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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
