[ntp:questions] Frequency Offset

unruh unruh at invalid.ca
Fri Feb 24 17:44:30 UTC 2012

On 2012-02-24, David Lord <snews at lordynet.org> wrote:
> unruh wrote:
>> On 2012-02-23, Alby VA <albyva at empire.org> wrote:
>>> On Feb 23, 4:29?pm, unruh <un... at invalid.ca> wrote:
>>>> No, that is a very typical figure for the frequency offset. Remember
>>>> that the crystals used to control the timing in computers are not
>>>> supposed to be terribly accurate. (They are chosen to be cheap, not
>>>> great).
>>>> It is because such frequency offsets exist that ntp was invented to
>>>> correct.
>>>   When you say "crystals used to control the timing in computers",
>>> are you referring to the parts that make up my actual FreeBSD
>>> Server (ie: Motherboard)?  Or Parts in the GPS Device (ie: Sure Elec.
>>> GPS)?
>> On your motherboard. 
>>> Or in the Satellites receiving the GPS signal?
>>>  I'm just trying to gauge, what hardware should be looking to obtain
>>> to
>>> improve that frequency offset. Or is it a moot point unless I'm going
>>> to
>>> invest in some high dollar atomic clock?
>> An atomic clock.
>>>  Or better yet, can I buy quality crystals from DigiKey and do some
>>> soldering in whatever devices that need crystal upgrading?
>> No.
>> And why do you care? As I said this is waht ntp was invented for. It is
>> doing its job. 
> When I started using ntpd it didn't work with some of my
> systems as the clocks were too far out, >> 50ppm, and/or

Do you mean >500PPM? 
If you were running linux, you could use the adjtimex program and the -t
or --tick adjustment to change the tick value of your system clock. 
each value of 1 adjustment speeds up or slows down the clock by about 100PPM. 
You can use that to get the clock within the +- 500PPM range that ntpd
can adjust. chrony uses it automatically. 

> too temperature sensitive. Some of my motherboards had a
> 3-pin jumper already fitted so that an external clock
> source could be connected.

Temperature sensitivity is usually in the "less than 1PPM per degree C"
so you would have had to be expriencing quite a heat wave (500 degrees
C) to have temperture be a factor. I suspect other things might have
been more urgent worries then.

> One of my desktop systems, p4-2667, has just taken two
> days to get to an offset of under 2 ms after a kernel

ntpd is slow, but not that slow. Since for greater than 128ms offset it
does a step, and since it fixes things by about 1/2 per hour, half a day
is more like it to get it down to microsecond, not millisecond ranges.

> change. I suspect because that coincided with the hottest
> day in February since 1998. All my pcs were affected
> except the one connected to PPS from a Sure GPS. The one
> with an MSF source, no PPS, was also fighting to keep sync.

It sounds to me like network problems and ntp server problems were
playing a role. 

> David
>> Note that even all of the time standard labs around the world use
>> programs like ntp to track their frequency offsets.
>> And all atomic clocks have them.

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