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LMX2592: Spurious frequency at an offset from LO

Amy Panton
Prodigy 20 points
Part Number: LMX2592


Hello,

We have been using the LMX2592 in our product's prototype quite successfully to date (~4 months), however it has suddenly started to output a concurrent second tone approx. 700MHz down from the LO. This second tone occurs concurrently with the expected output LO frequency, looks stable and locked, and tracks with the LO when the LO frequency and power are varied.

We have checked the output of the 50 MHz Ref TCXO (E7116LF) and Clock Buffer (CDCV304PW) combination that feeds the OSCin+ pin of the LMX2592 through a set of resistors/capacitors, and this 3V3 pk-pk signal seems clean and unremarkable. 

Checked the power supplies to the LMX2592 and Clock Buffer etc., and again nothing untoward. 

There was a small rework to an area of the PCB that is located at a quite a distance from the LMX2592, to replace a connector to sample off the clock buffer. Apart from that, the PCB was unchanged.

Would anyone have any ideas what could cause this sudden appearance of this second tone? The LO frequency is 6.5 GHz and the tone appears at 5.8 GHz. 

Thanks!

  • 0
    TI__Genius 12560 points

    Hi,

    What is the level of second tone? Is it same as LO?

    As you already mentioned the buffer output seems clean, have you checked the buffer output in oscilloscope?

    I would be suspecting some other tones/ higher spurs be present at OSCin. Could you probe the buffer output in spectrum analyzer and see if any other spurs present?

    Thanks!

    Regards,

    Ajeet Pal

  • 0 in reply to Ajeet Pal
    Prodigy 20 points

    Thanks for your reply Ajeet.

    The second tone is 15 dB down from the LO, and this tracks when increasing/decreasing the LO power level.

    Have probed a sample of the signal present at the OSCin port: 

    Do you feel these higher tones could be causing the problem? This part of the PCB and circuit haven't been changed from before the second tone made an appearance. 

  • 0 in reply to Amy Panton
    TI__Genius 12560 points

    Hi,

    OSCin input seems to be good.

    I could probably do understand, it can be some coupling at the input of the LMX part. Do you have an option to bypass the buffer and can try to provide the reference input external or direct from TCXO?

    Thanks!

    Regards,

    Ajeet Pal

  • 0 in reply to Ajeet Pal
    TI__Genius 12560 points

    Hi,

    I haven't heard long time from you and assumed you may have resolved the issue.

    I am closing this thread and feel free to write again on the same thread or open new for further support.

    Thanks!

    Regards,

    Ajeet Pal