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LMX2541: Spurious emmission at VCO frequency + 430 MHz (measured with SQ2380E VCO variant)

Reinier Seuren
Prodigy 10 points
Part Number: LMX2541

Tool/software:

Hi,

I am using the LMX2541SQ2380E in full chip mode to generate a 2400-2450 MHz RF signal. I noticed in the spectrum measurement of there is a large spurious peak at the VCO frequency + 430 MHz (for all measured VCO frequencies) in both bypass-mode (VCO_DIV=1) but also in divided mode (VCO_DIV != 1). 

I measured this spurious on multiple board designs, tested with various configurations of PLL_R, PLL_N, and loop filter settings, but none seem to have a significant impact and different reference clocks. 

Is this a known spurious from the VCO? Or is there maybe a settings that I missed?

Due to the proximity it becomes quite hard to filter out this spurious to meet certification for use in transmit applications. 

  • 0
    TI__Guru* 94510 points

    Hi Reinier,

    I cannot understand the plots.

    Would you provide the configuration like below? This GUI is available here (www.ti.com/.../TICSPRO-SW).

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hi Noel, 

    thank you for the relpy! 

    I see now that the plots x-axis say it is in MHz, but it should be GHz. I am sorry for the confusion. The plots show the measured spectrum at the RF output pin of the LMX2541 for 2.42 GHz (or 2420 MHz) and 2.85 GHz  (or 2850 MHz). The left most plot shows the two measurements in one figure, the other two show the individual measurements.

    The link you provided does not work for me, but I have added a screenshot of the PLL settings from the clock design tool.

  • 0 in reply to Reinier Seuren
    TI__Guru* 94510 points

    Hi Reinier,

    I don't believe this configuration will generate a spurs at 430MHz offset. This is an integer channel and all the frequencies are multiple of 20MHz, there is no way to generate a 430MHz spurs. I don't know where is it came from but I am sure it is not going to be coming from the synthesizer. I will check if I have a board to try in the lab. Stay tuned. 

  • 0 in reply to Noel Fung
    TI__Guru* 94510 points

    Hi Reinier,

    Here is the test data, as expected, there is no spurs at 430MHz offset.

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hi Noel, 

    Thank you for taking the time to do a measurement. Based on your results I was motivated to see if I could do another measurement. I managed to use a different spectrum analyzer and now I get the same result as you do (so no spurious peak at +430 offset). Seems there is something wrong with our spectrum analyzer. 

    Thank you for your help!