Design of MF RC500 Matching Circuits and Antennas

Directly Matched Antennas
One of the proposals in table 2 is a directly matched antennas. The recommended circuit can be used to reach operating distances up to 100 mm. The operating distance depends primarily on the size of the antenna as well as the correct values for antenna’s matching circuit. The needed components, the EMC filter, the receiving circuit and the antenna matching itself will be described as well as their necessity for a proper functionality of the MIFARE® system. Figure 3-3 shows the recommended circuitry for directly matched antennas.

EMC CIRCUIT
The MIFARE® system is based on an operating frequency of 13.56 MHz. This frequency has to be generated by a quartz oscillator which will also generate higher harmonics. To conform with the international EMC regulations the third, fifth and higher harmonics of the 13.56Mhz have to be suppressed adequately. Beside a multi-layer layout, it is strongly recommended to implement a low pass filter as shown in Figure 3-3. The low pass filter consists of the components L0 and C0. The values are given in Table 3.

RECEIVING CIRCUIT
The internal receiving part of the MF RC 500 uses a new receiving concept. It uses both side-bands generated by sub-carrier load modulation of the card’s response. It is recommended to use the internally generated VMID potential as an input potential of the RX pin. To reduce disturbances a capacitance to ground has to be connected to VMID. The receiving part of the reader needs a voltage divider connected between the RX and the VMID pin. Additionally, it is recommended to use a serial capacitance between the antenna coil and the voltage divider. Figure 3-3 shows the recommended receiving circuit. The receiving circuit consists of the components R1, R2, C3 and C4. The values are given in Table 3.

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Figure 3-3. Complete directly matched antenna configuration

The values for the filtering and receiving components L0, C0, R1, R2, C3 and C4 are fix.
Table 3. Values for the EMC- Filter and Receiving Circuit

Components Value Remark
L0 2.2mH – 10% Magnetic shielded e.g. TDK ACL3225S-T
C0 47 pF – 2% NP0 material
R1 820W – 5%  
R2 2.7kW – 5%  
C3 15 pF – 2% NP0 material
C4 100 nF – 2% NP0 material

Note: To achieve the best functionality the used capacitors and inductors should have at least the performance and the tolerances of the recommended ones.