[ntp:questions] Very large jitter and offsets on GPS ref clock after upgrade to "p5"

David Lord snews at lordynet.org
Wed Feb 8 17:45:06 UTC 2012


David J Taylor wrote:
> "David Lord" <snews at lordynet.org> wrote in message 
> news:gnbbu8-qne.ln1 at me6000g.home.lordynet.org...
> []
>> I've not put a scope on the pps but I added an extra section
>> of cable from my Sure GPS, 18 m of 4 x pair telephone cable.
>> I will need that length to reach from the GPS down to my PCs.
>>
>> In the process the offset jumped from 0.004ms to about 12ms
>> but fairly quickly went back to a few usec.
>>
>> PC = me6000 (VIA ME6000), NetBSD-5, ntpd 4.2.6p5
>>
>> From my ntp-stat file:
>> time     offset(ms)
>> 15:06        -0.004
>> 15:12         0.000
>> 15:18         0.001
>> 18:24         0.006   extra 18 m cable added
>> 15:30       -12.938
>> 15:36        -3.672
>> 15:42        -0.814
>> 15:48        -0.186
>> 15:54        -0.054
>> 16:00        -0.012
>> 16:06        -0.006
>> 16:12        -0.002
>> 16:18        -0.002
>> 16:24        -0.001
>>
>>
>> David
> 
> David,
> 
> I suspect that the offset jump was purely from losing the PPS signal, 
> and nothing to do with the extra cable as such.  BTW: I think your 
> editing accidentally included the time - from 15:24 to 18:24.  Must have 
> had "18" on the brain!  <G>

I didn't spot that until after I'd posted.

> 
> My comments on wiring are as before - use one line from each pair as 
> ground, and then the other four "active" lines become TXD RXD PPS and 
> +5V power.

Nearly that, but I'm splitting the power and planning on
having +9V and -Ve on two of the lines and a 5V regulator
since some of the GPS modules with interfaces take  over
200mA so common OV with the -Ve at the far end should
remove any effect from voltage drop over that length of
cable.

I might try rs422 for the PPS (and TxD lines) but they
are 16-pin chips and have unused sections. The 8-pin
versions with single Tx + Rx are much too expensive.


David



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