Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Magnus, At 10:31 AM + 1/10/09, time-nuts-requ...@febo.com wrote: Message: 5 Date: Sat, 10 Jan 2009 11:06:39 +0100 From: Magnus Danielson mag...@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-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. 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
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 ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
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-boun...@febo.com [mailto:time-nuts-boun...@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 + 1/10/09, time-nuts-requ...@febo.com wrote: Message: 5 Date: Sat, 10 Jan 2009 11:06:39 +0100 From: Magnus Danielson mag...@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-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. 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. ___ 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
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Or 1553, for that matter -Original Message- From: David C. Partridge david.partri...@dsl.pipex.com Get 'em to use twin-ax (twisted pair inside screen) like the IBM AS/400 terminals (5250?) send differential signal down the cable. Dave ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Magnus, At 6:02 PM + 1/10/09, time-nuts-requ...@febo.com wrote: Message: 4 Date: Sat, 10 Jan 2009 19:02:09 +0100 From: Magnus Danielson mag...@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
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
In message of3277ac5a.f5d1fae8-on85257537.008059cf-85257537.00817...@mck.us.ra y.com, Joseph M Gwinn writes: That's technically speaking not triax, that's double shield. Triax would have the conductors and one shield. No, I think that's twinax: http://en.wikipedia.org/wiki/Twinax_cable. Triax is a center plus two concentric shields: http://en.wikipedia.org/wiki/Triaxial_cable. Sorry, I fumbled what I wrote there. I would say wiki is wrong here, the usage I am used to is: coax: single conductor + shield twinax: twisted pair + shield triax: the wires + shield (Who once lost all ethernet interfaces, the access control system and a few minor computers when a moron first created and then cut a 600+ A ground loop). Was there a big bang? What was the source of the 600 amps? They replaced the separation transformer with a UPS, and they connected the two sides ground together at the UPS. Unfortunately the grounding on our secondary side was much better than the power companys grounding on the primary side, which was the entire point of having the the transformer in the first place. Yes, there were a significant bang and his two-hand wire-cutter was recategorized from tool to industrial art. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
In message 49657762.5060...@rubidium.dyndns.org, Magnus Danielson writes: Was there a big bang? What was the source of the 600 amps? I think there (with some delay) was some awfull scream of dispare. The cost of Ethernet interfaces where much more significant back then. The most expensive one we lost was in a UNISYS 2200, where three microprocessors worked together to limit bandwidth to 100 kB/s. I belive the sticker prices as $15k. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Poul-Henning Kamp wrote: In message of3277ac5a.f5d1fae8-on85257537.008059cf-85257537.00817...@mck.us.ra y.com, Joseph M Gwinn writes: That's technically speaking not triax, that's double shield. Triax would have the conductors and one shield. No, I think that's twinax: http://en.wikipedia.org/wiki/Twinax_cable. Triax is a center plus two concentric shields: http://en.wikipedia.org/wiki/Triaxial_cable. Sorry, I fumbled what I wrote there. I would say wiki is wrong here, the usage I am used to is: coax: single conductor + shield twinax: twisted pair + shield triax: the wires + shield (Who once lost all ethernet interfaces, the access control system and a few minor computers when a moron first created and then cut a 600+ A ground loop). Was there a big bang? What was the source of the 600 amps? They replaced the separation transformer with a UPS, and they connected the two sides ground together at the UPS. Unfortunately the grounding on our secondary side was much better than the power companys grounding on the primary side, which was the entire point of having the the transformer in the first place. Yes, there were a significant bang and his two-hand wire-cutter was recategorized from tool to industrial art. Similarly for Quadraxial cable there are 2 interpretations: 1) an inner conductor surrounded by 3 coaxial tubular conductors all insulated from each other. 2) 2 twisted pairs with an outer tubular shield used in some high speed network cabling. Both meanings are in common use. Quintaxial cable seems only to be mentioned in texts on cable shielding. In which it consists of a central conductor surrounded by 4 coaxial tubular screens all of which are insulated from each other. Bruce ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
In message 49665a6d.2030...@xtra.co.nz, Bruce Griffiths writes: I have been unable to find a reference to triax consisting of 3 conductors within a shield, however such confusion is understandable given the confusion over quadrax:- I have only ever seen it used for very old 3-electrode condenser microphones. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Magnus, 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. 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. 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. 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. 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. 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
