[time-nuts] Switching 1 pps signal

[jimlux]

On 7/6/19 8:34 PM, Glen English VK1XX wrote:
> Hal, Good point.
> 
> and I have never seen a spec for phase stability for Cat-7 cable !
> 
> for RG58, OR OTHER polyethylene, might be up to 150ppm /deg C. maybe as 
> good as 10 ppm/deg C for some LMR.
> 
> With 1000 feet of cable might be an issue for fine stuff. a few nano 
> seconds each way over temperature might not matter for his project...
> 
> I forget it is not a microwave  RF  VNA application. Easy to resolve 
> anyway with a difference balance term somewhere.
> 
> g

The propagation velocity variation is from:
1) change in the density of the dielectric with temperature, which 
changes the epsilon
2) change in the dimensions of the transmission line, which changes the 
"per unit" inductance and capacitance, changing the propagation constant 
(proportional to sqrt(LC))

If you need to deal with the propagation velocity variation, then what 
you do is look for the reflection coming back, time that relative to the 
sending, and use that to correct the timing of the far end.

Of course, an edge has all frequencies, and the propagation constant is 
not the same for all frequencies (some dispersion), so that brings in 
another set of problems.

People who do stuff like coherent combining of large microwave antennas 
for interferometry do stuff like run the coax/fiber in a temperature 
controlled conduit. (and even then, there's "post processing")