John, et al,
You are not going to like this.
I;ve been running several WWVB clocks since 1981 using Spectracom 8170
and Netclock/2 receivers with the NTP WWVB driver. NIST Time and
Frequency Services, NIST Special Publication 462 (Revised 1990), claims
nominal timecode accuracy of 100 microseconds, and that's what I got in
the beginning.
The WWVB signal has become degraded in recent years and now all WWVB
receivers here have been retired. Here in Delaware we are on the 100
microvolt per meter contour (before the WWVB transmitter upgrade) and
normally this would be sufficient for good accuracy. However, at least
here in Newark, there is another strong signal on 60 kHz that interferes
with WWVB. I chased this down to power-line conducted EMI and found the
interferiing signal was well above the noise several miles from campus.
It's not clear where it originates, but the prime suspect is a power
inverter for an arc welder at the Chrysler plant in town.
This was bad enough, but then we started installing banks of UPS units
in the machine room and they scream like banshees on 60 kHz, even with
the antenna on the roof of an outbuilding. The result was total failure
of all our WWVB radios, at least on campus.
Until recently I was getting good results at home, but now I see the
signal has failed there as well, probably due to increased crud at 60
kHz due some incidental radiator. I conclude WWVB is no longer useful
here and I suspect in any machine room, unless the antenna is far away
from the UPSes and with suitable ferrite decoupling.
A solution that would probably work for business is to locate the
antenna and radio in a doghouse on the roof and run serial cable to the
machine room. Lemme tell you how hard that is with a plastic sheet roof
and a landlord that believes rooftop space rental for tenant machinery
is a significant revenue stream.
Dave
John Ackermann N8UR wrote:
Ry said the following on 04/15/2006 02:14 AM:
I'm 1462.5 km (4.8 ms) from the WWVB transmitters, which is well inside
the coverage area maps on the NIST site. I figure WWVB is my best
(only?) option for a refclock at this location. Is sub-milisecond
accuracy possible using a WWVB refclock? The NIST site says 0.1 to 15
ms phase error, which is two orders of magnitude. I see ~5 ms average
offsets when I sync with nearby stratum-1 servers via NTP. Would WWVB
perform any better?
I can't answer your question about current availability, but I can show
you some WWVB results. I have an old Spectracom 8170 and the
accompanying large ferrite antenna is mounted on my roof. I'm about
1770km from the station.
At http://www.febo.com/time-freq/ntp/stats/index.html I have a set of
plots derived from the peerstats file of one of my NTP servers,
databox.febo.com. It shows the offset of my other internal NTP boxes,
as well as three external stratum 1s, versus the system time (ie, the
offset field from peerstats). databox itself is a stratum 1 machine,
using a shared memory PPS driver to sync to an HP Z3801A GPS disciplined
oscillator. So the offset of the other servers is being measured
against a fairly stable baseline, and if you're sneaky you can back out
any wander in databox's time by looking at common-mode wiggles in the
plots.
(Unfortunately, at the moment I'm doing some reconfiguration and two of
the servers that should be stratum 1 are actually running at stratum 2,
and one of them has a very strange wander pattern over 5ms or so that
makes the rest of the plot harder to read than it should be; all the
other machines are typically grouped well within 1ms.)
In general, toe.febo.com, which is hooked to the Spectracom 8170, holds
within several hundred microseconds of the other clocks. However, every
now and then, perhaps once every couple of weeks, there is a downward
excursion of perhaps 20ms and a recovery that takes a couple of days to
fully complete. I suspect this occurs when the 8170 loses lock and its
10MHz oscillator goes into free-run mode for a little while. It may be
that a receiver alignment would reduce the magnitude of this effect, but
my longer term plans are to use an external reference oscillator.
So, other than that occasional anomaly, the WWVB receiver provides very
good time, usually well within 1ms.
By the way -- I also have some plots of WWVB received signal strength at
http://www.febo.com/time-freq/wwvb/sig-strength/index.html, comparing
the performance of a couple of antennas, and showing the strength change
from day to night.
Having said all that, I think you may have problems getting decent
reception if your WWVB antenna is in a machine room near the bottom of
an urban canyon, but it's worth a shot. Another thing you might
consider is one of the CDMA refclocks that get their time from the
cellular network. It won't be as traceable as using an NIST or USNO
source, but you shouldn't have trouble getting a signal.
John
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