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LMX2594: LMX 2594 LOCKING ISSUE WITH PFD CHANGING

Part Number: LMX2594

I am using lmx 2594 to generate frequencies in the range 4-7GHz with reference freq 100MHz and pfd 200 MHz.

when i am changing pfd  as 160MHz for removing 1mhz spurs through serial interface some frequencies like 4601 and 4701 MHz are not locking.Please suggest me to resolve this issue

  • Hi Mohan,

    did you use MULT to change the fpd to 160MHz from a 100MHz reference clock? If this is the case, what is your setting of OSC_2X, MULT, Pre-R, etc?
  • These are the settings for 160MHz fpd
    Doubler value:2
    Mul : 5
    Pre-R : 4.
  • Hi Mohan,

    MULT and doubler cannot be used at the same time.
    You can set:
    doubler = 1
    Pre-R = 5
    MULT = 7
    Then you will get 140MHz fpd.
    Even though the spurs due to original 200MHz fpd can be removed with the new fpd. The crosstalk spurs between OSCin (100MHz) and the VCO or fout still exist. So you will still see some spurs at 1MHz and 2MHz offset.
  • Hi Noel,
    I kept doubler=1, Pre-R=5, Mul=7 for fpd 140MHz.The spur levels are further increased at 1mhz and 2mhz offset i.e -40dBc .Please suggest me a fpd that will reduce my spur levels at 1mhz offset aroud 75dBc because my system requirement spur levels is 75 dBc within 20mhz bandwidth of my center frequency.PLL Range is from 4570-6700Mhz
  • Hi Mohan,

    1MHz spurs is likely the crosstalk spurs between fout and OSCin. What is the 100MHz reference clock came from and how did you connect it to OSCin?
    Also try set CAL_CLK_DIV =1, this will reduce the state machine clock to 50MHz. This may help reduce the crosstalk spurs if the state machine clock is one of the spurs contributors.