[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 18:20:46 UTC 2012


"unruh" <unruh at invalid.ca> wrote in message 
news:yb%Oq.28020$d52.7176 at newsfe22.iad...
[]
>> I was expecting that, for timing purposes, the four pairs would be used
>> as:
>>
>> - TXD/ground
>> - RXD/ground
>> - DCD/ground
>> - unused
>>
>> i.e. that every relevant RS-232 line was carried with a ground on the
>> twisted pair.  This would be better than some random "pushing of pins"!
>
> The problem of course is that all of those grounds are common grounds at
> the DB9 interface ( and probably at the gps as well) which means that
> the ground current would be carried on all three ground wires, giving a
> common mode signal on each pair.

But I think that would be less of a problem as:

- the signals are carried on individual twisted pairs
- ground is now 1/3 of the resistance it was before

If possible, I would not be earthing the GPS other than via the cable (and 
use the unused pair to carry +5V to the GPS).

I would, of course, be interested in any actual waveforms, which have been 
unfortunately absent in this discussion so far.  We have no idea whether 
the problem is related to the cable length or not, and if so, why.

Cheers,
David 



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