[time-nuts] 1pps isolation

[Bob kb8tq]

Hi

For something like a 1:1 transformer, you actually don’t need all that super a ferrite.
Essentially all the current flow balances between the windings so you don’t have a 
lot of (effective) mag field in the core. Going with a lower frequency / higher permeability 
core “forces” any residual field to balance the current. Yes, it’s a bit more complicated 
than that, but you’ll get pretty close. 

Obviously you are going to risk getting a positive going spike followed some time later
by a negative going spike. If you have a 100 ms wide 1 PPS signal …. that’s what 
you will see. Since the front edge will be nice and fast, that’s not really a problem. Some 
sort of clamp diode to protect a logic input might be a good idea though. 

The same “balance” thing that helps with the core also helps you with the temperature 
coefficient. Looking at ferrite data, you would guess that the tempco would be really 
rotten. They can be pretty good. 

So yes, give it a try. Jitter and ADEV should not be all that bad.

Bob

> On Sep 1, 2017, at 4:24 PM, walter shawlee 2 <walter2 at sphere.bc.ca> wrote:
> 
> to get galvanic isolation, how about just using a pulse or RF transformer?
> the 1pps tick itself is very narrow, even though the PRF is slow, so a good quality
> pulse transformer/balun, or a few turns on a good fast ferrite core or similar should work fine.
> 
> I can't speak to the resulting adev degradation, but fast ferrites are common, and hopefully the
> resulting rise/fall time degradation would not be too ugly or asymmetrical.  I have used off the shelf MCL 50 ohm 1:1 isolation transformers to pipe 10Mhz around the lab where ground loops and distance are an issue, and seen no obvious timing problems as a result.
> 
> by the way, I have several racks of multiport 1pps buffers, from some kind of networked timing standard, they are looking for a new home if anybody can use them.
> 
> all the best,
> walter
> 
> -- 
> Walter Shawlee 2, President
> Sphere Research Corporation
> 3394 Sunnyside Rd.,  West Kelowna,  BC
> V1Z 2V4  CANADA  Phone: (250) 769-1834
> walter2 at sphere.bc.ca
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