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LMX2581EVM - Reference / PFD spur

Hiroshi Katsunaga
Mastermind 6480 points
Other Parts Discussed in Thread: LMX2581EVM

Hello,

My customer have some question about LMX2581EVM.

<Background>

My customer evaluated the LMX2581EVM.

Their configuration is as follows.

  • OSCin is driven by the SG through the OSCin SMA.
  • They modified the resistors related to OSCin as follows.
    • Remove R6 and R14.
    • Change R13 and L3 to 0 ohm.
    • Make R15 51 ohm.

<Issue>

They observed big reference / PFD spur in about -65dBc in RFoutB+/-.

[Q1]

Is my understanding correct ?

Is there any other cause ?

<My understanding>

The causes of the reference / PFD spur are as follows.

  1. Configured as Fractional mode in spite of integer mode
  2. Effect of the spurs on VccVCO
  3. Effect of the spurs on VccCP
  4. Leakage Dominated Spurs
  5. Pulse Related Spurs

[Q2]

Is my suggestion correct ?

Are there any other good suggestions ?

<My suggestion>

  1. Set FRAC_ORDER to integer mode, Set PFD_DLY to the minimum setting, Disable Dithering.
  2. Change R28 to ferrite bead.
  3. Change R22 to ferrite bead.
  4. Change the capacitors of the Loop Filter to Film Capacitors from Ceramic Capacitors.
  5. Change the PFD frequency to low frequency and Change the Loop BW to nallow BW.

Best Regards,

Hiroshi Katsunaga

  • 0
    TI__Guru* 81340 points
    Hi Hiroshi san,
    if the application does not require fractional channel, right, set FRAC_ORDER to integer mode and disable Dithering.
    There is an issue with the loop filter capacitor C3. We suggest this value should be in the nF range. See datasheet section 8.3.7 for details.
    BTW, if you can provide a screen shot of the measurement, that will help us identify the problem more easily.
  • 0 in reply to Noel Fung
    Mastermind 6480 points

    Hi Noel,

    Thank you for your fast response.
    I understood your comments.

    I want to communicate with your team by off-line.

    Therefore I close this post.


    Thank you for your support.

    Best Regards,
    Hiroshi Katsunaga