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K
K2R
Visitor II
October 12, 2023
Solved

ST25R3911B, range of air gap between transmitter and receiver antenna

  • October 12, 2023
  • 5 replies
  • 5325 views

Hello, commuity,

We want to estabilsh a data link between two devices over NFC. This is, because we want to power a kind of sensor device over the air via a gap of abt 1 mm.

The sensor consists of a ST25DV04KC and a STM32L031 with a low drop regulator.

On the other side or transmitter is a ST25R3911B driven by a STM32L031 (the controller will change later). Antenna matching network was designed by using the ST's Antenna Matching Tool. Anntenas itself are PCB coils of 4 windings of about 55mm diameter

Testing the power transmission we found, that this works fine in the range of more than 5mm. But the closer the antennas are, the more the field of the transmitting antenna drops, until there is no supply voltage available on the sensor board.

Are there limitations in the use of power transfer in this small ranges or did we miss any other restriction of this?

Thanks in advance

Kai

 

    This topic has been closed for replies.
    Best answer by Travis Palmer

    Hello Peter,

    I hope, I do not miss any of your questions - otherwise please repeat them.

    Yes, when being un-powered the RFO's are in high-Z and do not disturb the smith chart measurement.

    Yes, if the device is un-powered also the AAT pins are in high-Z and you see the matching as without AAT-caps populated. That's, why we recommend to power the chip and put the driver in high-Z mode.

    Your approach of measuring the smith chart and comparing powered and un-powered state of the chip are completely correct. But your smith chart look somehow distorted. If you still have an un-modified eval board ( I assume you are talking about the ST25R3911B-DISCO) can you please measure it un-powered. There is an AAT application note (AN4914) and the un-powered smith chart should look like figure 2 (or very similar). 

    I measured the antenna parameters (AN4974 - Figure 7) in the past. It should look very similar to figure 7.

    Regarding your antenna measurement (first figure in your post): It looks distorted to me. Maybe there is a problem with the calibration of your VNA? Did you also compensate the cable?(Autoport extension)? Can you measure O-S-L as a reference? You could also measure a de-soldered inductor (like EMC inductor) and maybe a 10Ohm resistor. Also your antenna might have a resonance below 1MHz. Typically the curve starts in the left part ( = 0R spot) of the real axe. Yours is starting from the right side ( = infinite Ohm spot). Maybe your antenna is not a closed loop. You can also check the resistance with a normal multi meter.

    Regarding your ST25R3911B-DISCO antenna measurement (second figure in your post):

    It looks better, but your beginning of the curve (1MHz marker) is very high in the inductive region. Again there is a strange distortion.

    Please let me know if your problem is solved.

     

    br Travis

     

    5 replies

    K
    K2RAuthor
    Visitor II
    October 23, 2023

    No one an idea what we can do?

    U
    Ulysses HERNIOSUS
    Technical Moderator
    October 23, 2023

    Hi,

    which antennas are used on both sides? Can you share a picture, please?

     

    Best Regards, Ulysses

    K
    K2RAuthor
    Visitor II
    October 23, 2023

    Hi Ulysses,

    find bitmaps of footprint attached. Both anntenas share the same shape: 4 windings with a diameter of about 65mm.
    As far as I know, the matching has been done using the ST25R Antenna matching tool.
    As written above, the necessary energy transfer does work, but even on distances more than 5mm. All below that lead to a dropping field, therefore to dropping voltage.

    Regards Kai


     

     

     

    U
    Ulysses HERNIOSUS
    Technical Moderator
    October 23, 2023

    Hi Kai,

    It sounds you didn't do the matching and above is an effect of non-optimal matching.

    I think you need to go back to your colleague having done the antenna matching. I think he will need to measure the antenna matching (not only calculate it) and re-match according to the recommendations in our AN.

    In case he requires help here on the community  he will need to share measurements and smith charts of the antenna in its target environment.

    Also just the antenna rendering may not be enough if there are PCBs/metal/... close by. That was why I was asking also for actual photos.

    Best Regards, Ulysses

     

     

    K
    K2RAuthor
    Visitor II
    October 24, 2023

    Hi Ulysses,

    okay, the last point was a misconception: in my test setup there is no metal but only the PCB-antennas.

    So I will go to the hw developer and lead him to here for getting the needed support.

     

    Regards Kai

    T
    Travis Palmer
    ST Employee
    October 25, 2023

    Hello Kai,

     

    could you share the smith chart of the (un-powered) reader with and without tag present?

    I think your antenna matching is perfectly fine if no tag is in the field, but once the sensor tag is being put on top the matching de-tuns and very little power is transferred.

    Thanks, BR Travis

    T
    Travis Palmer
    ST Employee
    January 22, 2024

    Hello Kai,

     

    your receiver circuit is loading and de-tuning the antenna. You could start by measuring the smith chart when the receiver is in different distances (0mm, 5mm, 10mm, 20mm) to see the effect. Please feel free to share the measurement with the community.

    Most likely the antennas have a strong coupling in 0mm or 1mm.

    I see two possibilities:

    1.) tune the antenna with the receiver within the desired operation distance (e.g. 1mm).

    2.) increase the transmitter antenna to lower the coupling in close distance.

    please let us know, if this helped.

     

    br Travis

    P
    Peter_H
    Visitor II
    February 13, 2024

    Hello Travis,

    many thanks for Your reply.

    The question with the insufficient energy at the power receiver is nearly solved. This must be caused to the sub-optimal SW configuration of the initiator. If I change to other screen in the GUI, the voltage at the power receiver is much higher.

     

    Could we please get also some reply to my questions, which I posted on 04.12.2023? (In which I write about different Smith-diagram of the evalboard antenna compared to the appnote.)

    Many thanks in advance!

    Peter

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