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Original question:

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

LMX2492-Q1: LMX2492

Matthew Richardson
Prodigy 10 points
Part Number: LMX2492-Q1
Other Parts Discussed in Thread: LMX2492

Tool/software:

I am also using the LMX2492 for an FMCW radar and have found that undesirable spurs that are created from using the chirp feature.

Like the previous designer, I am also seeing spurs that are ~11dBc that create a haystack effect:

Before reaching out, I have check my clock and power supplies as potential sources of the spurs, but now that I have seen this issue in another thread, I am convinced that this is primarily a chirp function behavior.

I am using a very clean, low phase noise 100MHz reference and have very clean < -80dB power supply noise.

Are there settings, that can be configured to minimize the fractional N spurs created while using the chirp feature or a set of recommended chirp pulse width/duration/frequency range for optimal performance?

Kind Regards,


Matthew

  • 0
    TI__Guru** 107030 points

    Hi Matthew,

    When we do FMCW, we are changing the VCO frequency with a very small step size. That means, the PLL/VCO is operated at fractional frequency most of the time. As such, there will be spurs unless you have set the loop bandwidth to a very small value. However, a narrow loop bandwidth will slow down the possible rate of frequency ramping. 

    There are couples of register field we can use to optimize phase noise/spurs.