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

[David C. Partridge]

Get 'em to use twin-ax (twisted pair inside screen) like the IBM AS/400
terminals (5250?) send differential signal down the cable. 

Dave

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Joe Gwinn
Sent: 10 January 2009 15:23
To: time-nuts at febo.com
Subject: Re: [time-nuts] Standards sought for immunity of shielded cable
links to power-frequency ground loops

Magnus,

At 10:31 AM +0000 1/10/09, time-nuts-request at febo.com wrote:
>
>Message: 5
>Date: Sat, 10 Jan 2009 11:06:39 +0100
>From: Magnus Danielson <magnus at rubidium.dyndns.org>
>Subject: Re: [time-nuts] Standards sought for immunity of shielded
>	cable	links to power-frequency ground loops
>To: Discussion of precise time and frequency measurement
>	<time-nuts at febo.com>
>
>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.

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

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.

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


>  >>>> 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.

True.


>  > 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>

Joe

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