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LMX2595: VCO fundamental operation is to 15 GHz, but 0.5*F present at output for F < 15 GHz

John Haraguchi
Prodigy 250 points
Part Number: LMX2595

The LMX2595 datasheet indicates that the VCO operates fundamentally from 7.5 – 15.0 GHz, so I did not anticipate seeing any sub-harmonic content at the output (i.e. 0.5*F) when tuning below 15 GHz, but it is there. The 0.5*F level is less than 50 dB down from the tune frequency when tuning above 13 GHz.

I was hoping to use the LMX2595 without a filter at the output given that the VCO operates fundamentally to 15 GHz. The 0.5*F and 1.5*F levels are lower than other PLL ICs that use a doubler to cover the same frequency range, but they would still require filtering.

Is there some internal divide-by-2 being enabled that is causing this or is it just a function of the VCO design?

I did find that setting SEG1_EN (R31[14]) to a ‘1’ dropped the level at 0.5*F by a few dB.

Are there some other register value settings that could be changed to improve the sub-harmonic performance?

Thanks,
John

  • 0
    TI__Mastermind 28011 points

    John,

    SEG1_EN would be the first bit I would try.  I assume that you are not using the output divider if you set SEG1_EN=0.

    Also, ensure that both output dividers are set to OUTA_MUX=OUTB_MUX=VCO.  Note that if you are not using an output and power it down, this MUX setting could still have an impact for crosstalk if OUTA_MUX<>OUTB_MUX

    Aside from that, I'm not really sure what is going on.  You might try changing CAL_CLK_DIV to see if this has any impact.  If so, then it could be some harmonic of the state machine clock.

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 250 points

    Dean,

    Changing OUTB_MUX from ‘High Impedance’ to ‘VCO’ in R46 reduced nearly all the 0.5*F and 1.5*F content into the noise floor of my measurement (> 80 dB down). I did have some 5/2*F start creeping up between 12.5 and 14.0 GHz but nothing greater than 70 dB down.

    Thanks for the suggestions, it would have taken me a while to stumble across the solution to this one. Since I have OUTB powered down, I wouldn’t have thought to try adjusting the OUTB_MUX setting. 

    Thanks,

    John