142 lines
7.5 KiB
HTML
142 lines
7.5 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>The Clock Mini-HOWTO: Radio Clocks</TITLE>
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<LINK HREF="Clock-3.html" REL=previous>
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<LINK HREF="Clock.html#toc4" REL=contents>
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</HEAD>
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<BODY>
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Next
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<A HREF="Clock-3.html">Previous</A>
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<A HREF="Clock.html#toc4">Contents</A>
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<HR>
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<H2><A NAME="s4">4. Radio Clocks</A></H2>
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<H2><A NAME="ss4.1">4.1 CHU and the "Gadget Box"</A>
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</H2>
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<P>CHU, the Canadian shortwave time station near Ottawa, is similar
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to WWV in the US but with one important difference: in addition
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to announcing the time in both French and English, it also
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broadcasts the current time once per minute using the old "Bell
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103" (300 baud) modem tones. These tones are very easy to
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decode, and Bill Rossi realised that you don't even need a
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modem-- just a shortwave radio and a sound card. If you're able
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to receive the signal from CHU, this may be the cheapest radio
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clock available. Shortwave reception varies throughout the day,
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but Bill claims that by changing frequencies twice a day (morning
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and evening) he gets almost 24-hour coverage. CHU broadcasts on
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3.33, 7.335, and 14.670 MHz.
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<P>For more information see Bill Rossi's website at
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<A HREF="http://www.rossi.com/chu/">http://www.rossi.com/chu/</A>. The source file is also
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available at the usual Linux archive sites. For information on
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CHU's time services see
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<A HREF="http://www.nrc.ca/inms/time/ctse.html">http://www.nrc.ca/inms/time/ctse.html</A>.
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<P>The NTP website has plans for a "gadget box" that
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decodes the CHU time broadcast using an inexpensive 300 baud
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modem chip and any shortwave radio, at
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<A HREF="http://www.eecis.udel.edu/~ntp/ntp_spool/html/gadget.htm">http://www.eecis.udel.edu/~ntp/ntp_spool/html/gadget.htm</A>.
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The plans include a Postscript image of a
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2-sided custom printed circuit board, but you have to make the
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board yourself (or find someone who can make it for you).
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<P><CODE>Ntpd</CODE> includes a driver (type 7) for CHU receivers, which
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works either with modems like the "<CODE>gadget box</CODE>",
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or by feeding the audio directly into the mic input of a Sun SPARCstation
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(or any other machine with "compatible audio drivers").
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<H2><A NAME="ss4.2">4.2 WWV and the "Most Accurate Clock"</A>
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</H2>
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<P>You may have heard about Heathkit's "Most Accurate Clock",
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which received and decoded the time signal from WWV and had an
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optional serial port for connecting to a computer. Heathkit
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stopped selling kits a long time ago, but they continued to sell
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the factory-built version of the clock until 1995, when it was
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also discontinued. For Heathkit nostalgia (not including the
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clock) see
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<A HREF="http://www.heathkit-museum.com">http://www.heathkit-museum.com</A>. The
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Heathkit company still exists, selling educational materials. See
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<A HREF="http://www.heathkit.com">http://www.heathkit.com</A>.
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<P>According to Dave Mills, Heathkit's patent on the "Most Accurate
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Clock" is due to expire soon, so maybe someone out there would
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like to clone it as a single-chip IC.
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<P>The NTP website has a DSP program (and a PDF file describing it) at
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<A HREF="http://www.eecis.udel.edu/~mills/resource.htm">http://www.eecis.udel.edu/~mills/resource.htm</A>
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that decodes the WWV time signal using a shortwave radio and the
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TAPR/AMSAT DSP-93, a DSP kit which is no longer available. It was
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based on the Texas Instruments TMS320C25 DSP chip. The TAPR
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website at
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<A HREF="http://www.tapr.org">http://www.tapr.org</A> includes a lot of
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information on homebrew DSP programming.
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<P><CODE>Ntpd</CODE> includes a driver (type 6) for the IRIG-B and
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IRIG-E time codes, using <CODE>/dev/audio</CODE> on a
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Sun SPARCstation, with a note that it is "likely portable
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to other systems". WWV uses the IRIG-H time code.
