[ntp:questions] Re: WWVB 60kHz Receiver

David L. Mills mills at udel.edu
Sun Apr 16 03:31:00 UTC 2006


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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