The characterization of the effect of stray capacitance between a victim radio and ground is only a theoretical construct. In fact, all the measurements were made in an electrically small unlined shielded chamber with the radio sitting on the floor, which was sheet metal. So the capacitance, whatever it was, was higher than what would be encountered in the real world, and the the fact remains that the radios were 20 dB more sensitive to cm than to dm, and also the average value of the cm to which they were sensitive was precisely 48 dBuV, the FCC Class B limit back in the day.
Ken Javor Phone: (256) 650-5261 From: Richard Marshall <[email protected]> Reply-To: Richard Marshall <[email protected]> Date: Mon, 4 Apr 2016 21:22:36 +0100 To: <[email protected]> Subject: Re: [PSES] Common mode current vs. differential mode current and LISN Ken, I do not see any disagreement on the conversion of DM to CM, only differences about what happens AFTER the conversion has happened. Then there are two possible views; FIRST VIEW This is the ³What is on the Circuit diagram² view that is only valid at low frequencies . if I understand it properly, that is your argument and from that position I agree with everything that you say, but consider further the implications of the last sentence, repeated below; It is only when the signal is referenced to ground that the radio receiver becomes sensitized to it, because it has no line-to-ground filter elements (Y-caps). SECOND VIEW The high-frequency radio engineer¹s view takes account of the transmission line properties of the phase-to-neutral pair, and also of the capacitance to ground of the of the victim (radio in your example). I affirm that; - Once the DM interference has been partly converted to CM as we have all agreed then that CM will be reflected back along the phase-to-neutral pair towards the source in common-mode. That is the phase & neutral are effectively in parallel and ground is the return path. Waves travelling along transmission lines are reflected at points of mismatch. This is how ³ Standing Waves² are produced and they are well-known phenomena. Since CM current flow radiates that was unchallenged on the first day of this thread the unbalanced mismatch at the LISN will necessarily cause the radiation of interference FROM THE INCOMING CABLE. My original post did not mention LISNs. I commented on the real-world situation but it is much the same-or worse. - If we introduce to the scene a victim radio connected across the phase-to-neutral pair, then the FIRST VIEW correctly says that the radio¹s bulk cap does its job of filtering DM. However, the radio sees the shift in the reference level of the DM signal, because components of the radio have some stray capacitance to true ground. The radio may be a very small box on a wooden table, in which case that capacitance is only a few 10¹s of pF, or it may be huge if the ³radio² is a Hi-Fi radio with connection to a roof-top antenna and to several loudspeakers. This stray capacitance allows the radio to sense CM on its mains cable. A ³shift in the reference level of the DM signal² is another way of saying ³CM² and so that now has the capability of creating interference if the receiver design is inadequate. There are many ways for stray currents to get into low-level analogue circuitry. I did not mention radios either: the analysis is valid across the board. So it is the circuit components that are not on the equipment circuit or on its cabling layout that matter at Radio Frequencies and that is where EMC gets seriously interesting. Richard Richard Marshall Laboratories, 30 Ox Lane, Harpenden, Herts., AL5 4HE, UK +44 (0)1582 460815 www.design-emc.co.uk <http://www.design-emc.co.uk> Member of the EMC Industry Association From: Ken Javor [mailto:[email protected]] Sent: 03 April 2016 17:48 To: [email protected] Subject: Re: [PSES] Common mode current vs. differential mode current and LISN I think there is a fundamental misunderstanding of the mechanism converting dm into cm, both on the part of Michel Mardiguian back in the day and those who repeat his criticism in this thread. What people are saying, (if I understand it properly) is no different than if I used two LISNs, one with a different inductor value than the other. In that case, at frequencies where at least one LISN did not look like 50 ohms, the EMI port measurement for the dm part would be different on each LISN, and that would make it look like a cm signal, and therefore we have a problem. But that is flatly incorrect. The issue is not the relative value of the dm portion at each LISN, but rather the reference for the signal. For dm, regardless of ³balance² both dm LISN contributions sum and they are referenced from phase-to-neutral, not conductor-to-ground. As long as the dm contributions are phase-to-neutral, the bulk cap in the victim radio power supply does its job of filtering and doesn¹t care how much dm is measured to ground at each LISN. It is only when the signal is referenced to ground that the radio receiver becomes sensitized to it, because it has no line-to-ground filter elements (Y-caps). This point was made abundantly clear in the report that was presented to TC77 back in 1998 (or thereabouts). Ken Javor Phone: (256) 650-5261 From: Richard Marshall <[email protected] <mailto:[email protected]> > Reply-To: Richard Marshall <[email protected] <mailto:[email protected]> > Date: Sun, 3 Apr 2016 12:54:43 +0100 To: <[email protected] <mailto:[email protected]> > Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN Rich I¹m sorry! I did notice the X when it should have been Y when I received my list copy. I decided to wait to see who was alert enough to correct me. Well done and Thank you. The usual class Y value of 4,700pF presents an impedance of only 1.13 ohms at 30MHz, so it provides a very substantial unbalance to create CM from a neighbour¹s DM Richard Richard Marshall Laboratories, 30 Ox Lane, Harpenden, Herts., AL5 4HE, UK +44 (0)1582 460815 www.design-emc.co.uk <http://www.design-emc.co.uk> <http://www.design-emc.co.uk> <http://www.design-emc.co.uk> Member of the EMC Industry Association From: Richard Nute [mailto:[email protected]] <mailto:[email protected]]> Sent: 02 April 2016 18:24 To: [email protected] <mailto:[email protected]> Subject: Re: [PSES] Commom mode current vs. differential mode current and LISN - A device with a single class X capacitor from neutral to ground. Safety standards require the capacitor to be Class Y. Rich - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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