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<P>WWV is run by NIST, which has a website at
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<A HREF="http://www.boulder.nist.gov/timefreq/index.html">http://www.boulder.nist.gov/timefreq/index.html</A>.
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This site includes the text of "Special Publication 432",
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which describes their time and frequency services, at
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<A HREF="http://www.boulder.nist.gov/timefreq/pubs/sp432/sp432.htm">http://www.boulder.nist.gov/timefreq/pubs/sp432/sp432.htm</A>.
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WWV broadcasts on 2.5, 5, 10, 15, and 20 Mhz.
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<H2><A NAME="ss4.3">4.3 GPS and the "Totally Accurate Clock"</A>
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</H2>
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<P>GPS signals include the correct time, and some GPS receivers have
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serial ports. <CODE>Ntpd</CODE> includes drivers for several GPS
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receivers. The 1PPS feature ("One Pulse Per Second",
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required for high accuracy) usually requires a separate interface
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to connect it to the computer.
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<P>TAPR (Tuscon Amateur Packet Radio) makes a kit for an interface
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called "TAC-2" (for "Totally Accurate
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Clock") that plugs into a serial port and works with any
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GPS receiver that can provide a 1PPS output-- including some
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"bare board" models that can be mounted directly to
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the circuit board. For more information see their website at
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<A HREF="http://www.tapr.org">http://www.tapr.org</A>. The price (as of June 1999) is around
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$140, not including the GPS receiver. The kit does not
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include any enclosure or mounting hardware.
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<P>The CHU "gadget box" (described in another section)
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can also be used as an interface for the 1PPS signal. The NTP website
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has a discussion of this at
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<A HREF="http://www.eecis.udel.edu/~ntp/ntp_spool/html/pps.htm">http://www.eecis.udel.edu/~ntp/ntp_spool/html/pps.htm</A>.
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<H2><A NAME="ss4.4">4.4 Low-frequency Time Signals: DCF77, MSF(Rugby), WWVB</A>
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</H2>
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<P>These low-frequency stations broadcast a time code by simply
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switching the carrier on and off. Each station uses its own
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coding scheme, and summaries are available on the NTP website at
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<A HREF="http://www.eecis.udel.edu/~mills/ntp/index.htm">http://www.eecis.udel.edu/~mills/ntp/index.htm</A>
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(near the bottom of the page). DCF77 in Germany broadcasts on
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77.5kHz. MSF in England (also called "Rugby", which
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apparently refers to its location) and WWVB in Colorado both
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broadcast on 60 kHz.
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<P>Reception of WWVB varies, but there are plans to increase its
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broadcast power, in several stages. You can follow its progress
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on NIST's website at
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<A HREF="http://www.boulder.nist.gov/timefreq/wwvstatus.html">http://www.boulder.nist.gov/timefreq/wwvstatus.html</A>.
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<P>Inexpensive receivers that can plug into a serial port are
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reported to be available in Europe. <CODE>Ntpd</CODE> includes
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drivers for a couple of MSF receivers.
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<P>A number of companies in the US sell relatively inexpensive
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clocks that have built-in WWVB receivers (including several
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analog wall clocks), but I'm only aware of two that can be
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connected to a computer:
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<P>The Ultralink Model 320 sells for about $120 (as of June
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1999) and has a serial interface and a straightforward ASCII
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command set, so it shouldn't be too hard to program. It draws 1mA
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from the serial port for power. The antenna can be up to 100 feet
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away from the computer, and the unit contains its own clock to
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maintain the time if it loses the signal. They also sell a
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"bare board" version for about $80 that is
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designed to work with the "BASIC Stamp" series of
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microcontrollers. See
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<A HREF="http://www.ulio.com/timepr.html">http://www.ulio.com/timepr.html</A>.
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<P>Arcron Technology sells a desk clock with an optional serial
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port for about $130, including software for Windows. See
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<A HREF="http://www.arctime.com">http://www.arctime.com</A><HR>
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Next
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<A HREF="Clock-3.html">Previous</A>
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<A HREF="Clock.html#toc4">Contents</A>
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</BODY>
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</HTML>
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