RFID distance issue with ST25R3916B

[DejanLesnik]

We are building an RFID reader. I copied the schematic and RF + antenna from the X-NUCLEO-NFC08A1. The design works, but I have an issue with the reading distance.

Some RFID tags are detected over 5 cm away, while others need to be placed less than 1 cm from the reader antenna. Where should I look to solve this issue?

I tried some commercial readers, and they all read tags from the same distances. I cut the antenna from the NUCLEO-NFC08A1 and replaced the one on our reader, but the result was the same. I also replaced all the RF components on our reader with those on the NUCLEO, and there was no change.

I tried many things related to RF design and antenna matching, but nothing helped. How can I achieve consistent reading of all RFID tags at the same distance, and ensure that the distance is more than 3 cm?

I'm now suspecting the variable capacitors or some settings in the ST25R3916B.

Best

Dejan

[Travis Palmer]

Hello Dejan,

Although the same antenna is being used, the matching could be different due to parasitic's.

This could originated by using a different PCB layer stack.

Therefore i would recommend to verify first the smith chart using a VNA.

Afterwards we need to check the RFI voltage (between 2Vpp and 2.7Vpp) followed by the supply voltages (VDD, VDD_TX, VDD_RF, VDD_DR, VDD_AM, AGD, VDD_A and VDD_D). Since you already have a NFC08A1, the easiest is to compare between those two.

You can also compare the power consumption of the ST25R3916B on both boards.

You mentioned that some cards are ok, others are not ok. What is the difference between these tags?

For example: NFC-A => ok, NFC-B => not ok?

BR Travis

[DejanLesnik]

Thank you for the answer. I measured both antennas and compared smith charts. They are not the same. Now I'll match the antenna with the circuit and see if this helped.

[Travis Palmer]

Hello Dejan,

could you share the smith chart plots?

Many Thanks, BR Thomas