[time-nuts] cable delay variation (was: Question about the PLL of Trimble Thunderbold)

[Bob kb8tq]

Hi


> On Oct 29, 2018, at 3:42 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> On Mon, 29 Oct 2018 12:03:25 -0700
> Hal Murray <hmurray at megapathdsl.net> wrote:
> 
>> If I go looking for good cables, do they specify temperature coefficient?  I 
>> don't remember ever seeing it when scanning specs, but I probably wasn't 
>> looking for it so I could have skimmed over something.
> 
> Yes. The key word here is "phase stable coax." If you search for that,
> several cables will pop up. They wont be cheap, though.
> 
> 
>> What determines the dispersion of a cable?  Is it as simple as bigger wire 
>> (more copper) is better?
> 
> For us, it's mostly the frequency dependence of the dielectric.
> If you go for higher frequencies, it also becomes a matter of
> the different modes a cable supports, which all have different
> velocities. The latter is the reason, why GHz cables and connectors
> are becomming thinner and thinner as we go up in frequency.
> 
> 
>> Why do I care about the dispersion as long as all cables match?
> 
> Because it degrades the slew rate of the pulse. If you have
> a very bad case of dispersion (combined with long cables),
> the rising slope will look more like a jagged mountain range
> than a step (though it's unlikely you hit that with modern
> cables, unless you go for several km of cable). Attached is
> a picture of what it might look like.
> 
> If you remember the old telegraph and telephone lines, they
> all used to have inductors placed on them every few km, to
> compensate the dispersion. As most of our communication these
> days is digital and we have relative short lengths before a
> regeneration step happens, there isn't much need for dispersion
> compensation anymore. Unless you go for submarine cables or
> optical fibers, both of which have elements with negative
> dispersion inserted.

Even worse things can happen when you have an air dielectric cable and 
evenly spaced supports. Putting a pulse into something like that can 
be *very* messy. 

Bob



> 
> 			Attila Kinali
> 
> -- 
> <JaberWorky>	The bad part of Zurich is where the degenerates
>                throw DARK chocolate at you.
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