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LMX2820: PLL spurs at 1khz, 2khz...

Aizkibel Salcedo
Prodigy 180 points
Part Number: LMX2820
Other Parts Discussed in Thread: , USB2ANY

Hi,

We are using the LMX2820 to generate a frequency of around 18250 GHz, w hich the following configuration:

Layout and components are the same that TI used in the evaluation board.

Spurs are more or less the same that the SW calculates, with the expeption of low frequency spurs (1khz or 2khz around LO):

We have tested three different references (OCXO, Keysight signal generator, PNAx generating a tone).

We have tested different power supplies ( DC board, Keysight power supply R&S Power supply.

We have switched off the controller.

And anything changes... Any help?

Thanks in advance,

PS: 100 hz could be lamps frequency, we are more worried about the khz spurs.

Best regards,


Aizkibel Salcedo

  • 0
    TI__Genius 12560 points

    Hi Aizkibel,

    For the attached plots (1.45GHz, with 50MHz reference), your device is operating in integer mode and it doesn't seems to be generates any spurs at few KHz range.

    I did the quick check on the LMX2820EVM setup and doesn't shows any spurs at these range. 

    This indicates to me that either there is something going on with your reference introducing the spurs, or else you have another noise source mixing into your device (is USB2ANY connected?). In some other cases, we have seen some spurs from USB2ANY / reference etc.. If reference already checked, would be suggested to remove the USB2ANY after programming and see the performance.

    Regards,

    Ajeet Pal

  • 0 in reply to Ajeet Pal
    Prodigy 180 points

    Hi Ajeet,

    Thanks for your support, I didn't provide all the information:

    The module is a downconverter, input 19.XXX GHz-> output 1.xx GHz, so the LO is 18.250. The captures are done at the output, however we have checked that the spurs are in the LO path. 

    The unit is programmed used a TMS570 integrated in the board, however I will try to switch off after set the PLL.

    Please, can you check if the LMX2820EVM generates any low frequency spur at 18.250?

    Thanks in advance,

    Aizkibel

  • 0 in reply to Aizkibel Salcedo
    TI__Genius 12560 points

    Hi Aizkibel

    Aizkibel Salcedo said:
    Please, can you check if the LMX2820EVM generates any low frequency spur at 18.250?

    I'll have a check on the EVM and will get back to you on the measured results at 18.25GHz.

    Regards,
    Ajeet Pal

  • 0 in reply to Ajeet Pal
    Prodigy 180 points

    Thanks Ajeet, I will wait for your answer.

    Best regards,

    Aizkibel

  • 0 in reply to Aizkibel Salcedo
    TI__Genius 12560 points

    Hi Aizkibel,

    I'll update you soon.

    Regards,
    Ajeet Pal

  • 0 in reply to Ajeet Pal
    TI__Genius 12560 points

    Hi Aizkibel,

    Thanks for your patience.

    I have the check on my LMX2820EVM for the requested frequency performance and it looks pretty clean at 18.25GHz.

    You may see the noise floor up, that could be my reference is not so clean, but we don't have any spurs at kHz offset. See below:

    You can have a look on your setup, while isolating each section and can debug the spurs.

    Used TICS pro config file attached here for your reference.

    LMX2820_18.25G_50MREF_100MPFD.tcs

    Regards,

    Ajeet Pal

  • 0 in reply to Ajeet Pal
    Prodigy 180 points

    Hello Ajeet,

    Thanks for your support, the tcs file will be very useful, we will check again our design.

    Best regards,

    Aizkibel