[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
Sat Jun 13 10:02:52 UTC 2009

David Woolley wrote:
> David J Taylor wrote:
>> In the RS-232 implementations I have seen, the line is /never/
>> correctly terminated, as I use the term.  Correct termination
>> implies, if the
> I'm not disputing that.  I was really pointing out that by using RS232
> you couldn't follow the earlier advice to use terminated lines.  RS232
> is designed to be used on bandwidth limited, electrically short lines,
> both driven and terminated above the characteristic impedance, so that
> the line behaviour approximates a discrete capacitor.

Thanks for clarifying that.  Agreed - that's why David Lord is proposing 
line drivers for his PPS signal over co-ax.  Had I the equipment and the 
time, I would like to see how well the GPS 18x LVC drives that amount of 
capacitive load.

>> cable impedance 50 ohms (such as the often used Belden 8777), that:
> RS232 cables tend to have a high frequency impedance closer to 100
> ohms, although at the frequencies involved, the impedance is variable
> and far from the high frequency limit.

Yes, that's what I thought until I looked again at the specs. for the 
cable we used to use!

>> - the driver output impedance is 50-ohms
> Commonly not true, even for controlled impedance systems.  Radio
> transmitters rarely reverse terminate the line properly.

But there, the power loss inside the transmitter is important.  Take a 
look at fast data systems, sending video over cables etc. - they will be 

>> - the cable impedance is 50-ohms
>> - the receiver input impedance is 50-ohms
> Whilst optimum for data transmission, for low noise radio receivers it
> can be the wrong thing.



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