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This post shows the finished design for a MFRC522 antenna circuit. Calculations were done using the NXP application note AN1445. The antenna used for this project is a MOLEX 142360021. Antenna parameters (from datasheet) are: L(a)=2,6uH; R(a)=3,2Ohm; C(a)=2,3pF; Q=68,4.
The antenna circuit is divided in 4 functional blocks, each indicated by the down facing inductors.
(From left to right)
The first block is simply the output of the chip and will, in reality, only be 3 pins to connect to.
The second block is an EMC filter which is tuned to 14.339MHz, thus close to the upper sideband frequency. (13,56MHz (carrier) + 848 kHz (sub carrier)).
The third block is the matching circuitry, matching the ~230 Ohm antenna impedance to the 40 Ohms required on the chips output. The impedance transformation is not only done in this block but also in the EMC filter.
The last block is the equivalent circuit of the antenna and additional damping resistors which are necessary because of the required Q=35 specified in the chips datasheet.
The receiving side of the circuit is not included yet, as most of its components are predefined and the rest need in circuit measurements which I won’t be able to do until I finished the prototype PCB.
Note: Modulation of the signal is obviously not implemented in sim and the chip, most certainly won’t use square waves on the output. These are just for visualization.
Does anyone here have any experience with multiplexing RFID antennas? If so, what are the key design choices to look out for?
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