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Signal Amplitude Symmetry DP83848I

Fokke Hoekstra
Prodigy 20 points
Other Parts Discussed in Thread: DP83848I

Dear Reader,

During ethernet compliance tests using the DP83848I Phy we passed all the tests except the "ANSI X3.263-1995: 9.1.4 Signal Amplitude Symmetry". Specification: 0.98 <-> 1.02, test result 1.041. Checked the datasheet and found the phy to have 2% tollerance, so theoratically even using 0.1% resistor I'd not be able to meet the specification..... Phy specification from datasheet: VTPTDsym PMD Output Pair 100M Transmit Voltage Symmetry + 2 %

Anyway 4% is way passed to 2% so can you advise how to improve the performance?

Thanks, Fokke

  • 0
    TI__Mastermind 31600 points

    The datasheet value matches the IEEE specification value.  The intention is to indicate that the device meets the specification.  In our in-house testing as well as in external 3rd party testing at the UNH Interoperatbility Lab, the DP83848 has been shown to pass this specification with margin. 

    If you would like to provide your schematic for review and the details of your test methodology, I will make some recommendations on how to improve the measurement.

    Patrick

  • 0 in reply to Patrick OFarrell
    Prodigy 20 points

    Attached the schematics.

    The tests were performed at an external test lab in Belgium "Testonicslabs".7750.Phy layout WT130225.docx

    Fokke

  • 0 in reply to Fokke Hoekstra
    TI__Mastermind 31600 points

    Fokke,

    Capacitors from the MDI pins (TD+/-, RD+/-) to ground are not recommended.  What value of capacitor is used for CET41, CET42, CET43, and CET44? 

    We recommend de-coupling the transformer center tap pins with 0.1uF capacitors to ground near the transformer.  It does not appear that this was done.  We also recommend 0.1uF de-coupling capacitors to ground from the common point of the 50 Ohm termination resistors.  It is not clear if this was implemented.

    We do not generally recommend ferrite beads in between the 3.3V supply and the transformer.  What component was used for LET1? 

    Lastly, could you identify the transformer (TET1) and the connector (JET1) that were used? 

    Patrick