[time-nuts] Discipline an oscillator with NTP?

Sun Jul 24 00:22:42 UTC 2011

Hi Don,

ntpd is both server and client.

For a ublox with usb i did the following a few days ago.

     sudo ln -s /dev/ttyACM0 /dev/gps0

add this to the /etc/ntp.conf

     server 127.127.20.0 mode 81

and then

    /etc/init.d/ntp restart

This particular setup without 1pps gives a lousy time (some 150ms late
with 50ms jitter).

I apologize if I was not clear enough in my previous answer.

--

   Björn

> Hi Bjorn: Thanks very much for the sources! I was afraid I was not clear
> when I sent my question(s). I want to generate a network server rather
> than a client. I looked up man ntpd, and it seems at first glance to be
> a Linux/Unix client that will sync up a local computer. I want to
> generate a network server that ntpd or other clients can access. Perhaps
> I did not read closely enough, and if not, I apologize.
> Don
>
> bg at lysator.liu.se
>> Hi Don,
>>
>> 1PPS is useful. 10MHz is not direcly useful. You also need some kind of
>> timecode, telling ntpd which second the 1pps indicated.
>>
>> The software side is normally ntpd configured with one of its drivers
>> (called refclock) for the GPS protocol your receiver has. Again the most
>> used is probably refclock_NMEA.
>> (http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html) look
>> if
>> there is anything else fitting your receivers.
>>
>>    http://www.eecis.udel.edu/~mills/ntp/html/refclock.html
>>
>> Then wire a (preferably) serial cable with rx, tx and gnd in the usual
>> places, add your 1PPS on DCD (pin 1). Depending on your serial port you
>> could get away without levelshifting the 1pps to rs232-levels.
>>
>> Last but not least, for OS select your favorite unix derivative.
>>
>> --
>>
>>    Björn
>>
>>> Poul and others?
>>> As usual, I suddenly had a thought ripple across my otherwise placid
>>> cortex, and have forgotten if there was a previous answer on this
>>> thread. What does a NTP server look like on the network? There are now
>>> several small net appliances/eval boards available. I run a program
>>> called NMEAtime on my Windoze computers that currently get info from a
>>> net NTP server. I also have some simple net appliances. So, using my
>>> GPS
>>> 10 MHZ or 1 sec signals, or Rb 10 MHZ, how might I generate a local
>>> NTP
>>> server?
>>> I hope I don't put this awkwardly; what is the protocol? What does the
>>> supplicant client see and how does it ask for service?
>>> Best to all of you,
>>> Don
>>>
>>>
>>> Poul-Henning Kamp
>>>> In message <022101cc4970$b24a4b80$16dee280$@com>, "Jose Camara"
>>>> writes:
>>>>
>>>>>After
>>>>>one year of NTP queries, assume you have a 100ms jitter on the
>>>>> network
>>>>> time,
>>>>>you could at most tweak your oscillator, based on past performance,
>>>>> to
>>>>> 6E-9.
>>>>
>>>> It is a lot more complicated than that, we need to talk
>>>> allan-deviation
>>>> here, not scalar numbers.
>>>>
>>>> The main problem here is that the 'default' NTPd software is not
>>>> really
>>>> written for something like this, and has attributes which makes it
>>>> truly sucky for the task.
>>>>
>>>> If you want to do this, you want to write your own software and you
>>>> want to give it an entirely different modus operandi.
>>>>
>>>> As with all oscillator discplining, what you are looking for is
>>>> the so called "allan intercept" where the two sources allan deviation
>>>> cross.
>>>>
>>>> With a NTP reference, its location varies depending on stochastic
>>>> network properties, which depends what's between the server and you.
>>>>
>>>> If you control the network topology (as in: Can make sure there is
>>>> no other traffic), you're fine, normal PLL style stuff works.
>>>>
>>>> If you don't control the network topology, the RTT between you and
>>>> the server becomes a BIG problem, because the fundamental NTP
>>>> assumption that it is symmetric is almost always wrong.
>>>>
>>>> You can average over long tau's, but then your ISP upgrades their
>>>> routed and a systematic change in RTT screws your integrator over
>>>> for several weeks.
>>>>
>>>> Alternatively, if your LO is stable enough (=Rb/Cs), you can operate
>>>> on first derivative of the RTT, which turns the routed upgrade into
>>>> a single spiky sample, but the cost is an overall higher noise in
>>>> your error signal.
>>>>
>>>> --
>>>> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
>>>> phk at FreeBSD.ORG         | TCP/IP since RFC 956
>>>> FreeBSD committer       | BSD since 4.3-tahoe
>>>> Never attribute to malice what can adequately be explained by
>>>> incompetence.
>>>>
>>>> _______________________________________________
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>>>>
>>>
>>>
>>> --
>>> "Neither the voice of authority nor the weight of reason and argument
>>> are as significant as experiment, for thence comes quiet to the mind."
>>> R. Bacon
>>> "If you don't know what it is, don't poke it."
>>> Ghost in the Shell
>>>
>>>
>>> Dr. Don Latham AJ7LL
>>> Six Mile Systems LLP
>>> 17850 Six Mile Road
>>> POB 134
>>> Huson, MT, 59846
>>> VOX 406-626-4304
>>> www.lightningforensics.com
>>> www.sixmilesystems.com
>>>
>>>
>>> _______________________________________________
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>>
>>
>>
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>
>
> --
> "Neither the voice of authority nor the weight of reason and argument
> are as significant as experiment, for thence comes quiet to the mind."
> R. Bacon
> "If you don't know what it is, don't poke it."
> Ghost in the Shell
>
>
> Dr. Don Latham AJ7LL
> Six Mile Systems LLP
> 17850 Six Mile Road
> POB 134
> Huson, MT, 59846
> VOX 406-626-4304
> www.lightningforensics.com
> www.sixmilesystems.com
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>