Joseph, > time-nuts-boun...@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. >>>>> I imagine that the shield is grounded at both ends, if only for >>>>> safety reasons. >>>> That is actually a very unsafe practice, unless there is another >>>> much thicker and reliable ground connection between the two domains. >>> There is a very heavy grounding grid, and such systems almost always >>> ground the (outer) shields at every connector. >> Which would imply that if the signal passes through a connector jack or >> through a wall, much of the current would be sent back to its EMF source > >> locally in the room. This does have its merits. > > Yes, but that isn't the reason. It's really a safety and EMC rationale. As suspected, but this is really just another of these EMC rationales. >>>> 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. > 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. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.