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LMX2592: LMX2592 Output Power Level Became Minuscule After Temperature Testing

Randy4TIics
Prodigy 165 points
Part Number: LMX2592
Other Parts Discussed in Thread: LMX2594

After hot temperature testing at 85C, the output of the LMX2592 went from a healthy 2dBm to -48dBm. The output frequency of 8510 MHz can be consistently programmed, except that the power level is now very low. Other boards are fine.

  • 0
    TI__Mastermind 33830 points

    Hi Randy,

    Was this test performed on an EVM? It's possible that the thermal design of the EVM is marginal when supporting long-term temperature testing, as the thermal mass of the board is relatively small.

    Do you get signal out of either end of the differential pair? Have you confirmed that the pull-up components are undamaged and still soldered to the PCB in the correct locations?

    Do you observe any difference in current between the working boards and the board with the output issue? Also, does the other output on the board still work?

    Regards,

    Derek Payne

  • 0 in reply to Derek Payne
    Prodigy 165 points

    Hi Derek,

    Thanks for your reply. The board is an X-band transceiver and the circuit used is exactly like the LMX2592 EVM except for the loop filter. This is a production board whose components are all rated -40 to 85C.  Actually, I already changed the pull-up inductors (2.4nH 0402 Coilcraft) hoping that would solved the problem. Current drawn by the chip of course is now lower. 

  • 0 in reply to Randy4TIics
    TI__Mastermind 33830 points

    Randy,

    It sounds to me like the output buffer was damaged, and the -50dBm you're seeing is capacitive coupling through a damaged output stage. Have you checked that the 2.4nH inductor doesn't have a resonant frequency at the 8510MHz or the harmonics?

    I have heard of the LMX2594 output buffer (similar structure as LMX2592) encountering stability problems with certain exotic loads e.g. filter stages at high temperature. This could be further exacerbated by a resonant frequency issue if present.

    Regards,

    Derek Payne