Magnus, [email protected] wrote on 01/07/2009 01:27:52 AM:
> Joseph M Gwinn skrev: > > 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. I left a useful detail out: The reference signal is a 10 MHz sinewave. > > 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 shieldis 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. > > For most purposes an isolation transformer would solve this issue. The > unfortunate signal characteristics of a PPS pulse makes this a little > more cumbersome, but not unachievable, but it is no longer a simple > passive device. For higher frequencies will RF chokes be an aid of > course, but the RF choke needs "bolting down" in order to be effective, > so that there is a common mode current for the RF choke to object to. > However, the RF choke is not as effective with lower frequencies and > essentially useless for DC. The receivers have built-in RF transformers. There is no 1PPS signal per se, although the transformer would probably pass such a signal well enough. What is being carried is 10 MHz. The problem is to devise a test and spec that ensures that the actual implemented circuit in the receivers suffice. There are many ways to botch this circuit. > > 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 theirharmonics. > > You should look into the telecom set of standards. If you think of it, > they have been addressing this particular problem for ages. The words > which probably get you right on the target is "bonding network" since > you bond to the ground. This is just the sort of lead I was hoping to find. > In short, there are two grounding strategies: all gear is floating > relative the safety ground or all gear is internally tied to the safety > ground. There is benefits and problems with both strategies. Regardless, > a hierarchial star ground strategy emerges. In our systems, everything is tied to ground for both safety and RF reasons unrelated to timing signals. And we do have a star of sorts, but the story always ends up more complex than that, so it always ends up being a somewhat random grounding grid. My problem is not safety, it is tolerance of conducted EMI. > One document to start with is the "Qwest Technical Publication > Grounding - Central Office and Remote Equipment Environment" at > http://www.qwest.com/techpub/77355/77355.pdf > > Not to say that it is the standard of any sort, but I think it is a good > document to start from as it is a public source of telecom bonding > practices to be used in many facilities, implementing existing > international standards and involving transmitting towers (which is > within your field). > > IEC 60950 should be a standard reference regardless. > > You should also consult Bellcore GR-1089. There are additional Bellcore > specs, but starting with GR-63 and GR-1089 is not totally off the mark > at least. Bellcore specs costs money, but if you need to comply there is > no alternative. > > ITU-T has a set of documents, such as the K-series of standards. You can > download these for free at: > http://www.itu.int/rec/T-REC-K/e > > The European telecom world uses ETSI EN 300 253 as basis. They require a > login which you can get for free and then pull down all the documents > you like. There is also alot of specific EMC documents for various > contexts etc and they are all there. ETSI EMC is the TB handling them. > > On the military side, MIL-HDBK-419 may be a guide: > http://tscm.com/MIL-HDBK-419A.PDF > > Old standard MIL-STD-188-124B: > http://www.tscm.com/MIL-STD-188-124B.PDF > > Newer standard MIL-STD-1310 for ships: > http://www.earth2.net/parts/basics/milstd1310g.pdf I will be doing some homework. Some of these are tomes. > In the end, all these documents forms a reference of standards and > practice in a varity of environments. I suspect that your environment > does has some bonding standard and practice and you need to figure out > what it is so that you know what you can expect, what you need to > fullfill (which is limiting freedom on what methods you may apply!) and > then it becomes easier to say what may help you. Also, you need to > figure out what is the type of problems you run into, how disturbances > actually induce into your lines. It could very well be that PSUs acts as > EMF due to bad conditioning for instance. > > There are many anecdotes and horror stories to be told on the subject. > There are also sucesses stories to be told. We do have a bonding story, one that sort-of follows MIL-STD-1310, even though the system is land based. > What makes the field a bit complex is that you need to think about > failures, EMC, bonding, interference, lightning strikes (on wire, in > tower, on building) which can cause a disparity of various indirect > effects. It's a bit like being a time-nut. We could probably have a > separate email list setup for that kind of discussions alone. Fortunately for me, I do not have to worry about lightning. That's handled elsewhere, as all these cables are within a steel-frame building with a lightning protection system built in. 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.