time-nuts-boun...@febo.com wrote on 01/08/2009 03:47:29 AM: In message OF3277AC5A.F5D1FAE8-ON85257537.008059CF-85257537. 00817...@mck.us.ray.com, Joseph M Gwinn writes: Was there a big bang? What was the source of the 600 amps? They replaced the separation transformer with a UPS, and they connected the two sides ground together at the UPS. Unfortunately the grounding on our secondary side was much better than the power companys grounding on the primary side, which was the entire point of having the the transformer in the first place. One assumes that there were too many cooks. Yes, there were a significant bang and his two-hand wire-cutter was recategorized from tool to industrial art. He probably needed a stiff drink after that. 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
On Thu, Jan 08, 2009 at 08:51:45AM +, Poul-Henning Kamp wrote: In message 49657762.5060...@rubidium.dyndns.org, Magnus Danielson writes: Was there a big bang? What was the source of the 600 amps? I think there (with some delay) was some awfull scream of dispare. The cost of Ethernet interfaces where much more significant back then. The most expensive one we lost was in a UNISYS 2200, where three microprocessors worked together to limit bandwidth to 100 kB/s. I belive the sticker prices as $15k. I'm somewhat confused about how this took out Ethernet transceivers or interfaces... from the beginning even vampire tap RG-8 yellow cable Ethernet transceivers were ground isolated from chassis ground of the computer system just exactly to avoid ground loops and back path ground currents. Both power and transmit/receive and control signals are isolated... and usually transformer coupled... and as I remember it rather a substantial voltage difference between shield on the cable and computer system ground had to be tolerated (hundreds of volts at least)... I guess, however, if someone grounded the yellow cable at more than one point enough current could flow on its outer conductor to induce substantial voltage between the shield and the center conductor which could trash the driver/receiver/carrier sense chips or protective clamp diodes ... One was never, of course, supposed to ground the yellow cable at more than one point... -- Dave Emery N1PRE/AE, d...@dieconsulting.com DIE Consulting, Weston, Mass 02493 An empty zombie mind with a forlorn barely readable weatherbeaten 'For Rent' sign still vainly flapping outside on the weed encrusted pole - in celebration of what could have been, but wasn't and is not to be now either. ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Joseph, Joseph M Gwinn skrev: Magnus, time-nuts-boun...@febo.com 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. 10 MHz into a transmitter. Should not be too hard to master. For some reason I feel confident in that environment. :) 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. I see. It is fairly easy to induce common mode currents and DC voltages. An isolational transformer from a source and then on the other side simply DC offset or apply signal through a transformer if not directly from an amplifier. 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. Great. 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. As always. My problem is not safety, it is tolerance of conducted EMI. The reason I mention safety is that some people suggest solutions which does not fullfill the safety criteria in spirit or standard. It gets you into the right category of solutions. 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. You could also look up ETSI EN 300 132-* and EN 300 386 which is relevant for telecom boxes. Further on is EN 300 199-* probably good to have around, but maybe not so applicable to this particular problem. What you want to transfer is similar to an E1 or E2 on an intra-office link. 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. Sounds good. Will think about levels. What makes the field a bit complex is that you need to think about failures, EMC, bonding, interference, lightning
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
In message ofade54b4f.d29dba7a-on85257537.00086866-85257537.00090...@mck.us.ra y.com, Joseph M Gwinn writes: The effect of differing ground potentials on a shielded cable is to pull a large current through the shield, [...] The correct enginering solution is to use twinax, ground the shield in one end only and transformer-couple the signal at least in the other end from the grounding. Look at IBM's 5250 terminal hookup for an school book example of getting it right. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Magnus, time-nuts-boun...@febo.com 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:
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Poul-Henning, time-nuts-boun...@febo.com wrote on 01/07/2009 04:25:04 PM: In message OFADE54B4F.D29DBA7A-ON85257537.00086866-85257537. 00090...@mck.us.ra y.com, Joseph M Gwinn writes: The effect of differing ground potentials on a shielded cable is to pull a large current through the shield, [...] The correct enginering solution is to use twinax, ground the shield in one end only and transformer-couple the signal at least in the other end from the grounding. Yes, I know of this. Shielded twisted pair is also widely used in audio, for the same reasons. But my system is coax and was that way before I arrived. I know of some similar but ship-board systems that use twinax for time reference distribution, as you suggest. Don't know if they use transformers though. Could be a differential TX and RX. I recall that they send a RS422 signal. I imagine that the shield is grounded at both ends, if only for safety reasons. Fortunately, my system is not so noisy as a ship. If I had it to do over, I might well use multimode fiber. Look at IBM's 5250 terminal hookup for an school book example of getting it right. A blast from the past - shades of the 1970s! A parallel story: Some years ago I was working on shipboard systems that used 10BASE5 ethernet over thick coax (nominally RG8). The problem is that there is no real ground on a ship, and there can be 7 volts difference between bow and stern because the hull is used as the power system neutral. Well, 10BASE5 ethernet uses 2-volt signals, so 7 volts offset would prevent communications. The solution was to use triax. The outer shield was grounded at both ends. The inner shield and center conductor together formed the ethernet media. The inner shield was connected to the outer shield in exactly one place. For safety, this connection had to be able to handle 1,000 amps, to ensure that breakers would pop before ground links opened. (One of our young engineers was going to use a AWG #30 wire-wrap link.) The outer shield stopped at the cabinet I/O panel, with only the inner shield and center conductor continuing (as a bit of RG58) to the etherent transceivers. This worked flawlessly. 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
In message of56303512.93b049a7-on85257537.0079cde3-85257537.007cc...@mck.us.ra y.com, Joseph M Gwinn writes: Could be a differential TX and RX. I recall that they send a RS422 signal. Depending on the speed, RS422 works fine with transformers. 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. 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. If I had it to do over, I might well use multimode fiber. Yes, never roll copper more than 100m or between buildings if you can get away with installing fiber. The solution was to use triax. The outer shield was grounded at both ends. The inner shield and center conductor together formed the ethernet media. The inner shield was connected to the outer shield in exactly one place. That's technically speaking not triax, that's double shield. Triax would have the conductors and one shield. But yes, double shielding works great, provided you don't have morons with screwdrivers around. Poul-Henning (Who once lost all ethernet interfaces, the access control system and a few minor computers when a moron first created and then cut a 600+ A ground loop). -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Poul-Henning, time-nuts-boun...@febo.com wrote on 01/07/2009 05:56:19 PM: In message OF56303512.93B049A7-ON85257537.0079CDE3-85257537. 007cc...@mck.us.ra y.com, Joseph M Gwinn writes: 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. 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. 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. If I had it to do over, I might well use multimode fiber. Yes, never roll copper more than 100m or between buildings if you can get away with installing fiber. The solution was to use triax. The outer shield was grounded at both ends. The inner shield and center conductor together formed the ethernet media. The inner shield was connected to the outer shield in exactly one place. That's technically speaking not triax, that's double shield. Triax would have the conductors and one shield. No, I think that's twinax: http://en.wikipedia.org/wiki/Twinax_cable. Triax is a center plus two concentric shields: http://en.wikipedia.org/wiki/Triaxial_cable. The terms are very similar. But yes, double shielding works great, provided you don't have morons with screwdrivers around. Poul-Henning (Who once lost all ethernet interfaces, the access control system and a few minor computers when a moron first created and then cut a 600+ A ground loop). Was there a big bang? What was the source of the 600 amps? 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
Poul-Henning Kamp skrev: In message of56303512.93b049a7-on85257537.0079cde3-85257537.007cc...@mck.us.ra y.com, Joseph M Gwinn writes: Could be a differential TX and RX. I recall that they send a RS422 signal. Depending on the speed, RS422 works fine with transformers. You want a DC balanced encoding if you send data down the line, otherwise you will saturate the transformer and saturation works very well as method of damping. This was actually used for modulation of high powers and then the transformer was called a transductor, essentially a transformer with too small core. This is the core at the Grimeton long wave transmitter (a world heritage site) in south of Sweden. Lovely thing to visit. The 127 m high and 1,8 km long antenna is not easy to miss. I beleive the output effect was 200 kW at 17,2 kHz. It has a definitive steam-engine feel to it. Lovely. Usually thought the signal is just dampend out. Only transitions survive. A pure 10 MHz is not a problem at all, but generic RS422 may not survive. 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. Which is what most bonding network standards will describe never the less. 