ST25R95 - Distance range, output power, inductor as antenna

[JPehl.1]

Hi.

I am developing an electronic lock using an STM32 L4 and a ST25R95 NFC transceiver.

According to document "ST25_product_overview-July2018.pdf" ST25R95 is supposed to be a 230mW transceiver. I would like to know what reading distance I can reach between the reader and the tag, in centimeters? 

In my project, for security reason and in theory, my reader would work with a maximum distance of 15cm from the tag, better to not work with more than 15 to 20cm.

Is it possible to make it work this way? it has a DPO...

And also, is it easy to use an SMD inductor instead of copper traces around the board as antenna? From which values of inductors should I start? Can recommend a part number? 7mm height max for example.

Consider that the antenna of the reader is placed inside a door lock, so there is a obstacle. My main doubt is if I can (or can't) select the output power of the reader via SPI registers.

Regards.

[Brian TIDAL]

Hi

the reading distance depends on the technology being used, in particular ISO15693 has an higher reading distance than ISO14443.

Which technology do you plan to use?

As far as I know, the ST25R95 does not have DPO and the output power cannot be changed via register settings.

For the antenna questions, I'll ask a colleague to provide answers to you

Rgds

BT

[JPehl.1]

Hi Brian.

Yes, now checking again I can see it does not have DPO. But anyway we would like to know the aproximate distance range for ISO15693 and ISO14443, considering that the board will be inside the lock of an ordinary door. Our goal is something like 10-15cm for maximum distance of operation between the tag and the reader.

And also, if we use an inductor as antenna, what would be the microHenry value of it, to start? in order we can select its part number.

Question: when I use inductor as antena, I can make my board in a way it can support multiple sizes of inductors, and then this would change the distance range... is that possible?

[Brian TIDAL]

Hi

the reading distance is dependent on many factor, in particular the size of the tag antenna

On my X-NUCLEO-NFC03A1 with the X-CUBE-NFC3 firmware, depending on the tag, I can achieve the following read ranges

• 3.2 cm and up to 5.2 cm for ISO14443 tags
• 4.6 cm and up to 9.2 cm for ISO15693 tags

As a preliminary answer regarding the antenna performances,  see AN5248 ST25R95 transceiver antenna tuning circuit with EMI filter. An RF expert will answer to you antenna questions.

Rgds

BT

[JPehl.1]

Thanks very much Brian.

We have space limitation for this project, then I wondered if I could use an inductor on the place of an antenna made of some turns drawn around the board.

You said you reached 9.2cm. Was this at open space? or with obstacle? Which tag was used in this case? Whats the size of it? can you send a picture please?

The EMI filter and antenna spreadsheet calculator (ST25R95_EMI_FILTER_CALCULATION) starts with antenna as 1.2 microHenry, 800 miliOhms...

We would like to know if we can reach the same 9.2cm when using an inductor as antenna... Or maybe, if we could even get a greater distance with an inductor.

When talking about distance range of communication, in our case we should consider that the electronics is to be placed inside the lock of a simple door (wood mostly). The lock itself will have metal box with hollows to permit RF to pass.

Do you have idea of when I will get the answer about the RF?

Regards,

Jeferson.

[Brian TIDAL]

Hi Jeferson,

All the reading distance measurements have been done in open field (no obstacle). I've used many, many different tags from the market with mainly class 1 antenna. Our RF expert will provide you more information and recommendation regarding the tag antenna class for your specific application. I expect an answer within this week for the various RF questions.

Regarding the tag, ST has a full range of solution for ISO15693 (ST25TV and ST25DV series) and ISO14443 (ST25TA series). I will ask one of the tag expert to provide you more information for your application.

Feel free to share with us more information regarding your application and the features needed on tag side (signature, etc.) so that we can provide appropriate answers.

Rgds

BT

[JPehl.1]

Hi Brian.

We need to know if can use inductor in place on pcb antenna. we finished a pcb layout that can use an inductor of up to 12x12mm x 5mm height.

If can use something like this: https://www.digikey.com/product-detail/en/B82464A4102M000/495-2001-1-ND/723990/?itemSeq=326427780

The inductor was placed at one of the edges of the board.

We need to know this to proceed to pcb prototype stage.

[Henry Crane · Accepted solution]

hello Jeferson,

you can use an inductor for reader antenna assuming it is not shielded.

However, compared to a few cm x few cm the read range will be shorten. Indeed, the smaller is the diameter of the reader antenna, the higher id the field strength at 0mm (wires generating magnetic field are closer, so the magnetic field is concentrated), but from the other end, it decreases more rapidly with distance.

Please keep us posted of your read range evaluation on first prototype.

Best regards,

Henry crane NFC/RFID support team.