[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

Joseph M Gwinn gwinn at raytheon.com
Thu Jan 8 19:51:50 EST 2009


time-nuts-bounces at febo.com wrote on 01/07/2009 10:47:46 PM:

> Joseph,
> >>> Could be a differential TX and RX.  I recall that they send a RS422 
> > signal.
> >> Depending on the speed, RS422 works fine with transformers.
> > 
> > Yes.  It would be 10 MHz or 20 MHz, depending on coding.  Or 5 MHz, so 
> > transitions are at 10 MHz.  I don't recall, or never knew.
> RS422 does not imply any encoding as such so it would be 10 MHz but 
> naturally there is twice that many transitions, but it is the frequency 
> of the signal you are interested in for this case.

I know that RS422 is not the encoding.  I cheated, and talked to the 
relevant engineer.

For digital signals (1PPS, various triggers), it's RS422 over 100 ohm 
twinax (fancy shielded twisted pair).

The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with the 
signals 180 degrees out of phase.  This is acheived with a pair of hybrid 
transformers which convert from one-cable to two-cable and then back to 
one-cable, where all cables are 50 ohm coax.

> >>> I imagine that the shield is grounded at both ends, if only for
> >>> safety reasons.
> >> That is actually a very unsafe practice, unless there is another
> >> much thicker and reliable ground connection between the two domains.
> > 
> > There is a very heavy grounding grid, and such systems almost always 
> > ground the (outer) shields at every connector.
> Which would imply that if the signal passes through a connector jack or 
> through a wall, much of the current would be sent back to its EMF source 

> locally in the room. This does have its merits.

Yes, but that isn't the reason.  It's really a safety and EMC rationale.

> >> But you should never let the screen float in the far end, you should
> >> terminate it with a 10M resistor and a sparkgap in parallel to the
> >> local ground.
> >>
> >> The resistor takes care of static electricity and the sparkgap will
> >> do lightnings.
> > 
> > I've done such things, but with a 100 ohm resistor (and a safety 
ground to 
> > ensure that the voltage doesn't get too large.  But this was 
> a lab lashup.
> The trouble with 100 ohm is that still can be a little low in relation 
> to ground loop impedances, it still allow some fair current to roll down 

> the cable. A capacitor in parallel would cut most of the transient 
> energy straight through and allow for a higher resistive path for the 
> low frequency energy.

The ground grid impedance between any two points is well less than one 
ohm, so 100 ohms will pretty much abolish all ground loops.  I've used 10 
ohms in like labs, with success.  I'll grant that this would not work with 
long wires outside.

By the way, I also finally talked to one of our most experienced EMI/EMC 
engineers.  He suggested using MIL-STD-461 test CS109, even though CS109 
was developed for enclosures.  It turns out he was involved in developing 
CS109 when he worked for the US Navy.



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