John, Really good information there regarding treatment of chassis ground & evacuation of other layers. I'd add the comment that the layout of the signal etch between the transformer & the RJ45 connector (UTP) is also important. Keep the +/- pairs as short, tightly coupled and as symmetrical as possible. We usually do use discrete CM chokes and have never tried backing them out.
Regards, Jack Cook Xerox EMC Engineer -----Original Message----- From: jrbar...@lexmark.com [mailto:jrbar...@lexmark.com] Sent: Wednesday, October 17, 2001 12:57 PM To: Jon Keeble; emc-p...@majordomo.ieee.org Subject: Re: 10/100 base interface in a plastic box Jon, My department has developed ten generations of Ethernet adapters (10BASE2, 10BASE-T, 100BASE-Tx) for IBM/Lexmark printers since 1990. We have looked at, but so far have not used, integrated-magnetics connectors because we like having the option of putting a common-mode choke in between the Ethernet magnetics (transformer-filter) and the connector. In our card layouts we: 1. Define a FRAME_GROUND to connect the metal bodies/shields of all connectors going to the outside world. 2. Connect FRAME_GROUND to GROUND with 4-or-more "ground ties", 0.025-inch wide traces on the topside and bottomside of the card. We use at least one pair of ground ties for every 3 inches of "beach front" with connectors going to the outside world. 3. For the Ethernet interfaces, put a void in all layers stretching from the center of the transformer-filter to the farthest pins of the RJ-45 connector, 0.2" wider than the transformer-filter, common-mode choke, and RJ-45 connectors. The *only* wires permitted in this area are the Ethernet transmit/receive signals. 4. Run FRAME_GROUND down the edge of the card, as wide as we can make it, ending in mounting pads for a metal bracket or the chassis. These pads have non-plated-through holes for the mounting screws circled by eight vias, and are plated with tin or tin-lead on the topside and bottomside. FRAME_GROUND has the same outline in all copper layers, although we sometimes have to leave it as a void in ground planes because of a quirk in Mentor Graphics. Put a via about every 1/2 inch along FRAME_GROUND to connect the layers together. 5. Place a ground tie at each mounting pad topside and bottomside, with additional topside and bottomside ground ties roughly equally spaced in between the mounting pads. During development testing, these groundties can be easily cut with an X-acto knife if it looks like separating FRAME_GROUND from GROUND, or having them connected at only one end, might improve radiated emissions and electrostatic discharge (ESD) immunity. Make sure that solder does not get onto the mounting pads during manufacturing. The mounting pads sit right on the lugs of the metal bracket/chassis. These contact points on the metal bracket/chassis must be bare metal. We recently discovered that a transparent phosphate wash applied to a chassis before powder coating, as a priming step, seriously affected radiated emissions and the ESD immunity. We now require these contact points to be masked off before the chassis goes through any cleaning/painting steps. A machine screw with a built-in belleville washer, and a nut with a captive star washer, hold the card and the metal bracket/chassis in tight metal-to-metal contact despite temperature changes, vibration, creeping of the plating, etc. For our External Network Adapters, the metal bracket is bent into L, and extends all the way under the card. This bracket ties the faces of metal connectors together, connects to FRAME_GROUND, and provides a ground plane all the way under the card to reduce radiated emissions and reduce our susceptibility to tabletop ESD (an IBM test). From the side the card, connectors, and bracket look like this: ! !+------+ !! ! !! ! !======================================== +---- ! <-- insulating spacer, or tab bent up to support card +---------------------------------------- here-- experiment to see whether having the card and bracket isolated or connected gives the best EMC/ESD results John Barnes Advisory Engineer Lexmark International ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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