ST25R3916B, ISO14443 and 15693 perfect, Card Emulation no Reaction.

GJaeg.1 · ‎2026-05-21

Hello,

I start testing with the STEVAL-M25R16B Board and use this Time the self compiled Demo from ST
to test it. All works find.

Then i made an own PCB with Reader IC and Antenna on it and connect it to your MCU /Display Board.
The Board is connected to 3.3V, GND and the SPI / IRQ Lines.

The Antenna on my PCB is very small, only 21x26mm. Inductance measured with Siglent VNA 770nH.
( 0.5mm Track Width, 0.3mm Distance, 3 Turns)

The from your Tools calculated Values and my final Values after tuning differ..

My Result: L=270nH, Cemv 680pF, Cs 220pF, CTune 430pF, Rs 0.68R

I tuned the Circuit using my VNA and get a Reading Distance of about 25mm for 14443 Keyfobs and Cards
and about 6cm for ISO15693 Card.

This all works fine. Smith Chart looks perfect.
The Rx Voltage is about 1.5Vpp. The Voltage Divider Capacitors are 33p/120p.ST25R

Problem: Tags are working but in Emulate Mode i got no Reaction. With the Original Antenna (66x66mm) all is fine.
Need a little help how to go on...

Travis Palmer · ‎2026-05-26

Hello GJaeg.1,

Your capacitive voltage divider 33pF:120Fp is suspicious to me.

Typically the capacitive voltage divider is in the range of 10pF:120pF to 10pF:220pF dependent on output power and q-Factor of the reader design.

If your capacitive voltage divider is 33pF:120pF and you are measuring 1.5Vpp on the RFI, then your antenna voltage is approximately 5.5Vpp.

Typically the voltage on the antenna is around 25Vpp to 50Vpp for a normal generic NFC reader.

Can you share your measurements and pictures of your setup?

You can also cross check the simulation results of the antenna matching tool and QUCS.

Besides this, the EFD (external field detector) is essential for the ST25R operating in card emulation mode.

You should try different settings of the EFD and observe its state when an external field is brought close to the device.

BR Travis

GJaeg.1 · ‎2026-05-26

Hello Travis,

i change the Voltage Divider to 10p/120p. No Result.

My Board incl. ST25 and Antenna should use (if possible) no Autotune Function because the
Varicaps used on your Board are NRND. But measuring at the AAT Outputs looks like you are
using different Tunings for R/W and Emulation Mode. Do you think it is possible to use both Modes
with one fix Tuning ?

I cant post Pictures because its an commercial Board. The Last 10 Years we use the CLRC66303HNY.
I tune the Circuit the same way as all Times and can reach very good reading distances with Keyfobs
and Cards so i cant be too wrong...i hope :)

GJaeg.1 · ‎2026-05-26

Hi Travis,

completly new from the Beginning.

Fix:
L=270nH and Cemv 680pF
Voltage Divider 10p/220p

Cs 180p
Ctune 82pf
C_AutoTune 180p (about 165 should be better, see Smith Chart.
Rs 0.33R

Ulysses HERNIOSUS · ‎2026-05-26

Hello CJaeg.1,

I just had a chat with Travis and he proposed to analyze also from SW side.

As I understood your latest experiments was now using the varicaps? The proposition would be to work without them and to use a fixed matching. According to my understanding this will be feasible on a small antenna like yours.

You can work by only removing the varicap devices. Additionally you can also remove the settings for RFAL. Provide an updated AnalogConfig / change existing AC to:

set aat_en=0 in CHIP_INIT section (rfal_analogConfigTbl.h)
(optionally) remove ST25R3916_REG_ANT_TUNE_A/B settings throughout the AC

If you continue to have issues, then it would be good to start looking first from SPI side what the ST25R3916B really sees. I_eon/I_eof interrupts, reception of PCD frames and see if ST25R3916B answers and reader potentially does not see it. If you share a logic analyzer trace of SPI + IRQ then we can help by inspecting and guiding you towards the root cause.

By the way: What is reader in this scenario? A mobile phone? It could be beneficial for debugging to try with a second ST25R3916B as reader, this way you could single-step through the NFC protocol.

BR, Ulysses

GJaeg.1 · ‎2026-05-26

Hello Ulysses,

no, i didnt use Varicaps and i hope to find a fixed Tuning. The Murata Varicaps are NRND.
The Device i use as Reader is a Smartphone.
About the SPI Analysation: I come back with this tomorrow. Our Software Developer also has done
some NFC Field Analysis. This Time i have only one ST25R3916B Board.

Can you confirm that Reader Mode and Emulation Mode should work with the same Tuning ?

Ulysses HERNIOSUS · ‎2026-05-28

Hi GJaeg.1,

what are then the above Ctune and C_AutoTune, which you are referring?

The different tuning was needed to achieve standard compliance. Even without it was working in desk-based interop testing. With your smaller antenna you should actually have here fewer challenges.

Looking forward to your traces.

BR, Ulysses

GJaeg.1 · ‎2026-05-28

Hi,

C_Tune is my Value for the 2x82pF in your AppNote, C_AutoTune is the Replacement für the
2x220p + Varicap in your AppNote.

Yesterday i ordered a new Version of the PCB. I increase the Antennas TrackWidth from 0.5 to 1mm
with 6 instead of 3 Turns to get the same Inductivity but higher Q.
I also place the capacitive Voltage Divider nearer to the ST25. Hope so to get a better Q and
clearer RX-Signal. As i told you it is a commercial Product, so i cant upload PCB Layout.
Parallel we are also testeing the PN5180 because the Software Support for DesFire from
NXP is also better.