[time-nuts] Comparing PPS from 2 GPS units

Hal Murray hmurray at megapathdsl.net
Mon Dec 17 21:34:07 UTC 2012

lists at rtty.us said:
> If you are trying to set up say a 1 us delay, you will get ~ 50 ps per
> degree C in your delay. That's a lot ..... 

A while ago, tvb at LeapSecond.com said:
> A long delay cable is fine too. If these are timing receivers you probably
> don't need more than 100 ns of delay, once you figure out which receiver is
> ahead of the other. The cable tempco is low enough not to worry about. 

100 ns is 50-100 feet.  That's a reasonable length to work with.  But I was 
curious about the temperature coefficient.  Google found this:
which says:
# Belden 8240 (solid) shows a temperature coefficient of around 
-0.252ps/m/deg in a temperature range between -20 and 30 deg. The coefficient 
becomes steeper beyond 30 deg.
# Belden 8219 (foam) shows a larger temperature coefficient of around -0.352 
ps/m/deg than that of 8240 in the similar temperature range. The coefficient 
becomes steeper beyond 30 deg, but less steeper than that of 8240.
# Fujikura RG58-A/U shows the smallest temperature coefficient of around 
-0.152 ps/m/deg, but in a narrow temperature range between -10 and 20 deg. 
The coefficient beyond 20 deg is much steeper than the others.

To pick round numbers, 30 meters and 3 C and 0.25 ps/m/C gives 25 ps.

These are my opinions.  I hate spam.

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