Hi
I have 2 ntp servers running Linux with LinuxPPS patch. Number one
uses a Meinberg PCI511 v1.00 as time source, number two uses a
Meinberg GPS170PCI v1.10.
On both machines ntp is configured to use the Meinberg refclock
(127.127.8.0) and furthermore the Meinberg PPS outputs are connected
to the serial interfaces and use the atom driver (127.127.22.0).
The strange thing is, that there is always a delay of 1-4 ms between
the refclock and the pps signal. In my opinion they should arrive at
the exact same time.

Here is an example graph of the dcf card:
http://img-up.net/img/dcf-delayY6wXrh.png

Here is an example graph of the gps card:
http://img-up.net/img/gps-delayHbRmW6U.png

And here the ntp conf entries:
DCF:
# meinberg dcf
server 127.127.8.0 prefer mode 2
fudge 127.127.8.0 time1 -0.003
fudge 127.127.8.0 refid DCFi
# meinberg dcf-pps
server 127.127.22.0

GPS:
# meinberg gps
server 127.127.8.0 prefer mode 7
fudge 127.127.8.0 refid GPSi
# meinberg gps-pps
server 127.127.22.0

Does anybody else experience this problem or know how I can fix it?

> From: "alkopedia@googlemail.com" <alkopedia@googlemail.com>
> Date: Wed, 17 Dec 2008 08:09:24 -0800 (PST)
> Sender: questions-bounces+oberman=es.net@lists.ntp.org
> 
> 
> Hi
> I have 2 ntp servers running Linux with LinuxPPS patch. Number one
> uses a Meinberg PCI511 v1.00 as time source, number two uses a
> Meinberg GPS170PCI v1.10.
> On both machines ntp is configured to use the Meinberg refclock
> (127.127.8.0) and furthermore the Meinberg PPS outputs are connected
> to the serial interfaces and use the atom driver (127.127.22.0).
> The strange thing is, that there is always a delay of 1-4 ms between
> the refclock and the pps signal. In my opinion they should arrive at
> the exact same time.
> 
> Here is an example graph of the dcf card:
> http://img-up.net/img/dcf-delayY6wXrh.png
> 
> Here is an example graph of the gps card:
> http://img-up.net/img/gps-delayHbRmW6U.png
> 
> And here the ntp conf entries:
> DCF:
> # meinberg dcf
> server 127.127.8.0 prefer mode 2
> fudge 127.127.8.0 time1 -0.003
> fudge 127.127.8.0 refid DCFi
> # meinberg dcf-pps
> server 127.127.22.0
> 
> GPS:
> # meinberg gps
> server 127.127.8.0 prefer mode 7
> fudge 127.127.8.0 refid GPSi
> # meinberg gps-pps
> server 127.127.22.0
> 
> Does anybody else experience this problem or know how I can fix it?
> 
> _______________________________________________
> questions mailing list
> questions@lists.ntp.org
> https://lists.ntp.org/mailman/listinfo/questions
> 

What you see is typical and the reason you use the PPS signal.

The time to process the time string from the clock is long and fairly
slow. The PPS is short and fast. As the documentation states, the PPS
signal "trains" the clock. 

While it is normal and not a real problem, you can use a "fudge" to
correct this a bit, but, as long as you leave the system up and
configure it reasonably, it should work fine. 

You should also set the GPS and PPS servers with "minpoll 4 maxpoll 4".
This will result in much faster convergence.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751

Hi,

alkopedia@googlemail.com wrote:
> Hi
> I have 2 ntp servers running Linux with LinuxPPS patch. Number one
> uses a Meinberg PCI511 v1.00 as time source, number two uses a
> Meinberg GPS170PCI v1.10.
> On both machines ntp is configured to use the Meinberg refclock
> (127.127.8.0) and furthermore the Meinberg PPS outputs are connected
> to the serial interfaces and use the atom driver (127.127.22.0).
> The strange thing is, that there is always a delay of 1-4 ms between
> the refclock and the pps signal. In my opinion they should arrive at
> the exact same time.

