[ntp:questions] Re: Question to NTP developers

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk
Thu Aug 24 07:17:50 UTC 2006


Eugen COCA wrote:
> David J Taylor wrote:
>
>> .. another long shot - but do you convert the GPS signals to true
>> RS-232 levels, or simply use them as-is from the GPS18?
>>
>
>
> David,
>
> this could be a possible problem as the GPS receiver deliver signals
> from 0 to 5V and not true TIA-232 levels (-9V to +9V). I thought at
> this situation several months ago but I'm not sure if such a converter
> will not add too much jitter to the PPS signal (as it must be supplied
> with a separate voltage source).

Eugen,

I would not expect a level convertor to add any significant jitter, as it 
will be in the tens of nanoseconds or less, compared to the perhaps 
hundreds of nanoseconds for the PPS signal and perhaps microseconds for 
NTP.  Indeed, by swinging the voltage over a wider level, it might 
actually reduce the effects of noise (which would translate into jitter) 
at the receiver.

On the other hand, serial ports usually have a much better threshold than 
the +/- 9V.  IIRC, +/- 3V is the actual threshold required.  However, it 
does mean that using 0V for the lower level is, technically, outside the 
RS-232 specification, but it's something which people do seem to have done 
successfully for the last 20 years or so!

It would be more important, of course, on longer connections or where the 
environment was electrically noisy.

Cheers,
David 





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