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

David Lord snews at lordynet.org
Tue Jan 10 11:24:45 UTC 2012


David J Taylor wrote:
> "David Woolley" <david at ex.djwhome.demon.invalid> wrote in message 
> news:jefokf$9m0$1 at dont-email.me...
> []
>> To the extent those constraints don't apply and both ends are 
>> terminated well above the characteristic impedance, the output voltage 
>> actually goes up in a staircase, with the steps being the round trip 
>> time.
>>
>> Some other mistermination conditions cause a ringing approach to the 
>> final value, which is why it is better to operate with a high 
>> impedance load, and therefore capacitive characteristics.
> 
> I wish Chris would just look at the remote signal with a 'scope!  A wide 
> pulse over the length of line he talks about should be no problem at all 
> (but a microsecond-wide pulse might).  The timing PPS I've seen are in 
> the tens of milliseconds wide.  If there is overshoot, perhaps a 
> capacitor to slow things down might help, or on the other hand, if the 
> edge is too slow providing a better match might help.
> 
> Of course, "clean" transmission does rely as I said before on each 
> signal being provided with its own ground in the twisted pair, a point 
> also made by Bill Unruh.  For PPS a screened 50-ohm coax cable is not 
> needed except in extra ordinary circumstances (exceptional noise or 
> sub-microsecond accuracy).  Even level converters may not be needed - 
> even though the specification says so.  Many RS-232 ports work just fine 
> with TTL levels.


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


> 
> Cheers,
> David



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