[ntp:questions] Re: Comparing offset of multiple refclock/servers

David L. Mills mills at udel.edu
Mon Aug 15 18:26:06 UTC 2005


I've done a lot of experiments like yours. First, decide whether the 
goal is to compare the clocks against each other (open loop) or the to 
assess the performance of the synchronized system (closed loop). If open 
loop, say "disable ntp" on all three systems so they run open loop. Then 
collect the peerstats on all the machines for each other and the clocks 
for a few days and write a matlab program to plot the differences. The 
program will assume the local system clock of each machine is simply a 
flywheel to compare the clock of one machine against the clock of 
another. You can of course connect all three clocks to one machine and 
watch them babble.

For closed loop performance, consider one of the GPS clocks a reference 
and calibrate the others from it, keeping track of the little wiggles 
that do occur between the individual referenc clocks and system clocks. 
We usually keep our public servers within 100 microseconds closed-loop.

 From my experience the GPS receivers are much better than WWVB 
receivers, certainly in my case and possibly in yours due to cochannel 
RFI. Both of my 8170 receivers were modified at the factory many years 
ago to conform with the behavior of the Netclock/1 which I also have 
online. So far as cochannel RFI performance, the original and modified 
receivers are about the same.

Twenty five years ago when the FCC was doing its job the WWVB 
performance was reliably within the millisecond. As time went on and the 
FCC did not enforce the limits specified in Part 15 of their rules, 
things have deteriorated. Once upon a time the power of an unintentional 
radiator was limited to 15 microvolts per meter at a distance lambda / 2 
PI, where lambda is the wavelength in meters. I know that very well, as 
I was called as consultant in a case where a campus carrier-current 
radio station was observed much above that limit and I had to measure 
the field strength and correct the problem. The Ann Arbor 560-kHz signal 
had leaked on the high voltage lines and was heard 59 in Ypsilanti, 
several miles away.

I just broke out a recent version of Part 15 and am astonished by what 
the special interests have done to make holes for wifi, vehicle 
location, RF tags and whatnot. The limits for all special interest 
devices have increased easily tenfold over the years, with current FCC 
proposals on broadband over power line (BPL) an absolute disaster for 
emergency, broadcast and amateur operations.

Consider unintentional radiators, say due to conducted RFI leaking into 
the power grid and the wires acting as antenna. Current Part 15.209 says 
300 microvolts per meter at a distance 2400 / frequency (khz) in meters. 
That's 300 microvolts per meter at 40 meters. Current WWVB signals here 
are in the order of one microvolt per meter at the receiver. Do the 
sums. A radiator like that would clobber WWVB if less than 4.6 miles 
from the receiver. The original limit was 15 microvolts per meter at 796 
meters which is about one microvolt per meter at 3.8 miles. This is huge.

Consider the case of conducted RFI, which  was originally targeted at CB 
receiver local oscillators leaking into the power line. Consider also 
the case where some high power welding machine at 60 kHz backs up into 
the power grid. FCC 15.207 says not more than a millivolt can leak into 
the power line. You're telling me a 500 kW welding machine leaks less 
than a millivolt at 560-V three-phase AC source? What say the Chrysler 
management if I show up at their front door with a field strength meter 
pegged at 60 kHz? Heck, the FCC consultants told them BPL wouldn't work, 
but nobody was listening.

The bottom line is that WWVB should be considered an endangered species 
and probably a hazard for DCF77 and HBG in Europe as well. I don't know 
why the FCC didn't make an exception at 60 kHz. They did make exception 
for 5 kHz either side of the WWV carrier frequencies, but then allowed 
50-kW redneck broadcasters in Arkansas with carrier frequencies right at 
the stopband edges. Sideband splatter kills WWV 5 MHz every night. Gotta 
forgive the FCC, since the engineers are long gone and replaced by 
E-street lawyers.


John Ackermann N8UR wrote:

> As part of my timekeeping fixation, I just put three FreeBSD-based NTP
> servers online on my home network, each driven by its own refclock (two
> HP Z3801A GPSDOs, and one Spectracom 8170 WWVB clock).
> In addition to plotting the internal offset and jistter performance
> (which I have on-line at http://www.febo.com/time-freq/ntp/stats), I'd
> like to compare and plot the relative offset of the three systems
> against one another.  It seems like that should be easy, but I'm having
> some trouble figuring out the best way to go about it, since even the
> PPS sources may show a slight offset in the ntpq -p results, and of
> course if one of the refclocks has an offset (e.g., right now my WWVB
> clock has about 3ms of systemic offset), the server that relies on it
> won't directly show that -- it won't know that the time it's syncing to
> is a little bit off..
> One option might be to go to a fourth machine that is looking at these
> three, and use it as a sort of transfer standard -- looking at the
> relative offset of other machines, and then subtracting them against
> each other to obtain the differences.  But that seems sort of klunky.
> Am I missing something obvious?  Any suggestions would be appreciated.
> Thanks,
> John
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