Evaluation Board M24SR64-Y - Inductance value and tuning frequency

[Niko97]

Good morning,

I am using the ANT7-T-M24SR64 evaluation kit based on the NFC front end M24SR64-Y, with a 14x14 mm antenna, and I would like to know what is the exact value of the COIL inductance, as from the ST sw I was not able to replicate the correct inductance value to be able to do a proper tuning.

Also, as far as tuning is concerned, should I rely on the 14.2 MHz frequency if I want to be able to increase the reception distance between the receiver and the NFC transmitter? Or if so, can I add capacitors for proper tuning?

Kind regards,

Domenico

[Henry Crane · Accepted solution]

Hello Domenico,

the measured equivalent inductance of ANT7-M24SR64 is 4.76uH, leading to a resonant frequency around 14.1MHz.

A tuning frequency of 13.56MHz will improve the powering distance and the communication distance. Feel free to adjust the frequency with external capacitors, we often recommend to our customers to put some capacitor footprint to optimize the performance of their PCB without need to redesign the complete PCB.

To decrease the resonant frequency, put an external capacitor (C0G material is prefered for higher Q factor) in parallel to M24SR64 (between AC0 and AC1). 

In case the tuning frequency is too low, series capacitors have to be connected between M24SR64 and antenna. A single capacitance can be used to adjust the frequency. Afterwards, two capacitors representing each twice the trimming capacitor should be connected between AC0 and antenna on one side, and AC1 and antenna on the other side, to balance the aditionnal capacitance.

App note AN6505 (https://www.st.com/resource/en/application_note/an5605-design-of-a-1356-mhz-doublelayer-antenna-stmicroelectronics.pdf) "Design of a 13.56MHz a double layer antenna" may be interesting for you: it describes the way to use our eDesign Suite tool for 2-layer antennas, combined with use of tuning capacitors.

Feel to contact us for more informations, or share you antenna design constraints with us.

best regards,

Henry Crane, NFC/RFID products support team.