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LMX2492EVM: Clock Reference with OSC In Issues

Jay Shenoy
Prodigy 30 points
Part Number: LMX2492EVM
Other Parts Discussed in Thread: LMX2492

Hi,

I'm trying to generate an FMCW chirp using a 10Mhz external clock. Currently I have this reference signal hooked up to osc_in.

I'm wondering how to configure the software and PLL settings in TICS pro so that I can properly generate the chirp. Right now,

nothing seems to be generate after I attached the reference clock. Also, do you really need to remove R21 and R28 from the EVM to

use the reference clock?

  • +1
    TI__Guru* 81250 points

    Hi Jay,

    The EVM user's guide has some examples, were you able to replicate the examples? Also the user's guide has a section regarding using different reference clock, please take a look.

  • +1 in reply to Noel Fung
    Prodigy 30 points

    Hi Noel, 
    Thanks for the response. I saw the EVM user guide and it mentions in appendix B the removal of two resistors on the EVM. In terms of software it doesn't have much regarding the reference clock. Let me know if there's a specific place in the user guide I should look.

  • 0 in reply to Jay Shenoy
    TI__Guru* 81250 points

    Hi Jay,

    There is no software setting for reference clock.

  • 0 in reply to Noel Fung
    Prodigy 30 points

    Doesn't the value of the PLL need to be changed in TICS Pro? Also, do the resistors need to be removed? Is there a simple way to do this?

  • 0 in reply to Jay Shenoy
    TI__Mastermind 28530 points

    If you are changing from on-board 100 MHz reference to off-board 10 MHz reference, you would need to change the OSCin frequency from 100 MHz to 10 MHz in TICS Pro.
      > With your 10 MHz reference, be aware of slew rate requirements.  For optimal phase noise a slew rate of at least 3 V/ns is recommended.  At very low slew rates, you may have locking issues.

    R21 and R28 should be removed from the on-board reference to prevent spurs and improve signal integrity.

    73,
    Timothy

  • 0 in reply to Timothy T
    Prodigy 30 points

    Hi Timothy,
    Thank you for this response. I have reduced the PLL in TICS Pro. However, I do not see any signal output from the LMX2492. I have not removed R21 or R28. Does the board automatically detect the reference clock input and switch from the onboard clock? Is there anything else I need to do to properly generate a signal output?

  • 0 in reply to Jay Shenoy
    TI__Mastermind 28530 points

    There is no automatic switching of the reference.  The LMX2492 has only one, although differential reference input.  Without removing those resistors, you've hard-wired the on-board XO reference with whatever your off-board reference is.  So it's probably confused.

     - Did you confirm all was well and locking using the on-board reference?  To become familiar with the device, I suggest following the EVM guide examples first.

     - Next I would then remove the resistors, provide you're off-board clock and proceed with updates as required for your configuration/objective.

    73,
    Timothy