M24SR64-Y Antenna Design

[ELTE-Suraj]

Hello,

We recently designed an antenna patterns for  M24SR64-Y chip. As per the spec, the central frequency of this chip is centered around 14.2MHz, (Please correct me if my understanding is wrong). Thus we proceeded to design the antenna with STM edesign suite with 5uH inductance

The target inductance was set to 5uH, with 35u copper thickness and 1.6mm PCB thickness, For reference, I have attached the image below.

We recently got the antenna manufactured and found that the inductance of the antenna is around 5.6uH. 

We can communicate with the NFC chip. Here are our queries

1. We are not sure why we have shift in inductance value from 5uH to 5.6uH ?

2. What would be implication of this shift in the performance of the chip ? 

Following is the PCB Layout, 

Your suggestions would help us fix the problem and improvise for the next spin.

Anticipating your response

Regards

Suraj

[Henry Crane · Accepted solution]

Hello Suraj,

the tuning capacitance value specified in datasheet corresponds to the value monitored in production at low voltage (1Vpeak). The typical tuning capacitance at minimum operating voltage that has to taken into account for antenna tuning is 27.5pF.  

The best compromise to maximize the M24SR64 powering distance and the backscattering signal is a tuning frequency between 14 and 14.4MHz. The corresponding antenna equivalent series inductance is 4.7uH.

5uH leads to  13.56MHz antenna tuning frequency (M24SR64 powering distance maximized).

5.6uH leads to 12.8MHz antenna tuning frequency; this tuning frequency will cause a performance loss by both decreasing the powering distance of the tag and also decrease the tag backscattering signal.

In case you want to restore a tuning frequency closer to 14MHz, a possibility is to populate SJ5 and SJ6 beads with CMS capacitors. the calculation is the folllowing:

- to get 14MHz tuning frequency 5.6uH, the total requested capacitor is Ctot=1/(5.6e-6*(2*pi*14e6)^2)=23pF

- the series capacitance Cadd to put in series with M24SR is given by Cadd=(Ctun*Ctot)/(Ctun-Ctot) where Ctun is the M24SR tuning capaictance (27.5pF) and Ctot is the desired total capacitance to achieve 14MHz tuning frequency). The result is Cadd=140pF. Cadd has to be spiltted between SJ5 and SJ6 by populating C_SJ6=C_SJ5= (140pF*2)*280pF .

Another possibility is to redesign the antenna to match 14MHz with 27.5pF which can be acheved by the following design:

- L=73mm, W=45mm

- nb of turn: 6

- antenna track width=0.6mm

- antenna track spacing=0.5mm

feel free to contact us for more details,

best regards,

Henry Crane NFC/RFID product support team.