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LMX2615-SP: Assessing the tolerance of the output power within 10 seconds and over 1 year

Mohammad Rajabi1
Prodigy 130 points
Part Number: LMX2615-SP

Dear all,

I am asked to do the WCA and assess the tolerance of the output put power LMX2615-SP  under two situations, and verify the usage and then purchase of the LMX.

1. RF system output signal power at the output interface shall be stable better than 0,01% within 10 seconds over the full specified frequency range?

2. When RF output power falls outside Pgoal_min_limt and Pgoal_max_limit the RF system output power shall be stable within +/- 0.01W over 1 year in orbit.

I do not exactly how to assess them.

I believe the first item should be assessed by checking the phase noise. But I do not know how to relate it to 10 seconds? Checking phase noise 0.1 Hz offset of the phase noise diagram is the expected variation of the power level within 10 seconds?

For the second one, I have no clue.

Please let me know, if you are aware of assessing these things in simulation or theoretically.

Best regards

  • 0
    TI__Mastermind 33430 points

    Hi Mohammad,

    I am unclear if the first requirement refers to the output power stability in dBm, or the output frequency stability.

    • If the requirement refers to frequency stability, you could use the frequency over time transient analyzer mode of an E5052 (or similar equipment), which tracks frequency changes over time.
    • If the requirement actually suggests output power stability, you will likely want to use an RF power meter or other equipment with the capability to measure signal power. I am unclear how to interpret "better than 0,01%" for output power, when output power is typically reported as a logarithmic value - does this requirement give units for the measured variable?

    For the second requirement, I'm unsure what environmental factors the requirement is trying to control for. You could test a single device at the specified frequencies where RF output power is outside the limits, across supply voltage and temperature, to get an idea of the variation in output power at each frequency that could occur for typical testable environmental reasons. But if the requirement is trying to suggest there may be some change in output power due to radiation-related events, I'm not sure how we can test or simulate this. I will ask if we have any simulation or test data that could help answer this question, but it will take some time to get a reply due to the upcoming US Holiday.

  • 0 in reply to Derek Payne
    Prodigy 130 points

    Hi Derek,

    The requirements refer to power level in watt.

    As far as I understood from the requirement is that 10 second to second the power level should not vary more than 0.01% of the P_out in watt.

    In measurements, it is almost straight forward what to do. However, my main problem is that I do not know how to assess this requirement by checking the datasheet or simulation.

  • 0 in reply to Mohammad Rajabi1
    TI__Guru** 107030 points

    Hi Mohammad,

    I think your requirement is not well defined and does not apply to a component, your requirement is more or less the overall system requirement "at the output interface". (I guess this requirement is used for high power transmitter)

    In addition, no component will specify "output power stability within xxx second".

    Furthermore, 0.01% is an extremely small power variation, I don't think we are able to measure it for a low power device.

  • 0 in reply to Noel Fung
    Prodigy 130 points

    Hi Noel and Derek,

    For the second requirement, as you mentioned there are many factors:

    The output power variation vs temperature is shown in Figure 17 in the datasheet that is helpful. Would you please let me know if you have any information about the aging influence? How much does it vary? Any information is considered valuable.

    Best regards,

    Mohammad

  • 0 in reply to Mohammad Rajabi1
    TI__Mastermind 33430 points

    Hi Mohammad,

    First, I notice that the power limit is ±0.01W over 1yr. I should point out that LMX2615-SP does not exceed 10dBm into 50Ω load with 50Ω pull-ups (including over temperature), so that is one way to satisfy the requirement. On the other hand, with inductive pullup it is possible to achieve higher output power at high frequency (>2GHz). At typical inductive pull-up power of around 12.5dBm (about 18mW into 50Ω), you could go as low as 9dBm or as high as about 14.75dBm before running into trouble. The OUT_PWR register value can be used to compensate the output power against temperature changes, though if you do use inductive pull-ups you would likely have to set the initial power lower to account for variation across temperature.

    We designed the LMX2615-SP to remain within rated specifications and support worst-case power conditions over the full warranted lifetime, which happens to be one full year. The aging effect of semiconductors due to temperature is modeled in the Arrhenius equation, implying that higher temperatures (and consequently higher output power) will decrease the usable lifespan of the device. Moreover, the typical limiting factor is electromigration, implying it is more likely to see high current density paths increase in resistivity over time. To some extent the circuit design mitigates these effects through the use of regulated current sources/sinks, though at some point the power dissipation in the high current path will ultimately fail. I don't have any hard data or numbers, but the general takeaway is: if you use the device at the lowest power your application can reliably support, and if you can keep the ambient temperature low, the likelihood of the device surviving one year without any noticeable change in output power increases exponentially over the worst-case.

    Regards,