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Original question:

[FAQ] *READ FIRST FOR NFC/RFID* Frequency Asked Question Guides and Answers for NFC/RFID Devices

RF430FRL152H: RF430 Antenna Tuning Production Process

Tom Herron51
Intellectual 320 points
Part Number: RF430FRL152H

Hello,

I am working on a design using the RF430.  It is my first RFID project, and I am reading the practical guide to RF430 antenna design (SLOA217A).  It is a very helpful document, but I'm unclear about the intended production process for tuning.  The document kind of makes it sound like once the design is finished, a single external resonance capacitor value can be chosen to achieve the proper resonant frequency on all boards.  However, section 4 of the document explains that the internal resonance capacitor is 35 pF ±10%.  Given this wide range, it seems each board would need to be tested and then have a resonance capactor value selected in order to meet the -0/+1% resonance frequency tolerance recommended in section 5.4.  Is this the expectation?

Thank you.

Tom Herron

  • 0
    TI__Mastermind 18265 points

    Hello Tom,

    your observation is correct. In case you need the absolute best read range the resonance frequency should be trimmed.

    But consider that the +-10% for the internal resonance capacitor are max values versus process and temperature.

    The active Q factor of the antenna is low which means that the exact resonance frequency is less critical due to the flat resonance curve.

    Depending on materials you have nearby your transponder you will also have a change in the resonance frequency.

    In our demo boards we use no trimming.

    My recommendation is to do some tests with different resonant capacitors and check the difference in read range and if this is tolerable in your application.

    Best regards,

    Helfried 

  • 0 in reply to Helfried Vollbrecht
    Intellectual 320 points

    Thank you, Helfried.  Your explanation is more what I expected.  In my application, the read range is quite short and I think I will be able to get adequate results without trimming.

    However, I did notice that by designing the antenna to have lower inductance, I can drive up the required external capacitance (over which I can have more control) and get less variance of the total capacitance.  For example, I have limited space for my antenna and unless I make it double sided, it is difficult to get much above 0.6 uH.  In that case, I end up needing a resonant capacitor or 235 pF.  So I end up with a 200 pF external capacitor.   Since this external cap can easily be ±1%, my calculated resonant frequency variation is only 0.3 MHz.   My question is what is the downside of being so far off the recommended 2.7 uH recommended antenna inductance in the 15xH datasheet?

    Thanks again for your help!

    Tom Herron

  • 0 in reply to Tom Herron51
    TI__Mastermind 18265 points

    Hello Tom,

    I can understand the way you think to reduce the tolerance. In principle this will work but I assume not with the best performance.

    The recommendation for NFC tag antenna coils range from 0.5uH to 4uH so you are at the lowest end. There is a optimum for the L/C relation but this depends also from the reader. In any case you should try to make the antenna area as big as possible this will help best for a good read range.

    Best regards,

    Helfried

     

  • 0 in reply to Helfried Vollbrecht
    Intellectual 320 points

    Thank you, Helfried.  That is helpful.  I think we can mark this closed.