[time-nuts] Comparing PPS from 2 GPS units

Bob Camp lists at rtty.us
Tue Dec 18 01:47:34 UTC 2012


At some point the whole get it onto the board / get it off the board thing becomes the main issue. Then it's easier to just make the delay line part of the PC layout.


On Dec 17, 2012, at 8:39 PM, David <davidwhess at gmail.com> wrote:

> I wish there was an source for helically wound shielded differential
> transmission line like the type used in later analog oscilloscopes.
> The only place I know where to find it is oscilloscope part mules.
> Essentially it was transmission line with a ridiculously low velocity
> factor.  It is great for building instant digital delay lines up to
> the low 10s of nanoseconds range in a small space.
> On Mon, 17 Dec 2012 20:04:15 -0500, Bob Camp <lists at rtty.us> wrote:
>> Hi
>> 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. 
>> Bob
>> 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:
>>> http://www.hepl.hiroshima-u.ac.jp/phx/notes/cable/cable.html
>>> 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.
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