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Original question:

CDCM7005-SP: STATUS_VCXO and PLL_LOCK abnormal

CDCM7005-SP: STATUS_VCXO going high without input

Todd Reed
Prodigy 130 points
Part Number: CDCM7005-SP

Hello,

We are able to turn the power to our VCXO off and on, and I have recently noticed that the STATUS_VCXO pin will change to high with no input.  I have put a scope on the VCXO output and there is nothing there, but the STATUS_VCXO pin stays high.  If I power up the VCXO, then the STATUS_VCXO pin goes high.  If I power the VCXO down, the STATUS_VCXO usually goes down, but then it switch back to high after a few minutes.  What could the cause of this be?

Thanks,
Todd

  • 0
    TI__Genius 12560 points

    Hi Todd,

    What is the SXOIREF bit setting in the configuration? For STATUS_VCXO select is should be (0). Otherwise it will reflect the I_REF_CP status at this pin.

    If possible, please share the used configuration file for debug.

    Thanks!

    Regards,
    Ajeet Pal

  • 0 in reply to Ajeet Pal
    Prodigy 130 points

    Hi Ajeet,

    Thanks for helping out!

    The SXOIREF bit is set to 0.  Here are the register settings:

    Word 0 - 0x00007000
    Word 1 - 0x208A8001
    Word 2 - 0xFA402012
    Word 3 - 0x00000027

    Thanks,

    Todd

  • +1 in reply to Todd Reed
    Prodigy 130 points

    Hi Ajeet,

    I may have found the problem.  Testing has been good so far.  I have a single-ended clock output that I convert to differential as such:

    I was wondering if when there is no clock the two signals are nearly identical and potentially noise could be picked up and interpreted as an input clock.  I adjusted R121 to 120 ohms to adjust one bias a bit and I haven't seen the VCO IRef go high by itself.  I see in the datasheet that the Vinpp min is actually 150mV, so maybe that is low enough to pick up some stray noise?

    Thanks,

    Todd

  • 0 in reply to Todd Reed
    TI__Genius 12560 points

    Hi Todd,

    That's correct. If they are using the single ended input signal and wants to maintain the noiseless input without AC input, Vinpp should be greater than 150mV.

    Thanks!

    Regards,
    Ajeet Pal