My comments were more related to the situation of a floating ground that was not well grounded to the main ground. It seems to me that you may have a common mode noise problem. Noise is probably being coupled to the ground plane and using the shield on the DC cable as an antenna. Looping the wires that feed the D-sub in your unit) a couple of times around a lossy ferrite toroid is one way to decouple the cable's shield from the ground in your unit.
Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Gary McInturff [mailto:gary.mcintu...@worldwidepackets.com] Sent: February 25, 2002 4:14 PM To: Robert Wilson; emc-p...@majordomo.ieee.org Subject: RE: PCB floating area layout Jeeez, I have just the opposite problem GROUNDING a PCB at a certain location. The device is a metal enclosed digital device, about the size of a video cassett tape. Its powered by one of those small AC/DC power supplies like you have on your laptop computer. The DC power cable does have a shielded cable, with a drain wire in contact with the foil. Both are connected at shielded 9 pin D and grounded to the case. The other end however, is just a drain wire into the AC/DC supply. Its a plastic box. The PCB is grounded in several locations to the bottom of the metal enclosure, at roughly 37 cm (5 inches). At about 800 Mhz the thing radiates above class B by 3 or 4 dB. If I remove one of the ground points the signal drops 5 to 8 dB. Re-ground and its up, unground and its down. Guess, I should feel lucky. I can remove the ring and via so that it doesn't get grounded there and the problem is gone. I just get real nervous with that answer, but it works so the engineer boss is not to crazy about making any mods. Gary -----Original Message----- From: Robert Wilson [mailto:robert_wil...@tirsys.com] Sent: Monday, February 25, 2002 12:19 PM To: emc-p...@majordomo.ieee.org Subject: RE: PCB floating area layout Floating "grounds" on PCBs tend to be problematic, especially at high frequencies. Minimizing the effective capacitive reactance between the floating "ground" and the real ground will ensure that the floating ground is "AC Cold". I don't just mean bypassing it with (say) a few thousand uF of electrolytic capacitance, but instead ensuring minimal capacitive reactance to ground across the entire frequency band of interest. This usually entails (as an example) paralleling something like a 0.1uF cap, and with an NP0 1000pF cap (or similar). If the floating "ground" not properly decoupled to the main ground, and it is a significant proportion of a HF wavelength, then it can have very high AC voltages superimposed and act as a marvelous antenna. Reducing the size of the floating "ground" is always a good plan. Increasing it merely means that you have a larger potential antenna. Bob Wilson TIR Systems Ltd. Vancouver. -----Original Message----- From: Paolo Peruzzi [mailto:paolo.peru...@esaote.com] Sent: February 25, 2002 7:34 AM To: emc-p...@majordomo.ieee.org Subject: PCB floating area layout Hi all, I'm dealing with a PCB that has a floating section isolated from the rest of the board for safety purposes (patient applied part). I found out some problems with emissions, due to the coupling between the floating part and of the PCB and the earthed one. My questions are concerning the layout design of the floating area: 1) Is it best to minimize the HF capacitive coupling between the earthed ground and the floating ground or to maximize it? 2) Is it best to reduce the amount of the floating ground or to increase it? Does it depend on the goodness of the "main ground", i.e. how much it is "cold" ? (I see the board as a dipole with one end connected to earth, and the other floating). Thanks, p.p. ------------------------------------------------------------- ESAOTE S.p.A. Paolo Peruzzi Research & Product Development Design Quality Control Via di Caciolle,15 tel:+39.055.4229306 I- 50127 Florence fax:+39.055.4223305 e-mail: paolo.peru...@esaote.com ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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