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explanation for discretes on ethernet PHY interface

Discussion in 'Electronic Design' started by Apparatus, Jul 4, 2004.

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  1. Apparatus

    Apparatus Guest

    Hello,

    I am looking at pg. 108 of 51471a.pdf (found on
    http://www.microchip.com), in particular at the FL1012 ethernet
    transformer chip. I see that before the transformers, there is a 200
    ohm resistor across the transmit lines. I also notice that on the
    primary sides of the transmit and receive transformers, there are
    0.1uF capacitors tied to the transformer center taps and ground. Why
    the need for these components?

    Cheers,
    Chandler
     
  2. This one?
    http://ww1.microchip.com/downloads/en/DeviceDoc/51471a.pdf

    Here is the relevant datasheet (for the Ethernet chip):
    ftp://152.104.125.40/cn/nic/rtl8019as/spec-8019as.zip
    This is an Ethernet interface, so it must (should) comply with the
    appropriate requirements:
    http://standards.ieee.org/getieee802/802.3.html

    The 200 ohm resistor is actually on the receive lines, and sets the
    input return loss, which is a requirement of 802.3 section 14.3.1.3.4
    The output return loss (also defined in 14.3) seems to be determined
    by resistors inside the '8019.

    The 100nF caps help reduce the common mode signal coupled across the
    transformer (by the few pF of stray cap between the windings). This
    helps to reduce EMI, and to reduce the rate of bit errors caused by
    common mode signals. See e.g. 802.3 14.3.1.3.5

    Regards,
    Allan.
     
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