Low cost synchronization
Now that cell phones are raised, it is possible to use a
two step process to synchronize a world-wide "Ommm" or to
focus hatred on a particular person. (That last is from an
old SF story about criminals being executed in an arena by
public hating. In practice, it doesn't work any better than
any other form of mental power except hypnotism.) You see,
speculation can run wild when there is no data. Perhaps
this is a means to coordinate an attack.
Assuming that everyone involved has access to a clock and a
telephone, it is possible to call a number minutes before an
event. The synchronization is from a tone on the telephone.
Telcos have numbers that many people can call to handle local
events. Negotiations could be made with telcos to set up the
synchronization number. The cost would be spread over many
individuals. Perhaps network broadcasts could be persuaded
to broadcast a beep (or pips) at the right time.
OTOH, if the problem is to establish a world-wide heart beat
at a certain frequency, then point synchronization is not
the answer.
Regards,
Bill Hawkins
Hoping this scheme is mostly harmless, as the Guide puts it.
On Sat, 20 Aug 2005, Tom Van Baak wrote:
- Crystal Modeling
Standard 32 kHz crystals won't work. TCXO aren't good enough
either. OCXO are too power hungry.
Yup.
A couple of quartz wrist watches are good to 5 or 10 seconds
per year. This may be close enough for your needs.
Yes, that would be very tempting, especially the 5 seconds
number. If we could do that, we'd probably go for it and live
with the error.
The Pulsar PRS10 is one example.
I found web reports of them being off 15-20 seconds per year, so
the claims might not be valid for this watch. I fear the same
issue with us, great research effort to develop a stable timing
reference in the lab, but it fails to deliver in the field.
There are just so many variables.
I think they use dual mode crystals to achieve their
exceptional accuracy and relative temperature insensitivity.
With the quantities you are talking about a dual mode crystal
may fit the requirement.
Dual-mode crystals are a niche market, however, so making
arrangements with a manufacturer will not be simple.
Time to expose my ignorance, what is a dual mode crystal? Can
you give me pointers to the manufacturers? If they would work,
we can invest the time to make the arrangements.
- WWVB Receiver
WWVB reception quality is not an issue since it's only used to
intermittently re-synchronize the internal XO. One decent
reception every couple of days or even weeks will take care of
your requirements.
Even once a quarter would be good enough in most cases. Once you
learn how your local XO is doing, you can apply that in the
future.
Note also that many WWVB chipsets are now "global", meaning
they will also receive signals from LF time services in Europe
and Japan
Ideally, the device would work anywhere, but WWVB (US 60 KHz),
JJY (Japan 40 and 60 KHz), DCF (Germany 77.5 KHz), MSF (UK 60
KHz) only cover so much of the world. We're still missing sub
equatorial Africa, western Asia, South America, Australia,
Hawaii, and Alaska. Still, we can probably cover 80% of the
world's population with what we can get. There are a few
"global" VLF time receiver chips, notably those from C-max and
MAS. They get three frequencies, usually chosen to be 40, 60,
and 77.5 KHz.
Due to the key fob size, we can't have a very large ferrite rod
antenna so our sensitivity will be poor. It's not even clear we
can get eastern US reliably.
One wonders if you can build some sort of long term reception
processing that would pick out the signal from the noise. Since
you know what you should be getting, you can overlay multiple
minutes of reception to cancel out the noise. I wonder how much
processing that will take. Would it be possible to recover
enough signal fro the noise to make VLF receivable worldwide?
- GPS Receiver
- GPS Time Receiver
As many cell phones now include GPS receivers sizes and prices
are dropping. But I'm guessing you are not going to meet your
fob-size nor power specs with GPS (or other satellite nav
systems).
Actually, size and power are not the limiting factor. Consider
this module:
http://www.u-blox.com/products/lea_la.html
Size wise, this will fit (we have one for another project, pretty
awesome, about a postage stamp in lateral footprint). Power
wise, using it once a week for one minute would use 43 mAH, or
about 20% of a CR2032 coin cell (our preferred power source).
