CHU, the Canadian shortwave time station near Ottawa, is similar to WWV in the US but with one important difference: in addition to announcing the time in both French and English, it also broadcasts the current time once per minute using the old "Bell 103" (300 baud) modem tones. These tones are very easy to decode, and Bill Rossi realised that you don't even need a modem-- just a shortwave radio and a sound card. If you're able to receive the signal from CHU, this may be the cheapest radio clock available. Shortwave reception varies throughout the day, but Bill claims that by changing frequencies twice a day (morning and evening) he gets almost 24-hour coverage. CHU broadcasts on 3.33, 7.335, and 14.670 MHz.
For more information see Bill Rossi's website at http://www.rossi.com/chu/. The source file is also available at the usual Linux archive sites. For information on CHU's time services see http://www.nrc.ca/inms/time/ctse.html.
The NTP website has plans for a "gadget box" that decodes the CHU time broadcast using an inexpensive 300 baud modem chip and any shortwave radio, at http://www.eecis.udel.edu/~ntp/ntp_spool/html/gadget.htm. The plans include a Postscript image of a 2-sided custom printed circuit board, but you have to make the board yourself (or find someone who can make it for you).
Ntpd includes a driver (type 7) for CHU receivers, which
works either with modems like the "gadget box",
or by feeding the audio directly into the mic input of a Sun SPARCstation
(or any other machine with "compatible audio drivers").
You may have heard about Heathkit's "Most Accurate Clock", which received and decoded the time signal from WWV and had an optional serial port for connecting to a computer. Heathkit stopped selling kits a long time ago, but they continued to sell the factory-built version of the clock until 1995, when it was also discontinued. For Heathkit nostalgia (not including the clock) see http://www.heathkit-museum.com. The Heathkit company still exists, selling educational materials. See http://www.heathkit.com.
According to Dave Mills, Heathkit's patent on the "Most Accurate Clock" is due to expire soon, so maybe someone out there would like to clone it as a single-chip IC.
The NTP website has a DSP program (and a PDF file describing it) at http://www.eecis.udel.edu/~mills/resource.htm that decodes the WWV time signal using a shortwave radio and the TAPR/AMSAT DSP-93, a DSP kit which is no longer available. It was based on the Texas Instruments TMS320C25 DSP chip. The TAPR website at http://www.tapr.org includes a lot of information on homebrew DSP programming.
Ntpd includes a driver (type 6) for the IRIG-B and
IRIG-E time codes, using /dev/audio on a
Sun SPARCstation, with a note that it is "likely portable
to other systems". WWV uses the IRIG-H time code.
WWV is run by NIST, which has a website at http://www.boulder.nist.gov/timefreq/index.html. This site includes the text of "Special Publication 432", which describes their time and frequency services, at http://www.boulder.nist.gov/timefreq/pubs/sp432/sp432.htm. WWV broadcasts on 2.5, 5, 10, 15, and 20 Mhz.
GPS signals include the correct time, and some GPS receivers have
serial ports. Ntpd includes drivers for several GPS
receivers. The 1PPS feature ("One Pulse Per Second",
required for high accuracy) usually requires a separate interface
to connect it to the computer.
TAPR (Tuscon Amateur Packet Radio) makes a kit for an interface called "TAC-2" (for "Totally Accurate Clock") that plugs into a serial port and works with any GPS receiver that can provide a 1PPS output-- including some "bare board" models that can be mounted directly to the circuit board. For more information see their website at http://www.tapr.org. The price (as of June 1999) is around $140, not including the GPS receiver. The kit does not include any enclosure or mounting hardware.
The CHU "gadget box" (described in another section) can also be used as an interface for the 1PPS signal. The NTP website has a discussion of this at http://www.eecis.udel.edu/~ntp/ntp_spool/html/pps.htm.
These low-frequency stations broadcast a time code by simply switching the carrier on and off. Each station uses its own coding scheme, and summaries are available on the NTP website at http://www.eecis.udel.edu/~mills/ntp/index.htm (near the bottom of the page). DCF77 in Germany broadcasts on 77.5kHz. MSF in England (also called "Rugby", which apparently refers to its location) and WWVB in Colorado both broadcast on 60 kHz.
Reception of WWVB varies, but there are plans to increase its broadcast power, in several stages. You can follow its progress on NIST's website at http://www.boulder.nist.gov/timefreq/wwvstatus.html.
Inexpensive receivers that can plug into a serial port are
reported to be available in Europe. Ntpd includes
drivers for a couple of MSF receivers.
A number of companies in the US sell relatively inexpensive clocks that have built-in WWVB receivers (including several analog wall clocks), but I'm only aware of two that can be connected to a computer:
The Ultralink Model 320 sells for about $120 (as of June 1999) and has a serial interface and a straightforward ASCII command set, so it shouldn't be too hard to program. It draws 1mA from the serial port for power. The antenna can be up to 100 feet away from the computer, and the unit contains its own clock to maintain the time if it loses the signal. They also sell a "bare board" version for about $80 that is designed to work with the "BASIC Stamp" series of microcontrollers. See http://www.ulio.com/timepr.html.
Arcron Technology sells a desk clock with an optional serial port for about $130, including software for Windows. See http://www.arctime.com