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LMX2492-Q1: The Spur in Ramp but not in CW

Liu Hao
Intellectual 330 points
Part Number: LMX2492-Q1
Other Parts Discussed in Thread: LMX2492

Tool/software:

Hi all

I use LMX2492 in my FMCW radar , i find that if i set LMX2492 to CW ,the spectrum after  ADC sampled is clean . it just has some sub fraction spurs，the level of the sub fraction is a little high if i do not open the dither.

but when i set LMX2492 to Ramp, i can always see nearly 200kHz spur near the spectrum of IF

i check the power , the clock and my configuration , i think they are correct.

So i need check with TI about that Does this chip has some problems just like my describection. because if you set this chip to Ramp, the FRAC_DEN will be set to 2^24, this is not the same as CW. 

Could you give me some suggestion?

FMCW

CW

  • +1
    TI__Guru* 94680 points

    Hi Liu,

    Whenever ramping is enabled, PLL_NUM will be incremented and the VCO frequency will change. As such, the PLL is running at fractional frequency most of the time. Since the frequency change per PLL_NUM increment is small, we will get a lot of spurs, this is normal.