How to use ST25R3911B for long reading distance(40cm)

[Hzhao.2186]

Hello,

I am new to NFC solutions and want to design a product that can read tags for 40cm distance with ST25R3911B , could you give me some suggestions ?

And how to adjust the antenna driver output resistance (register 0x27) to match the impedance,also what is the suitable value for the capacitive voltage divider?

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Thank you very much for your help.

Best regards

Sky

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[Travis Palmer]

​Hello Sky,

Following this two threads, you should succeed.

https://community.st.com/s/question/0D50X0000AvgiHoSQI/st25r3911b-with-antenna-size-30x40cm

https://community.st.com/s/question/0D50X00009XkWCSSA3/design-questions-around-st25r3911b

In the second thread we achieved a read range of 24cm using a 14x10 cm antenna. For you purpose I would recommend a 25x25cm or 30x30 cm antenna.

The Register 0x27 should be kept at 0x00. I would start with a target matching impedance around 15 to 20. The Q-factor should be around 35. The capacitive voltage divider should be setup in a way you achieve 2.7Vpp on the RFI (using the Measure Amplitude command).

Further information can be found in the application note "AN4974 Antenna matching for ST25R3911B/ST25R391x devices".

BR Travis

[Hzhao.2186]

Hi Travis,

Thanks a lot for your reply. This is some inputs and test set up:

Could you point out whether there is any mistake above？

I am not understand the following content wrote by you：

I did some tweaking on the receiver settings (AM only, 1st level gain set to boost). also due to the high output power it was necessary to bypass the internal Vsp_RF regulator (connect pin 9 to VDD)

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Another question is if the larger the coil （eg. 40*40cm）, the farther the reading distance？

Thanks.

[Travis Palmer]

Hello Sky,

all your inputs in theST25R Antenna Matching Tool look valid.

One small hint about measuring the Voltage on the RFI: Your scope probe (tip and GND-connection) is

doing a big antenna itself. It will pick up the H-Field of the Reader and add or subtract if from the Voltage on the RFI's. Its recommended to use a N4838A ground spring. Using it, you can measure both RFI voltages referenced to J301.

The easiest way to measure the RFI voltage, is to use the direct command "measure amplitude". You should setup the capacitive voltage divider to achieve ~2.7Vpp on the RFI.

The read range of the system (reader + tag) depends on the coupling between reader and tag antenna.

The reader is generating a magnetic field, which is picked up by tag's loop.

The tag then needs a minimum voltage to power up and operate. The power transfer will depend on the coupling between reader and tag antenna and the

emitted power of the reader. At some point, it will not help to increase the Reader antenna, because the fields density will get lower and the tag picks up

less field.

Before doing more optimizations (output power, register settings etc.) you should determine if your power or sensitivity

limited.

1.) To do so, you should put a pick-up coil (either a self-made wire-wound or ISO calibration

coil) on the tag. The tags and pick-up coils antenna should have nearly the same geometry to sense exactly the field the tag sees.

2.) Connect the pick-up coil to the oscilloscope using a high impedance probe.

3.) Put the tag+pick-up coil in an area where the tag can be read reliably. Using the ISO15693 tab's Inventory button you should be able to see the readers request

and the cards response.

4.) Move the tag+pick-up coil slowly out of the operating area. hold the position, once the Gui does not display the tag anymore.

5.) There are now two possibilities, why the communication stopped working:

A.) Power limited: The tag's answer disappeared or is very different (shorter) as before. The tag has not enough power to reply.

B.) Sensitivity limited: The tag shows still the same response, but the reader is not able receive the signal.

What is currently the read range you are able to achieve?

Br Travis

[Hzhao.2186]

Hi Travis,

Now i can achieve the reading distance about 22cm with the ST25DV_Discovery_ANT_C5 tag，but difficult to improve more，

I have use the ground spring to get the capacitive voltage divider to 2.7Vpp on the RFI---change the C401&C412 to 220PF+27PF;

I am not very clear about the pick up coil, and how to use it;

And is it the impedance more close to pure resistance, the performance will become better？

Does the values of matching circuit mast be same？ eg.C410=C403 , C408=C406

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Thanks.​

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[Travis Palmer]

Hello Sky,

I usually use two pick up coils. One is close to the reader and used for triggering my scope, the second one is used to pick up the cards response.

You can either use a self made pick up coil (red wire tapped to the ST25DV_Discovery_ANT_C5 PCB directly) or an ISO Calibration coil (green PCB).

Both methods should use a high impedance probe to connect to the scope.

This is my lab setup for such investigations:

The yellow channel is recorded via the pick up coil close to the ST25R3911B-DISCO board. You can clearly see the inventory request command sent by the reader. The inventory response is already very small and cannot be seen with bare eye anymore. Still the reader can decode the response.

The red channel shows the signal recorded very closely to the tag. We can therefore clearly see the request sent by the reader and also the response sent by the ST25DV_Discovery_ANT_C5 tag. It shows the H-Field close to the tag.

If you now increase the distance between reader and tag, either the reader stops seeing the tag (sensitivity limited) or the tag stops responding (power limited).

On your setup, the reader antenna is very big and the tag antenna is very small. This means, there is already a bad coupling between the two antennas. The coupling influence both factors:

The power transferred from reader to tag and the load modulation amplitude (LMA) the tag can do.

