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Communication line

Discussion in 'Electronic Design' started by Martin, Nov 29, 2005.

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  1. Martin

    Martin Guest

    Hi, I need to connect two devices with fpgas over 50 meters on about
    100-150Mbits full duplex. I'm thinking about LVDS, but I know that
    there could be a problem with different grounds - I cannot connect the
    grounds of the devices, that could create current loops. Can this
    problem be somehow solved? Or is there any other relatively cheap
    solution on this (in these days) slow communication, for example some
    cheap optics?

    Martin
     
  2. John_H

    John_H Guest

    Consider using USB2.0 magnetics. By using a transformer capable of
    delivering the differential signal while isolating the grounds, you get your
    signal across. You can use LVDS without grounding troubles. It may be
    beneficial to encode the data stream with an 8b/10b scheme to keep DC from
    pushing the transformer toward saturation. Other, simpler baluns than the
    USB2.0 magnetics could also get you what you want - I'm just thinking of
    availability.
     
  3. There are 100MBit magnetic couplers from Analog Devices.
    These plus LVDS should do.

    Rene
     
  4. Guest

    The baluns must be transmission-line transformers to work at the
    frequency desired.

    I may be wrong, but my impression is that you can have a balun that is
    not a transmission line transformer, and it is certainly possible to
    have a transmission-line transformer that isn't a balun.

    I've used exactly this approach, using twisted pair made with 0.15mm
    diameter enamelled (transformer) wire in a single layer on an RM6 pot
    core, to carry 8b/10b (TaxiChip) data. It worked fine, but we didn't
    need to cover 50 metres.

    The transformer winding was about one metre long, which meant that the
    transmission started rolling off above 130MHz.The number of turns (13)
    and the core inductance gave a low frequency roll-off starting at about
    50kHz.
     
  5. Martin

    Martin Guest

    Do you know a partnumber? Because I cannot find any magnetic coupler on
    Analog's site...

    Martin
     
  6. Martin

    Martin Guest

    Thanks, do you know who make these USB2.0 magnetics?
     
  7. Since when are magnetics used in USB2.0?

    Meindert
     
  8. Mac

    Mac Guest

    Maybe use a Fast Ethernet phy, together with all the usual Ethernet
    magnetics and so-on?

    Or look into a serializer?

    --Mac
     
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