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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jan 10 10:06:39 UTC 2009


Joseph,

> 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 
> the 
>>> 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.

That is to cheat! :)

> 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.

OUCH! The trouble with that arrangement is that the coax cables MUST be 
twisted or else H-fields will induce differential mode current. It will 
induce current into both directions which through the 180 degree will 
not cancel but add up. The 0/180 degree arrangement will save you from 
common mode problems. You would prefer a twisted cable over a twisted 
cable pair, as the later allows for installation procedure errors to 
have huge impact and the twisting properties will not be as good either 
and thus compromising the quality. A single ended coax is not as 
sensitive to H fields to induce diffrential currents, but can have some 
other problems.

>>>>> 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.

As suspected, but this is really just another of these EMC rationales.

>>>> 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.

Should be sufficient then. But remember that capacitive coupling helps 
you in the RF area and impulse protection.

> 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.

Need to look it up. Never had to do any of the MIL-STD-461 stuff.

Cheers,
Magnus



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