Radio Clocks

[Markus Kuhn]

"Richard L. Shockney" wrote on 1998-10-11 15:38 UTC:
> BTW, there are a few of these on the (U.S.) market that use radio
> frequencies to automatically adjust to the U.S. Atomic Clock. As I recall,
> the wrist watch was about $900, the analog wall clock about $200 and the
> digital clock/radio about $100 (all U.S. dollars).

These clocks are probably based on WWV, which is a short wave
transmitter, for which receivers are a bit more expensive than for
long-wave transmission. The German DCF77 is a very powerful (50 kW)
long-wave transmitter. Long-wave radiation has the advantage that it
penetrates buildings quite well, so you do not need external antennas
and your clocks work in every room. I used to have a DCF77 wrist watch
for around $50, and I have connected to the serial port of my Linux box
a DCF77 receiver for around $20. Since suitable long-wave receivers can
be implemented in a very simple chip design with practically the only
external component being the receiver coil/capacitor combination, you
find DCF77 more and more often in lowest cost products (< $50) in
Germany.

NIST also operates a WWVB 60 kHz long-wave service in the US, but I it
has a much weaker signal than DCF77 (50 kW). WWVB used to have 10 kW for
a long time and was only very recently upgraded to 23 kW, with plans for
further upgrades to 35-40 kW. The British Telecom transmitter MSF in
Rugby has 27 kW at 60 kHz.

http://www.boulder.nist.gov/timefreq/
http://www.ptb.de/english/org/4/43/433/disse.htm
http://www.npl.co.uk/npl/ctm/msf.html

Once these WWVB upgrades have been completed, I would expect that you
might also get in the US mass market radio clocks that are as cheap as
those in Central Europe.

Markus

-- 
Markus G. Kuhn, Security Group, Computer Lab, Cambridge University, UK
email: mkuhn at acm.org,  home page: <http://www.cl.cam.ac.uk/~mgk25/>