[time-nuts] Switching 1 pps signal

[Bob kb8tq]

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

> On Jul 6, 2019, at 2:06 PM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I am trying to come up with a viable design to encase TICC along with 10MHz -> 1 pps converter.  My plan is to share the input connector and switch in/out the converter. 
> 
> 1 pps, although technically a 1Hz signal, rise time and duration is awfully short on some sources.  I'm sure constituent signal will go way into RF range.  I am not capable of calculating this.  But, that means I really have to treat it as an RF signal, correct?  Otherwise, slower rise will cause error as it will cross the threshold later than initiation of the pulse.  Question is, how careful do I need to be?
> My plan is to use two 18GHz microwave relay per channel to bypass/place-in the converter.  I know this would be OK but am I over-doing this?  I also have a miniature relay in DIP size but I'm afraid it may not be sufficient.  There is not a frequency spec, and consistency may be an issue.
> Advise, please?
> 
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
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