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

David J Taylor david-taylor at blueyonder.co.uk.invalid
Tue Jan 10 07:35:28 UTC 2012


"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.

Cheers,
David 



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