[time-nuts] NMEA Time

[Bob Camp]

Hi

Doesn't everybody run OCXO's for the clock in their PC?

Bob


On Nov 25, 2009, at 4:41 PM, Lux, Jim (337C) wrote:

>> You aren't going to get nanosecond accuracy out of a ntp server running over
>> an ethernet[1].  On the other hand, sub-ms isn't hard with a good OS and/or
>> good software.
>> 
>> It's fairly easy to get a reasonable sanity check on a (s)ntp server.  Just
>> setup a known good ntp system and have it monitor the DUT.  The reference
>> implementation for ntp (http://www.ntp.org/) has lots of support for
>> collecting data.
>> 
>> The key step in making a PC keep good time is tweaking the clock frequency.
>> This is the software equivalent of the EFC on an oscillator.  ntpd calls it
>> drift.  You can use it as a thermometer.
> 
> I don't think the oscillator quality in the typical PC is good enough to ever get nanoseconds, even with tons of tweaks and temperature compensation, etc.  The short term variability/phase noise is too high.  
> 
> Microseconds, I think you could do.
> 
> 
> 
> 
>> 
>> ----------
>> 
>> 1) There is a group working on that level of accuracy.  It takes special
>> hardware that can put a time-stamp on a packet as it leaves or arrives.
>>  http://en.wikipedia.org/wiki/Precision_Time_Protocol
>> 
>> 
> 
> More than just a casual group.  You can buy IEEE 1588 products from, among others, Symmetricom.
> 
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