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Any experience with ethernet layout on a PCB?

  1. Feb 8, 2008 #1
    I designed an ethernet circuit, and I am trying to lay it out on the board. The app notes and data sheets specify that the ethernet ground be isolated from the system ground and to be connected through a 0 ohm resistor. There are to be no ground or power planes underneath the magnetics and RJ45 connector too. . so where would my ethernet ground be if i can't have a ground plane under the connector/magnetics? My particular IC/magnetic/connector companies provide no specific guidelines on pcb layout, but there is obviously some very important rules to follow that I can't find.

    I have been looking on other boards I can find, and there seems to be a very specific pattern on layout, however I can't see the internal layers, so I'm a little in the dark. It looks like a keep out zone around these components, and I have done some reading around and see trace lengths and widths on the differential Rx and Tx lines being stated specifically for other ethernet app notes, but nothing that I can go back to and say 100% that it applies to my design.


    Does anyone know if there is a standard that requires specific layout guidelines for 10BASE-T ethernet circuitry? I don't want to make up my own rules for layout if it has a standardized layout strategy.

    I hope someone here has done an ethernet layout and can point me in the right direction please.
     
  2. jcsd
  3. Feb 8, 2008 #2
  4. Feb 8, 2008 #3

    mgb_phys

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    Science Advisor
    Homework Helper

    I wouldn't worry too much about track lengths at 10base speeds.
     
  5. Feb 8, 2008 #4
    I just found a lot of good documents for my design specifically, but I'm still interested to know what anyone has to say about this topic.
     
  6. Feb 8, 2008 #5

    berkeman

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    Staff: Mentor

    There are a number of reasons for the different PCB layout guidelines for network transceivers. Some are related to data integrity, but others are related to transient immunity (like to ESD and Surge hits), while still others are related to keeping your own radiated EMI (electromagnetic interference -- what your device radiates) withing the legal limits.

    Here is a link to a User's Guide for a network transceiver that I've worked on:

    http://www.echelon.com/support/documentation/manuals/transceivers/078-0156-01G.pdf

    In chapter 2 there are some PCB layout guidelines for this (not Ethernet) transceiver, with explanations about the various guidelines, like what keepout areas are meant to accomplish. Chapter 5 also has a separate discussion of EMI-related issues, including PCB layout issues and package/product design issues.

    In general, you will use things like separate grounds and keepout areas to guide ESD and Surge transient currents back to Earth ground, without having those currents go through any sensitive circuitry. You also use keepout areas to keep noisy digital circuitry away from the network connections (transformer and its traces, etc.), so that digital noise crosstalk does not make it out of the device onto the network wires (where it will radiate away and cause EMI issues). It is also *extremely* important to follow "star ground" floorplanning and PCB layout of your product, or you will definitely have EMI qualification isssues down the road.

    Hope that helps. Post if you have more questions.
     
  7. Feb 8, 2008 #6
    What exactly is star ground? My guess from the name is to spread out the ground into directions from a center point. Our board I'm working on has a complete ground plane.
     
  8. Feb 8, 2008 #7
    it means all the ground is connected directly to a common point. For example, lets say you have 3 point, A,B and C all go to ground.
    You can connect A-B-C-ground and that is the same as A-ground, B-ground, C-ground. But by connecting A-B-C-ground it means you have to pass through further lines just to get to ground.
    Connect all local ground for each stage to a local/common point. Like digital ground and analog ground, isolate them but connect them at just 1 point to isolate noise generated by digital lines.
     
  9. Apr 19, 2010 #8
    The best way to describe star grounding is this.
    Signals and power generate noise through the copper they are travelling. To eliminate the noise from reaching the outside world you have to match the return path of the signal in to opposite direction to the signal. Power out match to ground in. The signal through a ground plane will take the shortest route to the start. If your signal follows a different route, then you will generate external noise.
    So, you have a signal route with it's ground underneath it for best noise suppression.
    This then has the grounds for different signals with thier own planes, route etc. and they all return to a single point (star point) to complete the connection.
    This stops signals taking different paths from the grounds and reduces emc noise etc.
     
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