[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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