[time-nuts] Low cost synchronization

Tom Van Baak tvb at leapsecond.com
Sat Aug 20 14:59:45 EDT 2005

Hi Mike,

Sorry for the late reply. You raise an interesting
question and here are some thoughts.

> 1. Crystal Modeling

Standard 32 kHz crystals won't work. TCXO aren't
good enough either. OCXO are too power hungry.

A couple of quartz wrist watches are good to 5 or
10 seconds per year. This may be close enough
for your needs. The Pulsar PRS10 is one example.
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.

> 2. WWVB Receiver

These are exceedingly cheap now and should fit
all your requirements. Contact Rod Mack who has
probably done more WWVB R&D than anyone on
the list (he did the Ultralink receivers using Temic
chips). Email me offline for his contact info.

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

Note also that many WWVB chipsets are now
"global", meaning they will also receive signals
from LF time services in Europe and Japan

> 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).

> 5. Cellular

What percent of your thousands to millions of
users world-wide already have a cell phone? To
me this is the obvious solution. I would guess
all cell phones know the time to a millisecond
internally and this means a billion people on the
planet are already carrying just what you need.
Battery life is not a problem because all users
already know how to recharge theirs.

Now if each brand of cell phone would just have
a standardized 1PPS output connector you'd be
all set.

> 6. TV Stations

Two methods come to mind. The XDS timecode
(used by PBS stations) is good in principle but
perhaps not in practice. The other approach is
to discipline a 32 kHz XO against the 3.58 MHz
colorburst frequency. This seems dated, though.

> 7. Atomic Reference

In 10 years maybe.

> 8. Other?

1) Look into an interface with Sirius/XM satellite radio.

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

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.

3b) For extra credit use DSP. Since AM/FM radio
and TV frequencies have assigned slots world-wide
you can simultaneously receive many local stations
and combine their frequency stabilities to a common
mean time/frequency. This would make a wonderful
project for someone; commercial or university.

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.

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?

At least with your requirements, you don't have
to worry about leap seconds!


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