[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 Lord wrote:
> David J Taylor wrote:
>>> My Oncore docs give a suggested circuit with a 74HC132:
>>>
>>>
>>>   GPS-6_1PPS --+-----------------+        +--- MAX232_T2in
>>>                |                 |        |
>>>                |               4 +--      |
>>>              1 +--     3             & o--+
>>>                |   & o-- R=10k --+--     6
>>>              2 +--             5 |
>>>                |                 +-- C=470p -- +5V
>>>                |
>>>              9 +--     8
>>>                |   & o-- R=820 -- D=PPS LED -- +5V
>>>             10 +--               K         A
>>>
>>>
>>> I think that the generated pulse is only about 5 usec and was
>>> doubtful it would work for me over a 15-20m cable run. I've had
>>> regular rs232 work without problem over the same distance in
>>> the past.
>>>
>>> I've not started on this yet but will test with a short cable
>>> run first and increase the pulse width as needed.
>>>
>>>
>>> David
>>
>> David,
>>
>> Difficult to read your diagram, even in a fixed spacing font.
>>
>> My point was that the individual active lines should be paired with 
>> ground lines for transmission.
>>
>> If the pulse is only 5 microseconds, I would definitely want Chris to 
>> see how mangled (or otherwise) it was at the end of the run he has.  
>> It may well be pushing the limits, especially if it's not using the 
>> cables as twisted pairs.  Perhaps your circuit could be modified to 
>> produce something nearer 100 ms by replacing the C with something 
>> about 20K times bigger?  A 1 uF or so?
>>
> 
> The circuit is from the "Oncore QuickStart Guide" and I
> guessed that the pulse shaper was to get the 5 usec pulse
> otherwise output from pin 3 of the 74HC132 could have been
> used for default pulse width.
> 
> When I get around to assembly of my 2-to-6 rs232 expander
> I'll scope with both short and long cables and I've also
> rs422 MC3486/7 to try.

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



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