[time-nuts] Low cost synchronization

[Mike Ciholas]

On Sat, 20 Aug 2005, Tom Van Baak wrote:

> > 1. 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.

> > 2. 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?

> > 3. GPS Receiver
> > 4. 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. :-)

> > 5. 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.

> > 8. Other?
> 
> 1) 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.

> 2) Or piggy-back on the existing paging networks.

Too spotty, too much testing.  Pagers are a dying service as cell 
phones take their business.

> 3) 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 at ciholas.com
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