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LMX2594: Doesn't lock at VCO boundaries at cold temperatures

Josh Merti
Prodigy 190 points
Part Number: LMX2594

Hello,

I am using the LMX2594 and I'm having an issue where the LO will not lock when it is tuning near the boundary of VCO 4 and 5 (11950 MHz to 12000 MHz) and also the boundary between VCO 6 and 7 (13900 to 13950 MHz) when the temperature is low (-10 C or below).  One unit is having the issue at both boundaries and the other is only having the issue between 6 and 7.  On the unit that is only having a problem between boundary 6 and 7 the LO will unlock at 13950 MHz if the previous LO frequency used VCO 7.  If the last frequency used VCO 6 13950 MHz will lock.  It doesn't seem to matter what I make VCO_SEL, VCO_DACISET_STRT, or VCO_CAPCTRL_STRT.  The behavior is the same.  All of the FORCE parameters are set to 0.  I don't have any information from the first unit yet, I wanted to see if it was an isolated issue or a systemic problem so I tested the second one without doing any debug.

Thanks for your help.

  • 0
    Prodigy 190 points

    Original failed settings for 13950 MHz:

    VCO_SEL = 1

    VCO_DACISET_STRT = 300

    VCO_CAPCTRL_STRT = 183

  • 0 in reply to Josh Merti
    TI__Guru** 107030 points

    Hi Josh,

    Were you having problem to lock at cold temperature or the PLL was losing lock when temperature drop below 0degC?

    What is your reference clock frequency and CAL_CLK_DIV setting?

  • 0 in reply to Noel Fung
    Prodigy 190 points

    The issue I'm seeing is having a problem locking at cold temperatures.  I haven't tried configuring for a problem frequency at room temperature and then dropping to cold yet.  The calibration seems to get stuck in a VCO that doesn't seem able to cover the range.  In the 13950 case it gets stuck in VCO 7 (which should cover that frequency but doesn't appear to at cold) if it's already in VCO 7 but if the frequency I tune from is in VCO 6 then it will lock and choose VCO 6.  

    The reference clock is 100 MHz and the CAL_CLK_DIV setting is 0 (Divide by 1).

  • 0 in reply to Josh Merti
    TI__Guru** 107030 points

    Hi Josh,

    Looks you have identified the root cause. Right, at cold, the VCO free running frequency will be higher, so it is possible that VCO7 no longer support 13950MHz. 

    However, since you have VCO_SEL=1, I expect VCO6 will be selected instead of VCO7. 

    Can you try again with CAL_CLK_DIV=1 (Divided by 2)? You can also try set VCO_SEL=4.