The PCI511 card decodes the AM signal from DCF77 only. Due to the
characteristics of the AM signal the accuracy of the PCI511 card is only in
the range of a few milliseconds, while the GPS card provides an accuracy
better than 1 microsecond.

So what you observe is not an NTP problem but just due to the limited
accuracy of the PCI511 card which is sufficient for many standard
applications. However, if you really want high accuracy you should rely on
a GPS time source.


Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany

On Dec 18, 10:08=A0am, Martin Burnicki <martin.burni...@meinberg.de>
wrote:
> Hi,
>
> The PCI511 card decodes the AM signal from DCF77 only. Due to the
> characteristics of the AM signal the accuracy of the PCI511 card is only =
in
> the range of a few milliseconds, while the GPS card provides an accuracy
> better than 1 microsecond.
>

Hi, you got me wrong. I did not wonder about the offset between gps
and pps but about the offset between gps refclock and it's own gps pps
(same for dcf refclock and it's own dcf pps).
Take a look at this picture: http://img-up.net/img/gps-delay0tHm49.png

On Dec 17, 6:57=A0pm, ober...@es.net (Kevin Oberman) wrote:
> The time to process the time string from the clock is long and fairly
> slow. The PPS is short and fast. As the documentation states, the PPS
> signal "trains" the clock.
>

OK, my first example has an error. Of course I need the same fudge at
the pps like at the refclock.
As you say it needs time to process the time string from the refclock.
This makes sense to me. So the time string comes a bit to late which
should result in a negative offset to the pps signal.

This is the case for my dcf server:
     remote           refid      st t when poll reach   delay
offset  jitter
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D
 LOCAL(0)        .LOCL.          12 l    5   64  377    0.000
0.000   0.015
+GENERIC(0)      .DCFi.           0 l    1   16  377    0.000
-0.569   0.687
oPPS(0)          .PPS.            0 l   14   16  377    0.000
-0.003   0.797

The time for processing the time string would be ca. 0.56 ms.

But on my gps server it's different:
     remote           refid      st t when poll reach   delay
offset  jitter
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D
 LOCAL(0)        .LOCL.          12 l    -   64  377    0.000
0.000   0.015
+GENERIC(0)      .GPSi.           0 l   10   16  377    0.000
1.727   0.015
oPPS(0)          .PPS.            0 l   11   16  377    0.000
-0.009   0.015

Here the reclock has a positive offset of ca. 1.72 ms. This means the
time string was processed faster than the pps signal?

On Dec 18, 12:47=A0pm, "alkope...@googlemail.com"
<alkope...@googlemail.com> wrote:
> I did not wonder about the offset between gps
> and pps

Should be:
I did not wonder about the offset between gps and dcf :-)

"alkopedia@googlemail.com" <alkopedia@googlemail.com> writes:

>On Dec 18, 10:08=A0am, Martin Burnicki <martin.burni...@meinberg.de>
>wrote:
>> Hi,
>>
>> The PCI511 card decodes the AM signal from DCF77 only. Due to the
>> characteristics of the AM signal the accuracy of the PCI511 card is only =
>in
>> the range of a few milliseconds, while the GPS card provides an accuracy
>> better than 1 microsecond.
>>

>Hi, you got me wrong. I did not wonder about the offset between gps
>and pps but about the offset between gps refclock and it's own gps pps
>(same for dcf refclock and it's own dcf pps).
>Take a look at this picture: http://img-up.net/img/gps-delay0tHm49.png
That your gps report is slow is not surprizing. It is delivered in a manner
that takes time to deliver. That you PPS has an offset of 20-40usec does
surprize me. It should be much better than that (2-4ms is more typical)

On Dec 18, 7:56=A0pm, Unruh <unruh-s...@physics.ubc.ca> wrote:
> That you PPS has an offset of 20-40usec does
> surprize me. It should be much better than that (2-4ms is more typical)

Running time was not long enough at that point. In the last 12 hours I
have these values:
Min: -4 us, Max: +6.9 us, Average: 319 ns