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LMX2491: question about LMX2491 locking and spurs

Part Number: LMX2491
first question :
I noticed the description of RAMPx_LEN and
RAMPx_INC in datasheet, in every phase detector cycle, the fractional numerator will be increase a certain value witch is
programmed in RAMPx_INC.
and I want to know that whether the PLL will be locked in every phase detector cycle? And HOW if it is locked.

second question:
I try to drive a VCO witch is working at 24GHz with LMX2491, and VCO out put will be through a frequency divider(1/16)first and then go back to LMX2491.
and the frequency of phase detector is 52MHz,loop bandwidth is about 100KHz,VCO output frequency is 24180.01MHz.
Based on my experience, there will be a integer boundary spurs at N * frequency of phase detector(in my design is 24180MHz,offset is 10KHz),but I did not see it .
I want to know WHY.

Which engineer can help me answer my doubts? Thank you very much!
  • Hi There,

    The chip will not monitor the lock state but keep changing the fractional numerator whenever RAMP is started. 

    Assuming your PLL_DEN is 2^24, fractional spurs frequency is 0.0062kHz. At VCO output, the spurs frequency becomes 0.0992kHz, there is no 10kHz spurs.

    Bare in mind, the "VCO frequency" to the PLL is 24180.01MHz / 16, that's why you don't have 10kHz spurs.