We could get by with a solid time hit every two months. I don't
care about the 1 pps output, just the NMEA date message.
The real killer is cost. This module is probably $30 in qty ($70
in qty 1). Maybe, if we are very lucky, we could source
something similar for $15 in very high volume. The WWVB style
receiver is probably under $2. The concept is fairly price
sensitive so we have to be under $5 total manufacturing cost.
Now if only I could "duty cycle" the cost, then the GPS would
cost me less than $0.01. :-)
- Cellular
Now if each brand of cell phone would just have a standardized
1PPS output connector you'd be all set.
We've done a lot of work on embedded cellular. It's a mess
building something that works everywhere or with everything. Do
cell phones even know UTC or do they just know "local" time? It
is important that a device in central time go off at the same UTC
time as one in eastern time, not at the same numeric local time.
- Other?
- Look into an interface with Sirius/XM satellite radio.
Hmmm. I'll research that. One imagines the chipset cost is on
par with GPS, however, and may not have had the commodity
development attached to it yet.
- Or piggy-back on the existing paging networks.
Too spotty, too much testing. Pagers are a dying service as cell
phones take their business.
- Lock onto the carrier of a high-power local AM or FM
station. If these stations use Rb or GPSDO referenced
carriers you'll get a long-term stable frequency for free.
Only if I track it all the time. And I bet not all have stable
carriers. This is the cost of the WWVB receiver at best (and
probably worse since it has to tune instead of being fixed) and
the transmission produces no numeric encoding. I'd have to run
it much longer and more often than WWVB.
For any solutions that give you stable frequency only (XO, RF
carriers, 60 Hz) you will need a way to set the initial time
and to reset the time when the batteries fail.
Initial time can be set at factory. Batteries are never customer
removed so there should never be a need to reset it. When
battery is dead, device is dead.
For any solutions that give you time only you will presumably
need to convert from UTC to local time. Also, are you concerned
with DST?
No, all timing we do will be UTC and local time is irrelevant.
At least with your requirements, you don't have to worry about
leap seconds!
Yes. All that matters is that the devices go off at the same
time (+/- a few seconds). If they all are a minute late relative
to the desired time, that's okay. But I don't think that helps
much.
--
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 Sat, 20 Aug 2005, Tom Van Baak wrote:
This sounds odd to me given that cell phones I've seen can
display the date & time and they appear to be accurate to a
second.
In my experience, cell phones are accurate to within a second as
compared to the WWVB sources. That is more than we need.
By the way, Mike, have you considered if your battery-operated,
fob-sized, world-wide, low-cost, synchronization device will be
allowed through US airports?
What sort of problems would you expect? It should be no
different than a WWVB wrist watch since that basically the same
internals without the display. It will look like a harmless
inert key fob, much like RFID key fobs used for keyless entry.
--
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
At 4:07 PM -0500 8/20/05, Mike Ciholas wrote:
One wonders if you can build some sort of long term reception
processing that would pick out the signal from the noise. Since
you know what you should be getting, you can overlay multiple
minutes of reception to cancel out the noise. I wonder how much
processing that will take. Would it be possible to recover
enough signal fro the noise to make VLF receivable worldwide?
Mike,
This is what I do for a living - integrating radio signals over long
time periods to get a signal from what looks like pure noise.
The basic formula is that the S/N increases by the square root of the
integration time, as long as the sample phase remains correct. This
is the limiting factor - the PPM of the fob's timebase determines
this and will be another watch crystal which is good to maybe 1PPM.
There are two ways to proceed: One is to try to actually learn the
time of day form WWVB, and the other is to just keep your local clock
synchronized to the one second pulses from WWVB.
The standard time signals such as WWVB use a one-bit-per-second
coding scheme where the signal is amplitude modulated, so you'd need
to measure the phase of this modulation to perhaps 5% of a cycle, or
0.05 seconds. Your 1PPM oscillator will be off by .05 seconds in
50,000 seconds or 14 hours. So you could integrate data for 14 hours.
That should give you sqrt(50,000) improved S/N, or 200 times better
S/N ratio (23dB).
