[LinuxPPS] DCF77-PPS

[Hal V. Engel]

On Tuesday 11 November 2008 14:46:19 Remco den Besten wrote:
> On my blog (remco.org) I published the idea to use DCF77 pulses as PPS
> source. I did some experiments,
> found out that PHK already figured this out, and was a bit disappointed by
> the performance then.
>
> However, a few months ago I gave a friend of mine a DCF77-receiver with the
> instructions 'plug it in' , direct it 'so and so', and I remotely installed
> and configured his machine to add him to the ntp.pool. Although the machine
> performed well,
> I was intrigued with the posting(s) of Hal concerning the timer options. I
> found out that the machine of my friend has a 'pit' timer, and I remotely
> compiled and upgraded a kernel (2.6.8 to 2.6.27.5 <- ouch! ;-) with
> LinuxPPS and compiled
> ntpd with STA_NANO. Although DCF77 misses the 59th second, I think the
> whole contrapsion behaves relatively
> well. The offset and jitter remain within the 1 ms range. Results can be
> seen at remco.org/ntp.

I think that +-1ms is about as much as you can expect from this type of setup.  
I think that there is some variation in the propagation delay because of 
variability in the propagation path.  I would think that since DCF77 and WWVB 
propagate via ground waves that this should give better results than using HF 
time broadcasts like the 10MHz, 15Mhz or 20Mhz WWVA or WWVH signals since the 
propagation path will be more consistent.  I read some where that using WWVA 
or WWVH about the best you could do was around +-20ms. 

>
> Helium = LinuxPPS (kernel 2.6.27.4) with Oncore UT+ and acpi_pm timer (very
> temp dependent!)
> Lithium = normal kernel with DCF77 via radioclkd2
> FreeBSD = eh.. FreeBSD 7.0 (and sorry... behaves better here than LinuxPPS
> + nano)
> ntp2.remco.org = DCF77-LinuxPPS (kernel 2.6.27.5) + STA_NANO
>
> Be quick, this weekend I have a party at this home, and I will connect an
> Oncore VP GPS receiver ;-)
>
> What is also very visible, is that helium and freebsd are located in the
> same room (20 cm seperated ;-) but react totally different on the (same)
> temperature variations. The outcome (i.e. offset and jitter) of PLL
> (in)stabilities of LinuxPPS vs
> FreeBSD is striking.

In fact the frequency curves are basically inverted which is something I would 
not expect.   The other thing that is inverted is the direction of the 
frequency correction. 

The oscillator on ntp2.remco.org appears to be the most stable of the lot.  Is 
this because the room temperature is more stable where it is located or is 
this because this machine happens to have an oscillator that is not as 
temperature sensitive?  In any case I suspect that this machine will give very 
good results with the OnCore GPS.

The lithium frequency curve is very similar to freebsd.   Are these located in 
the same room?

Hal

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