Magnus, At 6:02 PM +0000 1/10/09, time-nuts-request@febo.com wrote: > >Message: 4 >Date: Sat, 10 Jan 2009 19:02:09 +0100 >From: Magnus Danielson <magnus@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@febo.com> >Joe, > >>>> 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. > >I just want you to end up having that trouble instead. I think you >should consider a shielded twisted pair instead. Use the transformer to >go between 50 Ohm and 100-110 Ohm while also getting the common mode >isolation. A double-transformer approach can be used in which the >launch/receive-transformer has a center tap on the "inside" which is >wired to local ground (needs to be very low impedance). This improves >capacitive isolation for common mode currents. The inner transformers do >impedance matching. This is really an alternative to getting isolation >transformers, it might even be cheaper. Dual-shielded isolation >transformers is better thought, as capacitive coupling as spread out >over the coil is always terminated to each side own shield which reduces >common-mode to diffrential mode conversion. The engineer wanted to use catalog components, which means connectorized hybrid transformers, probably from Minicircuits or the like. He did use real twinax elsewhere, and the hum pickup issue has occurred to him. The connectors are Type N, and the cable will be some kind of robust double-shielded flexible type. He may already be twisting the two cables, which are about 30 meters long. > > 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. > >I have a bit hard [time] to realize how the common mode ground current would >saturate the hybrid transformers unless the current is so high that the >asymmetry in the transformers helps. Some form of DC blocker or LF >current limiting may be wise thought. The 60 Hz limit in MIL-STD-461 CS109 is one amp (120 dB over one microamp). The EMI guy said that this limit was arrived at for submarines in the 1970s, and the currents were primarily due to charging currents from capacitor-input power supplies, and the like. What saves us with the hybrids is that while the cores are small, the windings might have five turns, so it will take a very substantial current to have any effect, and the winding will blow out first. > > The problem is that the radar and the ship are not yet built, so we > > cannot yet make tests. > >So much better. You have a chance to get things right before it is too >late and too expensive. Yes and no. The drawings are done long before, and change is painful. But necessary. >I am sure we can send a sub to sink it late if needed. I'm not sure that solves the problem, but it certainly eliminates the problem. > >>>> 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. > >The reason I keep mentioning it is since that it is easy to focus and >make a design "optimum" for one case and forgetting about other aspects. >Signal integrity, safety and EMC needs too be considered at the same time. Can't say that I much worry about such things in lab setups. As I said, the 100 ohm resistor works fine, but there are lots of places I would not do it that way. > >> > 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> > >Another site which can't keep their certs up-to-date. I got the same message, but the certificate was still current (just looked - expires in June 2009). I suspect that the real problem is that US DoD has its own certificate authority, and does not use Verisign et al, and the browser maker forgot to include DoD. >By looking at it, it seems reasonably to use that or some suitable >variant. Notice how the 10 MHz input/out wires is not included so some >adaptation would be required. Essentially one where the 10 MHz generator >is floating through isolation transformer and the current is induced on >the generator ground. Or just use a heavy filament transformer driven by a variac to pull up to ten amps through the shield. What then floats is the secondary of the transformer. If it's necessary to isolate the oscillator or distribution amplifier, use a DC block that breaks both center and shield with capacitors. Or use an RF isolation transformer. The oscillator output is already isolated from chassis ground, now that I think of it. But the distribution amplifier outputs are not isolated. Joe