[ntp:questions] NMEA ref.clock better than my ISP's timeserver?

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Thu Jun 11 13:41:50 UTC 2009


David Woolley wrote:
[]
> When used with proper balanced drivers and receivers, twisted pair is
> often better than coax.
>
> For any type of cable, group delay characteristics may be more
> important than characteristic impedance match.

Whilst I agree with both points, for carrying what are TTL-level signals 
with rise-times probably greater than 100ns where you are using an RS-232 
receiver to detect pulses 0.2s wide, and over a distance or 30m (i.e. a 
time of around 100-150ns), almost any cable will do.  The only exception I 
might make is where the environment is electrically noisy - i.e. an 
industrial setting or running behind a photocopier, air conditioner, heavy 
motor or similar, where I would want at least shielded cables.

I say this from years - too many years! - experience with RS-232.

Cheers,
David 




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