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Original question:

LMX2594: several questions about LMX2594

LMX2594: LMX2594 inconsistent Vtune while Ramping

Sam Solondz
Prodigy 20 points
Part Number: LMX2594
Other Parts Discussed in Thread: LMX2491

I am using the LMX2594 evaluation board, the TI Reference PRO board and TICS software. While using the following settings, if the LMX2594 is power cycled and registers are rewritten, the output of Vtune does not stay consistent. Occasionally Vtune is an ideal triangle wave that is expected, but this is not always the case. Is there a way to make the output consistent?

  • 0
    TI__Mastermind 28011 points

    LMX2594 Calibration Free Ramping.pdfSam,

    In summary, the issue is that your 62.5 MHz is right on the edge of how far the VCO can go without calibrating.    When you calibrate the VCO, you get different settings, specifically VCO_CAPCTRL.  This shifts the tuning votlage slightly, which is making the difference.  The disruptions you see are likey the VCO calibrating.

    Likely you will see at hot that the disruptions will increase due to the VCO calibrating.  I have written this document on the VCO calibration free ramping to give more insight as to how far the VCO can go without ramping.

    In this specific case, what you could do is this:

    Lock the device and get it to work.

    Read back VCO settings (rb_VCO_SEL, rb_VCO_CAPCTRL, rb_VCO_DACISET).

    Next time you go to this frequency, force these settings

    But be aware, that this not production worthy over temperature or devices.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    TI__Mastermind 28011 points
    One more thing...

    If the VCO calibration is an issue then one other consideration is to try a device that supports ramping without integrated VCO, such as the LMX2491/92.

    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 20 points

    Dean,

    Thank you for that document, that's very useful.

    If the user does nothing to assist the calibration speed, are VCO_SEL, VCO_CAPCTRL, and VCO_DACISET set automatically due to calibration?

    Sam

  • 0 in reply to Sam Solondz
    TI__Mastermind 28011 points

    Sam,

    The initial values are set by the user in the VCO_SEL, VCO_CAPCTRL_STRT, and VCO_DACISET_STRT.   For no assist, the user just programs these to a fixed value.

     

    TICSPRO Defaults are:

    VCO_SEL = VCO4

    VCO_DACISET_STRT=300

    VCO_CAPCTRL_STRT = 1

     

    Regards,

    Dean

     

    Regards,

    Dean