[time-nuts] Switching 1 pps signal

[Glen English VK1XX]

I thoroughly agree with Bob.

Suggest converting 1pps into something hardy like LVDS or LVPECL  and 
using twisted pair , Cat 7 (individually shielded pairs and controlled 
skew cable) , or a pair of coax (balanced coax). The balanced signals 
will deal with the chance of power supply differences, and attach the 
shields of the balanced pairs to local ground with 10nF caps of something

If there are different earth potentials at each end (different power 
circuits), where the differential receivers end up running out of common 
mode headroom, the balanced signals will work nicely with ethernet 
transformers . (LF caveat) .

coax cables are in general NOT PHASE STABLE with temperature, either.

or if lightning  is a problem, convert and drive a fibre optic . that's 
another story, you can use the SFP modules that have GHz of bandwidth 
but be wary of their internal optical AGC and thus may not extend to DC.

glen.



On 7/07/2019 8:52 AM, Bob kb8tq wrote:
> Hi
>
> Indeed the fast rise time of just about any modern digital signal gets energy up into the GHz region. Isolation wise, the
> simple answer is often to just disable the signal when not in use. In a one at a time system, that takes care of cross
> talk / feed through. There are a lot of “always on" devices out there that suffer from blips on (say) the 10 MHz output
> each time the (maybe) 1 pps edge goes out …. disabling signals at the sourcre isn’t always an option.
>
> The other half of the problem is how to get the signal to its destination when it *is* in use. Coax (with some sort
> of drive that will handle a 50 ohm load) is usually the answer for long runs. For shorter runs simple twisted pair works ok.
> For very short runs a trace on a board or a jumper wire may be all you need.
>
> So now, what are the dimensions on all those really vague terms? Long (as in coax) is up into the 10’s of feet and down
> range. Shorter (as in twisted pair is in the 5 feet and down range, with a foot or two being pretty safe. Anything in the
> “couple inches” range is territory for a wire or simple trace on a board.
>
> The two issues (isolation and integrity ) are not completely independent of each other. In many cases you both need isolation
> *and* a good signal. In those cases, the mumble mumble mumble lengths above get a bit shorter. Just how short
> depends on just how good the isolation needs to be. Coax runs in the “under a foot” range are not unheard of.
>
> Further complicating this is the fact that you can get fancy twisted pair setups that are fine for longer runs. You
> can get really crummy coax that isn’t good for anything. There are always ways to route traces or bundle wires that will get
> you in trouble.
>
> Lots of fun and lots of rabbit holes to wander down ….
>
> Bob
>