[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 13:02:09 EST 2009


Joe,

>>>  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.
> 
> You are right about the twisting.  The cables are close and parallel, 
> and ground offsets are the big problem, versus magnetic fields.

I just want you to end up having that trouble instead. I think you 
should consider a shielded twisted pair instead. Use the transformer to 
go between 50 Ohm and 100-110 Ohm while also getting the common mode 
isolation. A double-transformer approach can be used in which the 
launch/receive-transformer has a center tap on the "inside" which is 
wired to local ground (needs to be very low impedance). This improves 
capacitive isolation for common mode currents. The inner transformers do 
impedance matching. This is really an alternative to getting isolation 
transformers, it might even be cheaper. Dual-shielded isolation 
transformers is better thought, as capacitive coupling as spread out 
over the coil is always terminated to each side own shield which reduces 
common-mode to diffrential mode conversion.

> My worry was that the ground currents might be enough to saturate the 
> tiny ferrite cores in the hybrid transformers.  The engineer's 
> reaction to this was on the following day to say that if this turns 
> out to be a problem, he will add DC blocks.  This would have to be 
> the kind that blocks both center and shield paths.

I have a bit hard to realize how the common mode ground current would 
saturate the hybrid transformers unless the current is so high that the 
asymmetry in the transformers helps. Some form of DC blocker or LF 
current limiting may be wise thought.

> The problem is that the radar and the ship are not yet built, so we 
> cannot yet make tests.

So much better. You have a chance to get things right before it is too 
late and too expensive.

I am sure we can send a sub to sink it late if needed.

>>>>  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.
> 
> True.

The reason I keep mentioning it is since that it is easy to focus and 
make a design "optimum" for one case and forgetting about other aspects. 
Signal integrity, safety and EMC needs too be considered at the same time.

>>  > 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.
> 
> It's available for free on the web. 
> <https://acc.dau.mil/CommunityBrowser.aspx?id=127373>

Another site which can't keep their certs up-to-date.

By looking at it, it seems reasonably to use that or some suitable 
variant. Notice how the 10 MHz input/out wires is not included so some 
adaptation would be required. Essentially one where the 10 MHz generator 
is floating through isolation transformer and the current is induced on 
the generator ground.

Cheers,
Magnus




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