If you just want to maintain your oscillator's 1PPS phase against
WWVB, then you just need to do a pulsar synchronization technique of
splitting a second into perhaps 20 bins and accumulating the signal
strength during each bin. You are looking for the rising edge of the
carrier which indicates the top of the second. That is, a bin with
very little signal followed by one with lots of signal is when the
pulse starts.
If you want to learn the time, then the next problem is that you're
integrating data which is changing, so you have to deal with that by
some fancy integration tricks such as using separate bins for 1 and 0
level bits.
I hope this makes some sense. If not, then read up on pulsar
detection and timing techniques.
--David Forbes, Tucson, AZ
http://www.cathodecorner.com/
Mike Ciholas mikec@ciholas.com writes:
On Sat, 20 Aug 2005, Tom Van Baak wrote:
This sounds odd to me given that cell phones I've seen can
display the date & time and they appear to be accurate to a
second.
In my experience, cell phones are accurate to within a second as
compared to the WWVB sources. That is more than we need.
CDMA phones (mostly US-ONLY) seems to have good timing
information. The FDMA system GSM (rest of the world except Japan) IIRC
does not use absolute time in the base station and has no time service
to the terminals (phones).
--
Björn
On Sat, 20 Aug 2005, Bill Hawkins wrote:
Perhaps this is a means to coordinate an attack.
Ah, the age of paranoia. Rest assured, the experiment's purpose
is as far from that as possible.
If you look at my requirements (accurate to a few seconds, lasts
more than a year, low cost, built in thousands), I don't think
that is consistent with enabling any evil purposes that you could
not have done more easily some other way that already exists.
--
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
From: "Tom Van Baak" tvb@leapsecond.com
Subject: Re: [time-nuts] Low cost synchronization
Date: Sat, 20 Aug 2005 13:35:05 -0700
Message-ID: 001701c5a5c6$a68ea100$8633f304@computer
Tom,
Just as with WWVB receivers, he does not have to have the GPS powered up
very long for and
then only once a week or so to keep the oscillator tuned up. Once a GPS
solution has been
found, the local time and the GPS solution time give a time-difference and
by remembering
the GPS solution time from the last time you have the /|t you need to
calculate the
frequency error. So, a GPS solution could be possible.
I'm curious what the power requirements are.
My Casio WWVB wrist watch works on one
battery for two years while my Casio GPS
wristwatch is lucky to run for more than a
two days, even when in intermittent mode.
It all depends on what you aim for. If you really wish to, you can run an
ordinary 32k oscillator, have a GPS receiver turned on for say 15 min every
week or so. With the GPS constellation in memory and the local time to within
a few seconds it should be possible to keep the hot-time of the GPS receiver
down to a minimum. Doing linear or exponential backoff on failure to grab
signal should make the failure handling less power-insensitive.
Depending on which standard you have, the phones only may have a sense of
"real" time.
In GSM for instance, the phones traces network time only in a relative
aspect, but there
is no real way to get an accurate UTC. The phones is being synchronised to
the base
This sounds odd to me given that cell phones
I've seen can display the date & time and they
appear to be accurate to a second.
As I said, this all depends on the particular technology (there is a so many
that I don't recall them all) and in some cases how the operator have chosen to
operate their network. All I am saying is that it may not be as simple and
straightforward as one first may think. When time synchronisation is there,
the relative effects are more important than absolute time. This is true for
CDMA too, infact it is imperative or else the network fails miserably as I have
understood it. In GSM it fails more gracefully, so much infact that
synchronising base stations is not always done.
All we need are some counter-examples. Does
anyone on this list have a cell phone that displays
the time of day with an error greater than a few
seconds? (if yours has a HH:MM-only display
compare the instant when MM changes). If so,
then Mike can scratch cell phones from his list
of accurate time sources.
I have. My GSM phone is not updated over the GSM network, since my particular
operator have chosen not to transmitt the necessary signals. The reason I know
all this is since I wondered why I could not have the service, and I was
enlightend on this particular issue. Remember, if you are a time-nut, you do
expect things to be synchronised just to learn that people doesn't care as much
as you do. On the other hand, I can correct time of my GSM phone when I feel it
is out of time. I usually only care about hours and rought minutes.
