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LMX2531: Lock Detect Signal is Not Asserted in LMX2531.

Shehzad Rasheed
Prodigy 105 points
Part Number: LMX2531
Other Parts Discussed in Thread: CODELOADER

I m programming LMX2531LQ1570E through an FPGA for 1600 MHz. I have composed  the registers values from codeloader software. But when I configure the LD pin as digital logic high or low it works but when i configure it for N/2 or R/2 it outputs incorrect values. When this pin is configured as digital lock detect then it never goes high. Kindly provide some suggestion that what could be the issue. 

  • 0
    TI__Guru* 81340 points
    Hi Shehzad,

    Did it lock? it looks to me that it did not lock because you get error with both N/2 or R/2. What did you get from R/2?
  • 0 in reply to Noel Fung
    Prodigy 105 points

    No it did not lock. I m getting some incorrect sequence when i program it for N/2 and logic low when I program it for R/2.

  • 0 in reply to Noel Fung
    Prodigy 105 points

    Register Values are shown above.

    When we configure PLL for R/2 then it outputs accurate frequency of 7.5 MHz. But when i configured it for N/2 then it outputs incorrect and unstable frequencies. Still the status of lock detect is same(never asserted).

  • 0 in reply to Shehzad Rasheed
    Prodigy 105 points

    let me know one thing that what should be the impedance of pin 17( VregVCO ) wrt ground??

    Because i have another evaluation board where impedance of pin 17 is in order of mega ohm and this board is working with same configuration. But my board is not working and my board  has impedance of only 17 ohm on pin 17 and it looks short with ground.

  • 0 in reply to Shehzad Rasheed
    TI__Mastermind 27581 points

    Shezhad,

    We don't have this measurement, but 17 ohm is wrong.  The typical current consumption is on the order of 30 mA and the voltage at this pin is in the range of 2.65V, if my memory serves me.  So If there was 17 ohms to ground, this would be much more than 30 mA.

    VregVCO is the output of a regulator, so you should be able to measure the voltage.

    If you see this is off compared to the working device, some things to consider are:

    1.For the pins that say "NC" this means do not connect.   These are for the bond wires for the VCO tank inductor and grounding them will cause the VCO to oscillate at the wrong frequency, or not at all.

    2. Check the soldering guidelines in datasheet.  As this pinout is asymmetrical, it is less forgiving to improper soldering.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 105 points

    Dean Banerjee
     Thank you for your help. Issue has been resolved as PLL IC was not stuffed properly. Now after re soldering its working fine.