Magnus, At 10:31 AM +0000 1/10/09, time-nuts-request@febo.com wrote: > >Message: 5 >Date: Sat, 10 Jan 2009 11:06:39 +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> > >Joseph, > >> time-nuts-bounces@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. You are right about the twisting. The cables are close and parallel, and ground offsets are the big problem, versus magnetic fields. 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. The problem is that the radar and the ship are not yet built, so we cannot yet make tests. > >>>> 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. True. > > 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> Joe

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. > 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 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 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. I am sure we can send a sub to sink it late if needed. >>>> 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. >> > 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. 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. Cheers, Magnus

Get 'em to use twin-ax (twisted pair inside screen) like the IBM AS/400 terminals (5250?) send differential signal down the cable. Dave -----Original Message----- From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On Behalf Of Joe Gwinn Sent: 10 January 2009 15:23 To: time-nuts@febo.com Subject: Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops Magnus, At 10:31 AM +0000 1/10/09, time-nuts-request@febo.com wrote: > >Message: 5 >Date: Sat, 10 Jan 2009 11:06:39 +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> > >Joseph, > >> time-nuts-bounces@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. You are right about the twisting. The cables are close and parallel, and ground offsets are the big problem, versus magnetic fields. 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. The problem is that the radar and the ship are not yet built, so we cannot yet make tests. > >>>> 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. True. > > 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> Joe _______________________________________________ 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.