[time-nuts] Comparing PPS from 2 GPS units
lists at rtty.us
Tue Dec 18 01:04:15 UTC 2012
The nice thing about a spool of coax is that it's got a bit of thermal mass. It will average out a lot of minor temperature ups and downs.
On Dec 17, 2012, at 4:34 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 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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