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LMX2594: LMX2594RHAT preamp output error

KAZUYA IKEDA
Prodigy 30 points
Part Number: LMX2594


Tool/software:

5857.LMX2594RHAT preamp output error.pdf

There are 3 out of 3 errors at the output of the LMX2594RHAT preamp.
Please review the attached file and let us know your opinion on the three questions from us.

 

  • 0
    TI__Guru** 111670 points

    Hi Ikeda-san,

    Output power is configurable by changing the bias current of the output buffer.

    Is the input impedance of the pre-amp purely 50Ω? 

    The output impedance of the LMX2594 is not 50Ω, if you use 50Ω pull-up, this would help make the output impedance closer to 50Ω. If the input impedance of the pre-amp is not 50Ω, the fix attenuator between LMX device and the pre-amp would definitely help stabilize the system as the fix attenuator is a pure 50Ω load. 

  • 0 in reply to Noel Fung
    Prodigy 30 points

    To prevent abnormal oscillation of the IC (Convergence time measurement results for LMX2594RHAT.xlsx), a fixed 3dB attenuator is inserted between RFoutAP and RFoutAM, which eliminates abnormal oscillation.


    However, at low temperatures (-20°C), the convergence time is delayed when the frequency is set.
    The frequency is delayed by about 300μs compared to the normal temperature (+25°C).

    See the attached document for details.
    (For example, when the frequency is changed from 30MHz to 3000MHz, it is 1.006ms to 1.270ms).

    When the output is loaded with a 3dB fixed attenuator, does the convergence time delay increase due to temperature fluctuation?
    What are the causes and remedies?







  • 0 in reply to KAZUYA IKEDA
    TI__Guru** 111670 points

    Hi Ikeda-san,

    It is normal that lock time is longer at cold temperature, this is due to the loop bandwidth is reduced at cold. 

    The reason for loop bandwidth change is due to the Kvco characteristic of the VCO. At cold temperature, Kvco is getting smaller, as a result, loop bandwidth becomes smaller. Hence, lock time is longer at cold.