So, I think this option should be taken out of the list since it may not be
universally applicable.
By the way, Mike, have you considered if your
battery-operated, fob-sized, world-wide, low-cost,
synchronization device will be allowed through US
airports?
What the hitch?
Cheers,
Magnus
Hi Mike:
It may not be possible to get what you are asking for as a stand alone
time keeper but I think could be done by periodically resetting.
Using a radio signal for resetting has problems with the coverage area
and/or power consumption.
Maybe the resetting signal could be an audio time code optimized for
this application. The user could call an 800 phone number and a
microphone in the device would hear the code and reset. Or at a public
meeting the audio code could be put on the P.A. system so all present
would be synchronized.
If the microphone can hear the beeper of another unit there might be a
way to use a button on a unit to cause it to send it's audio time sync
signal so other nearby units would sync to it.
The device could learn it's aging rate at each reset and so change the
divisor number to match the current aging rate.
After sync the device might emit beeps or Morse code telling the user
how far off it was allowing the user to gauge how often they need to resync.
A TCXO should work for this.
Have Fun,
Brooke Clarke, N6GCE
w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com
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!
On Sat, 20 Aug 2005, David Forbes wrote:
At 4:07 PM -0500 8/20/05, Mike Ciholas wrote:
One wonders if you can build some sort of long term reception
processing that would pick out the signal from the noise.
There are two ways to proceed: One is to try to actually learn
the time of day form WWVB, and the other is to just keep your
local clock synchronized to the one second pulses from WWVB.
I can't stay sync'ed to WWVB. I don't have the power to run the
receiver all the time, and you loose the signal during the day in
many places. So we're into trying to get the signal during some
period of time.
The standard time signals such as WWVB use a one-bit-per-second
coding scheme where the signal is amplitude modulated, so you'd
need to measure the phase of this modulation to perhaps 5% of a
cycle, or 0.05 seconds. Your 1PPM oscillator will be off by .05
seconds in 50,000 seconds or 14 hours. So you could integrate
data for 14 hours. That should give you sqrt(50,000) improved
S/N, or 200 times better S/N ratio (23dB).
A >20dB improvement is huge! You might almost be able to get
some station anywhere on the globe. Of course, we might start to
get mutual interference between WWVB, MSF, and JJY when we are
between those two stations. Maybe a single receiver on DCF 77.5
KHz is better?
If you want to learn the time, then the next problem is that
you're integrating data which is changing, so you have to deal
with that by some fancy integration tricks such as using
separate bins for 1 and 0 level bits.
Yes, this will be necessary. Not really that bad if you expect
to be within, say, 4 minutes. You don't need to bin all the
minute bits and then you can apply a moving window on the binary
pattern to find the best "match".
I hope this makes some sense. If not, then read up on pulsar
detection and timing techniques. --
This was helpful. Thanks.
--
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 Sat, 20 Aug 2005, Bill Hawkins wrote:
Perhaps this is a means to coordinate an attack.
And Mike Ciholas replied,
"Ah, the age of paranoia. Rest assured, the experiment's purpose
is as far from that as possible."
Well, no, not so much the age of paranoia as the age of extreme
self-interest and no empathy for others. This leads to too many
liars with public exposure, followed by erosion of trust. When a
salesman says "Trust me ..." these days, it is best to walk away.
Trust is the basis for civilization. It is sad to see it replaced
by paranoid behavior as people learn not to trust other people.
Mike, I've no reason to believe that you are lying, but you have
withheld information. OTOH, if you did tell us the reason for
these ticking key fobs, how would that differ from an email from
someone who's money is trapped in Africa unless I help out? I mean,
besides the fact that you aren't asking for money, although you
could profit. Then too, your client could have misrepresented
their purpose.
I apologize if my creative mind has caused you any distress. It
has caused me some distress as I try to work out what the fobs
are for. Given the size of your market and the high probability
of some religious or New Age significance, I'll never know until
I see them for sale, if they go public.
Regards,
Bill Hawkins