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LMH2120: RF power detectors

Krishnam Raju Marupu
Prodigy 70 points
Part Number: LMH2120
Other Parts Discussed in Thread: LMV221

Tool/software:

Dear Team,

We are designing a TDD control scheme for an SPDT switch in a Wi-Fi frequency shifter application.(2.4Ghz to 300MHz )

  • Front end: Wi-Fi 2.4 GHz  path is down-converted to 300 MHz using a dual mixer(upconversion in RX path).

  • Goal: Generate the TX/RX TDD control signal from a power detector in the TX path to drive an SPDT switch (through a comparator) In both 2.4GHz and 300MHz bands

We have identified three classes of TI power detectors (peak, logarithmic, RMS) and would like your recommendation on which detector type is best suited for reliably generating fast, noise-robust TDD control in this application.

Could you please advise on:

  1. Which detector type (peak, log, or RMS) is most suitable for generating a TDD control pulse from the TX path in a Wi-Fi application, and why (speed, accuracy, dynamic range, sensitivity to modulation, etc.)?

  2. Recommended part numbers (power detector, directional coupleri in 2.4Ghz and 300MHz) that match the application (preferred supply: 3.3 V).

  3. Comparator/interface suggestions (comparator part, thresholding approach, any required filtering or hysteresis) to reliably drive an SPDT switch.

  4. Example reference designs or app notes (if available).

    Thanks in advance

Best regards,
Krishnam Raju

  • 0
    TI__Intellectual 1130 points

    Hi there Krishnam, apologies for the late response.

    Below is an overview of the power detectors currently offered:

    Peak power detectors sample the peak instantaneous level at the input. This means that they respond fast and are suitable for pulsed systems or systems requiring fast control. But in modulated systems with high peak-to-average-power-ratio's (PAPR) such as WiFi, this might not give an accurate representation of the true power in the signal. This downside wont really matter in your case if youre just driving a comparator. 

    RMS power detectors sample a running average of the RMS power in the signal. They are robust to high PAPR modulation schemes for this reason, and therefore are ideal for controlling the envelope power of high PAPR signals at the output of the system through an automatic gain control feedback scheme. But due to the fact that they average, they usually take a bit longer to respond to power at the input.

    Log power detectors are what TI calls "linear-in-dB", and put out a voltage that is linear to the input power in dBm. This makes them well suited for systems requiring high dynamic range and good fidelity, such as, again, automatic gain control systems. The downside is, like peak power detectors, they can give an inaccurate power reading in highly modulated waveforms. 

    This app note talks a little bit about this use case for power detectors in FDD systems:

    https://www.ti.com/lit/an/snwa002a/snwa002a.pdf?ts=1755704960912&ref_url=https%253A%252F%252Fwww.google.com%252F

    For your application, where you are just feeding a comparator, and looking for fast response times, i would look at log power detectors or peak power detectors, depending on the dynamic range requirements. 

    Take a look at the LMV221, or the LMV2121, the log and peak power detectors, respectively, that will cover your frequency range. The LMV2121, the peak version, will be cheaper in this case, and probably best option to feed a simple comparator looking at power in band.

    A coupler and filter will be required at the input to the power detector, both of which TI doesn't make. Choose a coupler that has enough coupling factor to not put too much power into the power detector. Pick filters such that any interferrors will be rejected enough to be below the comparators threshold at the output of the power detector. For the comparator, pick one that will work quick enough for your TDD duty cycle. A bit of experimentation will probably be required to find the proper thresholding approach, we have an EVM that shows the LMV2121 that can be used to prototype in the lab before build. 

    https://www.ti.com/tool/LMH2121TMEVAL

    Let me know if you have any more questions,
    Gideon

  • 0
    Prodigy 70 points

    Hi Gideon

    Thank you for your reply.

    With Regards

    Krishanm Raju M