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Original question:

LMX2595: LMX2595 spurs

LMX2595: 3/4 RF spur

Thomas Noonan1
Prodigy 30 points
Part Number: LMX2595

I have a spur at 3/4 the RF frequency when the output frequency is above 15 GHz and the VCO doubler is enabled.  From the reply to a previous question on this issue I tried setting the RFB output MUX t 2 but this only lowered the spur by 5 dB. to -60 dBm.  Can you please explain what is causing this spur and if you have any other ideas how I can reduce it further.

  • 0
    TI__Mastermind 27831 points

    Thomas,

    OK, so it sounds if switching the RFB output changing the spur by 5 dB implies that this was the dominant cause before.  But still you have -60 dBm.  Still sounds like there is Fvco/2 frequency running around.  If you disable  SEG1_EN (R31[14]), maybe this helps.

    If you have SYNC mode enabled, it enables this divide by 2 segment as well and you can't set SEG1_EN=0.

    Regards,
    Dean

  • 0 in reply to Dean Banerjee
    Prodigy 30 points

    Dean,

    Thanks for the quick reply.  I have SYNC mode disabled as well as the SEG1_EN set to 0 but no real change to the 3/4 spur.  My understanding is that the SEG1_EN is the enable for the Channel divider, but I noticed that  when I disable the Channel divider if I change the value of the Channel divider  the spur level will change a few dB.  This leads me to believe that  that the divider is not disabled just the output of the divider to the output mux.  Is there a way to shut down the divider itself.

    Thanks again for your assistance,

    Tom

  • 0 in reply to Thomas Noonan1
    TI__Mastermind 27831 points

    Thomas,

    The spectrum analyzer I have acess to now only goes to 12.8 GHz, but what I did was take 8 GHz and double to 16 and then look at 12 GHz.  I definitely see your 3/4 spur and see that if I change from channel divider to "VCO" for the unused side, the spur drops 10 dB.  I also tried "SYSREF" for RFOUTB and it dropped another 20 dB.

  • 0
    Prodigy 30 points

    Hi Dean,

    I set the RFOUTB to SYSREF without enabling, SYSREF or SYNC and the 3/4 spur is well below -90 dBm.  I thought I had tried that already, but I must have had some other setting different.  Thank You for your help.  I did notice that when not using the VCO doubler and having the RFOUTB set to SYSREF I get spurs at 0.5 and 1.5 the VCO frequency.  But if I set the RFOUTB mux back to VCO these spurs go away.  I can work with this but was hoping you could explain what is happening.  Engineering curiosity.

  • +1 in reply to Thomas Noonan1
    TI__Mastermind 27831 points

    Thomas,  

    I know that the SYSREF path is different internally from the clock path.  So it seems that part of the clock path for RFoutB is active.  For fun, you can try:

    CHDIV_OUTB_EN (R33[12]) = 0.  This is the enable path for the channel divider for RFoutB

    But beyond that, I dont' have much to add.

  • 0 in reply to Dean Banerjee
    Prodigy 30 points

    Dean,

    Thanks Again for your help.

    Tom