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LMX2581 spurs

Anthony Williams1
Prodigy 40 points
Other Parts Discussed in Thread: LMX2581

Hello

We are using four seperate LMX2581 devices to generate 1.0, 1.25, 1.5 and 1.75 GHz; the output divide by 2 is used in all cases.

We are using the devices in Integer mode with phase comparison at 62.5 MHz. We see some low level spurs at harmonics of 62.5 MHz either side of the carrier....the worst of which are 250 MHz either side of the carrier at about  -60 dBc. Are these spurs caused by leakage across the chip or are they connected to the fractional circuitry in any way?

Is it possible to disable the fractional circuitry in LMX2581 by setting a bit in one of the registers?

Do you recommend a lower phase comparison frequency? I chose 62.5 MHz in order to minimize phase noise.

Thank you

 

David

 

  • 0
    TI__Mastermind 27981 points

    The spur that you are seeing is the phase detector spurs.   The two mechanisms that this spur can get to the output is through the loop filter, coupling on the chip, and also coupling on the board.

    In general, if you use a lower phase detector frequency, these spurs will be lower, but I think that you can do better than -60 dBc using a couple tricks.

    -> For this design, since it is in integer mode, do the following

    1.  Set FRAC_ORDER to integer mode

    2.  Set PFD_DLY to the minimum setting

    3.  Disable Dithering.

     

    Also, the bypassing on the board can have an impact on this spur.  If you use a ferrite bead on the VccCP and VccBUF pins, this can reduce these phase detector spurs.

     

    Regards,

    Dean