RFID Antenna Tuning Circuit And ST Tool Values?

[Ricko]

Hi,
I don't have any experience in RF design but I need to design the matching circuit for this RFID antenna (PDF attached) to work with the ST25R100 (PDF also attached).

(NOTE: IF for some reason that antenna cannot be used, can you please suggest some other [ideally] rectangular antennas that can be used and that are FLEXI-PCB TYPE because we need to attach it to a curved plastic surface).

As long as the RFID card can be read reliably every time from a distance of at least 20mm, then it is acceptable.

I saw the reference design but the values are different. And after trying to play with the ST tuning tool I still don't know what values to put into some of the fields: https://eds.st.com/nfc-tuningcircuit/

From the datasheet, I assume the values to input in the software are as follows: antenna self resonance = 13.56MHz, DC Resistance 2,000mR and inductance 2,000nH. Correct?

BUT... These are not clear to me in the tool input fields:
- should I use series or parallel resistors? What are the pros/cons of each and the criteria for choosing?
- what is the Parallel Resistance (whether I click on series or parallel resistors it still shows PARALLEL Resistance)? How do I choose that value (it is not in the datasheet)?
- I can put the most extreme and different values in the Antenna input fields (top area) but it does not seem to solve, it always gives error of either Q being too high or Cp being less than 0?

Also, do we need to add two ultra low capacitance TVS for ESD protection between each of the two antenna pins and ground (or after the resistors) or would that de-tune it?

I would very much appreciate if you could please help me find the right values?

Thank you

[Ulysses HERNIOSUS]

Hi Ricko,

please see the appnote AN6065: Antenna design for ST25R200 and ST25R100 devices.

BR, Ulysses

[Ricko]

Thank you @Ulysses HERNIOSUS 

I read through it (or tried to be precise) and 80% of it made no sense because my background is not RF.

All we need is to be able to read an RFID card placed next to the coil and distance is not an issue, literally just 5 to 10mm (maximum) is totally acceptable.

Could you please be so kind to help me and provide some safe values I could start with that (at least most likely) would work. Again, we are not interested in long or even medium distance, the card has to almost touch the reader antenna.

Thank you :)

[Travis Palmer · Accepted solution]

Hello Ricko,

There are no general or universal values you can input into the antenna matching tool.

Those inputs are specific to your antenna. On a differently manufactured PCB, e.g. different layer stack, different dielectric parameter, the antennas might be different and your result might also be different.

If you share your smith chart measurement of your antenna - it should look like Figure 12 of AN6065 - we can help you with the inputs and verification steps. 

br Travis.

[Ricko]

Thank you @Travis Palmer 

we are working on the mechanics and checking if some other off the shelf antennas can fit. If the datasheet has the smith chart, I can post it here. Otherwise no idea how to generate it (I'm not an RF engineer).

Thank you :)