically. The assumption here is that the two crystals, being of the same type in the same environment, would age and change at the same rate. An experiment can be performed to check this. Quick example: One crystal is 1ppm faster than the other. Then the two counters will "slip" 1 count every 30.5 seconds. You count the "slips" to record time, the slips per unit time being measured at the factory. My intuition says there is something mathematically wrong here, the slip rate probably won't be stable to <1ppm. I guess this would generalize to more than 2 crystals. We could put a dozen in there and average the results to reduce the error. Seems like it would reach diminishing returns fairly quickly, though. Random question: Any good books on the finer details of crystals? I'm looking for something that goes into all the gory details of how they vary, age, drift, etc. > Real time temperature compensation is actually quite common. In > several radio applications I have designed systems using two > techniques. > > 1.. Have the manufacturer supply AT crystals with carefully > controlled cut angles - for a given cut angle the tempco will > not vary much. Then use a simple compensation table based on > angle. > > 2.. Learn the crystal characteristics. In a couple of chamber > runs measure the characteristics of each crystal. You probably > only need to do 20 temperatures to cover -10 to +60C. Work out > the slope and using some nice interpolation algorithm work out > a correction table. Then voltage control the Xtal osc to keep > the frequency on target. I can get a start from the chart in the datasheet on the crystal: http://www.ecsxtal.com/pdf/ecs-3x8.pdf I don't actually need to adjust the crystal itself, all I need to do is accumulate the error mathematically and "fix" the answer past the counter output. So no need for additional circuits. > #2 works well enough that a pretty standard crystal will > deliver TCXO or better performance because very few TCXO's use > active compensation. Are we well under +/- 1ppm? If we can get down to +/- 0.1 or 0.2 ppm, I think that would do it, but 1ppm isn't enough. > But really, can we fit all of this ins key fob? Nope! I was thinking a tiny microcontroller, a small watch crystal, and a thermistor would easily fit into a key fob. The question is whether the embedded algorithm in the device can be good enough to keep it on time. Still, the WWVB approach seems like the best bet so far but the crystal only approach would be so nice. -- Mike Ciholas (812) 476-2721 x101 CIHOLAS Enterprises (812) 476-2881 fax 255 S. Garvin St, Suite B mikec@ciholas.com Evansville, IN 47713 http://www.ciholas.com

MC

Mike Ciholas

Thu, Aug 18, 2005 8:20 PM

On Thu, 18 Aug 2005, John Miles wrote:

Are the subjects all within major metro areas? I wonder if the
TV stations are sending usable time codes in their blanking
intervals these days.

No, not limited to metro areas. TV receiver probably just as
costly and power hungry as GPS.

--
Mike Ciholas (812) 476-2721 x101
CIHOLAS Enterprises (812) 476-2881 fax
255 S. Garvin St, Suite B mikec@ciholas.com
Evansville, IN 47713 http://www.ciholas.com

On Thu, 18 Aug 2005, John Miles wrote: > Are the subjects all within major metro areas? I wonder if the > TV stations are sending usable time codes in their blanking > intervals these days. No, not limited to metro areas. TV receiver probably just as costly and power hungry as GPS. -- Mike Ciholas (812) 476-2721 x101 CIHOLAS Enterprises (812) 476-2881 fax 255 S. Garvin St, Suite B mikec@ciholas.com Evansville, IN 47713 http://www.ciholas.com

B

buehl

Thu, Aug 18, 2005 8:27 PM

Mike:

The 1 Hz difference leads to the problems of 'is it +1 or -1; Or at zero,
there is no output to count.

With the 1 MHz referrec to in John's email, you can run a reliable counter.

Does anyone in the group have expertise in "forced aging" of crystals. In
the 'good old days' before precision trimmed parts, we commonly 'aged'
parts with temperature cycles and voltage cycles to get past all the first
year variations. This gave long term stability as well as sorting out all
the early life failures of parts. Would take care of the 'first year
drift' variable.

Tom Buehl

At 04:13 PM 8/18/2005, you wrote: