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LMX2595EVM: Very Spurious Output

Kyle Weidmann
Prodigy 130 points
Part Number: LMX2595EVM

While looking out the output spectrum of the device at 8GHz, we saw that the output has very large spurs at the 2nd and 3rd harmonics. The datasheet has it listed as -20dBc and -50dBc respectively, while we measured about -15dBc for both the 2nd and 3rd. I've provided the TICS Pro PLL settings, if there are any settings that can be changed to see performance in line with the data sheet. Thanks

-Kyle Weidmann

  • 0
    TI__Mastermind 28011 points
    Kyle,

    Output harmonics vary as a function of frequency and OUTA_PWR. Here's a few things to try:

    1. Ensure that if this is single-ended output, that the unused side is terminated with 50 ohm. Unbalanced outputs cause high spurs.
    2. I see that RFoutB is powered down, but OUTB_MUX="Channel Divider". This powers up the channel divider and draws current and creates unwanted spurs. Change this to "VCO".
    3. You might see some small impact of OUTA_PWR on harmonics; I think lowering it may help slightly.


    Regards,
    Dean
  • 0 in reply to Dean Banerjee
    Prodigy 130 points

    Dean, 

    Thank you for the help, I will implement your suggestions and come back here with the results. For your first suggestion I had already terminated the output with a 50Ohm termination, but I will try various other terminators to see if it was possibly the problem,

    Thanks, Kyle

  • 0 in reply to Dean Banerjee
    Prodigy 130 points

    Dean,

    I implemented all of the suggestions you had and harmonic performance increased by about 1-2dB but it still not in line with where the data sheet says the chip should perform. I've attached the spectrum analysis and PLL parameters as before. 

    Thanks,

    Kyle Weidmann

  • 0 in reply to Kyle Weidmann
    TI__Mastermind 28011 points
    Kyle,

    The datasheet claims a second harmonic of around -20 dBc and this seems consistent with what you are measuring.

    So let's focus on the 3rd Harmonic.
    1. Datasheet claims -50 dBc
    2. You show more like -25 dBc.
    3. I took an EVM and measured about -28 dBc.
    4. We do have data showing -50 dBc but at high frequencies, it bounces around a lot as a function of frequency. This was with a different spectrum analyzer than I used today
    5. I have seen plots from customers with high end test equipment showing -50 dBc third harnonics

    In summary, what I think is going on is a combination of setup issue, resonance of cables/setup, and the device. The output impedance of our buffer at 24 GHz is far from 50 ohms at 24 GHz, so personally, I would not take this -50 dBc number to the bank.

    Regards,
    Dean