How to measure RSSI on NFC Reader and tag?

[WHadr.1]

Hello everyone,

I am working on a project where I need to measure the RSSI value on an NFC reader and NFC tag. The RSSI needs to be the same (or proportional) on both sides and then stored on two microcontrollers on both sides for further processing.

I first tried to get the value in the software but then I realized that the tag (ANT7-T-ST25DV04K) cannot provide such information, unlike the reader (ST25R3911B on X-NUCLEO-NDC05A1). This is not helpful because the RSSI needs to be measured independently on both sides.

I then tried to measure the signals on the reader and tag antennas on an oscilloscope to analyze the strength of the signal sent by the tag and the load modulation amplitude on the reader side as suggested here question/0D53W00000zoFtwSAE/rssi-measurement-via-st-nfc-sensor-app and here question/0D53W00000XFfD1SAL/hi-i-was-wondering-if-there-is-an-application-to-test-the-signal-strength-between-st25r3911b-disco-and-stevalsmarttag1-many-thanks. The main problem I faced here is that the passive probe I use on the tag side and the BNC cable I use on the reader side are loading the circuit and the signals are not accurate and not strong enough to make good measurements (I used a near field probe to see the attenuation created by the probes.)

What do you suggest here? Is my approach feasible? And would active probes help me get accurate measurements? Or should I change my measurements approach?

Also, are there any evaluation boards that can give me access to more data or have specialized measurement points for the antenna?

Best regards,

Wacime.

[Travis Palmer]

Hello Wacime,

for ST25R(eaders) most devices (e.g. ST25R3911B/3916/3916B) provides a measurement function to report back the measured RSSI value of the cards back scatter.

For tags, the signal quality is mostly proportional to the readers field strength. (e.g. OOK modulation scheme).

So measuring the field strength could give an indication about the signal coming from the reader.

I need to forward this question to one of my tag colleagues.

BR Travis

[Henry Crane]

Hello Wacime,

sorry for this late answer.

our ST25DVXX product family features an energy harvesting pin. This pin is normally used to provide a DC voltage from AC0-AC1 RF input voltage to power a low power circuitry. If you connect a 10nF capacitor to filter the DC voltage and a high impedance mesurement system you will get a DC voltage very close to the ST25DV RF input voltage magnitude.

For RF measurement, you can either use two oscilloscope active probes with ground tips connected together and floating, and make the difference between the two signals. Better, you can use differential probes which featrue lower parasitic capacitance.

Feel free to contact us for more details.

best regards,

Henry Crane

RFID/NFC application Engineer.