Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!swrinde!ucsd!rutgers!mcnc!rti!dg-rtp!bigben!beeblebrox!philip
From: philip@beeblebrox.dle.dg.com (Philip Gladstone)
Newsgroups: comp.protocols.time.ntp
Subject: Cheap Radio Clocks (was Re: CTS-10 PC clock board)
Message-ID: <1990Oct5.155541.6488@dle.dg.com>
Date: 5 Oct 90 15:55:41 GMT
References: <2994@idunno.Princeton.EDU> <59758@bbn.BBN.COM>
Sender: usenet@dle.dg.com (Net News)
Organization: Data General, Development Lab Europe
Lines: 47

In article <59758@bbn.BBN.COM> clements@bbn.com (Bob Clements) writes:
>It can hear the WWV signal for at least two hours out of 24 (in
>Massachusettts).  It listens ONLY to the 100 Hz subcarrier, not
>the 1000/1200 Hz tones.  It is certainly not in the "Stratum 1"
>category.  It is somewhat worse than the Heathkit clock, since it
>listens only to one frequency (10 MHz) and one audio tone (100
>Hz). 

I would be really suprised if a board placed anywhere near a PC
worked even two hours out of 24. I have built a couple of 60 kHz clocks
(synced to MSF Rugby) and they are both stunningly sensitive to VDUs and
monitors!

However, once positioned reasonably, they give very good results. I seem
to get about 700us variance on the 1 second pulses. I think that with a
bit more fiddling around with the RF stage and the filters, I could
probably get it down to somewhat under 500us. Bear in mind that this is 
only 30 cycles of the carrier, and you must agree that it is not bad!

My stratum 1 clocks just consist of a radio demodulator that gives me
a 1 pps pulse train, and then I decode that in software -- taking an
interrupt off the leading edge (which corresponds to the second marker). 
The leading edge is timed using the computer clock (it is the variance
in this number discussed above). The pulse data can easily be sampled
from the computer and then decoded to give the actual time. 

I actually use the parallel printer port on my machine and take the
data stream into the /ACK and ONLINE pins. My hardware takes an interrupt
on rising edge of /ACK, and then I can sample the ONLINE pin! I would
have thought that the same approach could be taken on a PC -- the
transition of /ACK certainly can cause an interrupt. [I seem to recall
that which edge the PC triggers off depends on which adapter card you
use!]. 

The clock board that I used cost around #20 (pounds UK) which is under
$40 (US). I would have thought that you could do a nice stratum 1 server
with enough effort and a spare PC! Some considerable care might be 
needed to get the precision up to a decent level -- especially if it is
a slow PC.

Anyway, enough ramblings.


Philip Gladstone                        philip@dle.dg.com
Development Lab Europe                  C=gb/AD=gold 400/PR=dgc/O=dle
Data General, Cambridge                   /SN=gladstone/GN=philip
England.  +44 223-67600