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DP83867IR: Overshooting PHY signal on probing

Sanjeev Karaiyan
Expert 6740 points
Part Number: DP83867IR

Hello,

I am trying to probe TX_CTL and RX_CTL signals of the gigabit PHY DP83867. I use a 100MHz (10MOhm, 11pF front end) Mixed signal oscilloscope (Agilent MSO6014A) with a 500MHz, 11pF CAT I, II probe (Keysight N2834A). I see some overshoot on the rising and falling edges of the signal and I am not sure if this is due to probing or the signal itself has the overshoot.

 

How do I make sure the probing (parasitics in probe and scope) doesn’t affect the signal? What is the recommended setup to measure such fast transitioning signals?

Thanks,

Sanjeev

  • 0
    TI__Mastermind 19780 points
    Hi Sanjeev,

    The rise/fall time of RGMII signals is 750ps, which puts the estimated BW of the signal at ~450-500MHz(assuming a low-pass RC network).

    Are your probes properly calibrated? An 11pF probe on the circuit should not introduce wildly inaccurate results like you describe in your other thread (1V spikes on a 1.8V rail). How much ringing is being seen?

    Another thing to note for the TX_CTL is that a series resistor will likely be needed to match the output impedance of the driver to PCB traces. The RX_CTL signal has integrated termination resistors in the PHY.

    Best Regards,
  • 0 in reply to Rob Rodrigues
    TI__Expert 6740 points
    Hello Rob,

    I replaced the setup with a 500MHz scope and a 1GHz active probe. Now the results are much better.

    I was seeing 0.5V overshoot. Now I am not seeing it.

    The 1V pulse issue is still seen. These are not spikes at the rising or falling edge. The RX_CTRL remains at 1V level for the entire duration of reception of some frames.

    Thanks,
    Sanjeev
  • 0 in reply to Sanjeev Karaiyan
    TI__Mastermind 19780 points
    Hi Sanjeev,

    I hope this is the related to the other question you have open. I am going to close this thread and we will continue discussing on here.

    Best Regards,