In your case, I would recommend to increase the coupling by increasing the tag antenna size. Using a ID1 size antenna should already boost your results.

"And is it the impedance more close to pure resistance, the performance will become better？"

What is your current matching impedance? can you add a VNA smith plot?

"Does the values of matching circuit mast be same？ eg.C410=C403 , C408=C406"

Yes, the series caps (C410 = C403) and the parallel caps (C408 = C406) should have the same value. otherwise your matching and loading of the chip will not be symmetric.

Br Travis

[Hzhao.2186]

Hello Travis，

Thanks a lot for your detail reply!

I have tried the smaller coil 25*25cm(2 turns) with the smith chart and change the received settings as below:

Now the maximum reading distance is 25cm, do you have any other suggestions for improve the reading distance, such as add power... or this is the limited distance for my case, actually the tag coil size can't be increased more...

Another question is the coaxial cable connected to VNA, did i need disconnect the circuit behind？

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Thanks.

[Travis Palmer]

Hello Sky,

Still the question is, if the tag stops replaying to the reader command, or if the reader does not see the tags response.

If the tag stops replaying, we can bypass the internal regulator and decrease the matching impedance to 6-8 Ohm.

If the reader does not see the response anymore, we need to look at the internal receiver signals to optimize the settings.

Do you have the trim pins connected? At 15 Ohm matching impedance and a high Q-factor it is most likely that the limit of 20V on the trim pins is exceeded.

The result might be, that the ESD protection is getting active and reduces the voltage / power on the antenna.

Your matching and VNA measurement looks perfect.

br Travis

[Hzhao.2186]

Hello Travis,

I have disconnected the AAT in my previous tuning.

And I tuned the matching impedance to 7Ω and connected pin9 to VDD（J303 with 0Ω），the distance no improve，

could you tell me how to optimize the settings of internal receiver signals？

And do you think it can reach the 35-40cm reading distance with the ST25R3911B ？

Thanks�?

[Travis Palmer]

Hello Sky,

Yes i am confident that this can be achieved. Likely due to the bad coupling between reader antenna and tag antenna additional circuitry (amplifier, filter) might be required.

Internal receiver signals:

You need to connect to P301 (CSO) and P302 (CSI) on the bottom side of the board. There is a footprint for UFL connectors foreseen where a UFL to BNC or SMA cable could be plugged. You can also directly solder a cable to the pads. To capture a good signal, i recommend to remove / cut the traces leading to the capacitive patches (they could pick up some field).

You should then use a high impedance probe to feed the signals into the oscilloscope.

Referring to chapter "1.2.15 Test access" of the datasheet DS11793 - Rev 5:

By setting the registers as described in this section, you can e.g. output the analog signal of the AM channel and see how the digitizer decodes it.

several other combinations are possible as mentioned in Table 13.

Below you can see a Type A response showing:

Yellow: H-Field

Blue: Analog output of AM channel (before digitizer)

Green: Digital output of AM channel (after digitizer)

The screenshot shows a quite strong signal. After the first sub carrier pulses the AGC reduced the signal to a optimum level.

You can now alter filter and gain settings and instantly see their impact.

Br Travis

[Hzhao.2186]

Hi Travis,

Thanks for the reply.

Do you mean maybe need add an additional amplifier between IC's RFO pins and antenna matching network?

And another question is for tag (ST25DV64K),What values do you recommend for the inductance and impedance of the coil?

I didn't see any matching elements on the circuit board for ST25DV tag.

Thanks�?

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[Travis Palmer]

Hello Sky,

Adding a push pull driver stage, connected to a higher supply voltage (12V or even higher) could help here to transfer more power to the tag.

BR Travis

[JL. Lebon]

​Hello Sky,

Regarding ST25DV64K, the calculation of antenna inductance is explain in the AN2972 that you can find at this place: https://www.st.com/content/ccc/resource/technical/document/application_note/bc/ac/13/fe/69/fb/49/8a/CD00232630.pdf/files/CD00232630.pdf/jcr:content/translations/en.CD00232630.pdf

The ST25DV has an internal tuning capacitance of 28.5pF. If you design your antenna correctly you can have an antenna + tag resonating at 13.56MHz without external tuning components. It is still possible to add an additional external tuning capacitance in parallel to fine tune the resonance.

In order to increase the reading range on the ST25DV side, you can play on the level of energy received by the tag.

To increase the level of energy, the main factor is the antenna size, and the coupling factor with reader's antenna. In your case, considering the size and the distance of the reader's antenna, a large antenna on tag side should improve the distance (but I understand you need a small antenna ?),

Another thing you can try  is to power the ST25DV with it's VCC pin (if this is possible in your case). By doing this, some internal blocks of the ST25DV are powered by the wired VCC instead of being powered by the RF field, and the global power consumption of the device on RF is lowered a bit. This may slightly improve the reading range (but not a lot) if the limit was the powering of the tag.

Best regards.

[Hzhao.2186]

Hi Lebon，

Thanks for your help.

Yes，the tag antenna need a small antenna in my case，and need to receive four directional signals. Could you give me some suggestion about the shape of the tag antenna？

Now i'm studying AN2972， one question is how to define capacitance value which needed to fine tune the resonance？

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Thanks�?​