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

Joseph M Gwinn gwinn at Raytheon.com
Tue Jan 6 20:38:37 EST 2009


First the background: 

In some timing distribution applications, the primary source of 
interference comes from different ground voltages in different parts of 
the facility, such as a ship or a megawatt radar. 

The effect of differing ground potentials on a shielded cable is to pull a 
large current through the shield, so there is a significant voltage 
between the ends of the cable.  No matter how good the shield is at RF, 
one consequence is that the same power-frequency offset voltage appears on 
the conductors within that shield, because the skin depth at 60 Hz vastly 
exceeds the thickness of any reasonable shield.  Unshielded twisted pair 
will suffer the same common-mode offset voltage, perhaps more.   This 
offset often contains significant harmonics of the power frequency, 
nominally up to the seventh harmonic, not just the fundamental.

If the cable is shielded twisted pair, such as twinax, the offset appears 
as a common-mode voltage on the two conductors, and (if not too large) is 
eliminated by the CMRR of the receiver. 

If the cable is coax, the offset voltage appears added to the timing 
signal voltage, and if the offset isn't too large the signal receiver will 
be sufficiently immune to this conducted EMI. 


And now the question: 

What standards exist governing required immunity of signal ports to these 
ground-loop induced power-frequency (hum) voltages?

All the conducted suseptability standards I've found cover only 
frequencies exceeding 10 KHz, not power frequencies and their harmonics.


Thanks,

Joe





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