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. You most probably want to use a capacitor from shield to chassi both for providing a low impedance path for RF and static electricity blasts but also helps in reducing the RF emission. Clamping an external RF choke on the cable will be meaningfull when the cap is there as the RF choke is being properly terminated. If I had it to do over, I might well use multimode fiber. Yes, never roll copper more than 100m or between buildings if you can get away with installing fiber. So true. Not that you can't get it to work, but it is tedious to make it work under all conditions. The solution was to use triax. The outer shield was grounded at both ends. The inner shield and center conductor together formed the ethernet media. The inner shield was connected to the outer shield in exactly one place. That's technically speaking not triax, that's double shield. Triax would have the conductors and one shield. But yes, double shielding works great, provided you don't have morons with screwdrivers around. The ethernet habit of using vampire clamps provided a great opportunity for less insightful installation practices. Poul-Henning (Who once lost all ethernet interfaces, the access control system and a few minor computers when a moron first created and then cut a 600+ A ground loop). As I said... :) Remember, we need to support our morons in their daily task. :) 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
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 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. 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. If I had it to do over, I might well use multimode fiber. Yes, never roll copper more than 100m or between buildings if you can get away with installing fiber. The solution was to use triax. The outer shield was grounded at both ends. The inner shield and center conductor together formed the ethernet media. The inner shield was connected to the outer shield in exactly one place. That's technically speaking not triax, that's double shield. Triax would have the conductors and one shield. No, I think that's twinax: http://en.wikipedia.org/wiki/Twinax_cable. Triax is a center plus two concentric shields: http://en.wikipedia.org/wiki/Triaxial_cable. The terms are very similar. I have some triax cables and connectors, but not twinax... But yes, double shielding works great, provided you don't have morons with screwdrivers around. Poul-Henning (Who once lost all ethernet interfaces, the access control system and a few minor computers when a moron first created and then cut a 600+ A ground loop). Was there a big bang? What was the source of the 600 amps? I think there (with some delay) was some awfull scream of dispare. The cost of Ethernet interfaces where much more significant back then. 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.
[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
During my experiences involving audio/phone, video and data transmission, we were taught to ground the shield at one end only so we would not cause a ground loop. I ran into problems everywhere I went with this and as much as folks disdain transformers, they are your friend in this type of problem. Don White Consultants/Interference Control Technology published a whole series on EMI, Grounding, and EMC for the military. They are located in Gainesville, VA. Brian Joseph M Gwinn wrote: 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 -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
At 4:59 AM + 1/7/09, time-nuts-requ...@febo.com wrote: Message: 6 Date: Tue, 06 Jan 2009 21:54:41 -0600 From: Brian Kirby kirb...@bellsouth.net 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 Message-ID: 49642781.2020...@bellsouth.net Content-Type: text/plain; charset=ISO-8859-1; format=flowed During my experiences involving audio/phone, video and data transmission, we were taught to ground the shield at one end only so we would not cause a ground loop. Yes, it's impossible to do this in a system of any size. In my experience, the RF cables connect the arms of the star-grounding system, causing loops. So, the receivers had to be immune. The problem is to quantify and specify the required degree of immunity. I ran into problems everywhere I went with this and as much as folks disdain transformers, they are your friend in this type of problem. DC blocks (usually a series capacitor) also work at RF. But we would have a lot of them. And we would still need some kind of spec to require, to know when we are done. Don White Consultants/Interference Control Technology published a whole series on EMI, Grounding, and EMC for the military. They are located in Gainesville, VA. But do they publish formal and official requirements documents? That's what I need, versus training. Thanks, 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.
Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
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. 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. For most purposes an isolational 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. 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. 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. 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. 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 stdandard MIL-STD-1310 for ships: http://www.earth2.net/parts/basics/milstd1310g.pdf 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 sucsess stories to be told. 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
