Design of MF RC500 Matching Circuits and Antennas

W Matched Antennas
In table 2 two concepts are proposed to design a 50Ω antenna. In both concepts the EMC circuit and the receiving circuit are identical. Firstly, these parts will be described, followed by a description for the long- and the short- range impedance transformation circuits. The last part will show the recommended design for a matching circuit for 50Ω 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 3-7. The low pass filter consists of the components L0 and C0. The values are given in Table 5.

RECEIVING CIRCUIT
The internal receiving part of the MF RC500 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 3-7 shows the recommended receiving circuit. The receiving circuit consists of the components R1, R2, C3 and C4. The values are given in Table 5.

Table 5.Values for the EMC- Filter and Receiving Circuit

Components Value Remark
L0 1.0 mH – 10% Magnetic shielded e.g. TDK ACL3225S-T
C0 47 pF – 2% NP0 material
R1 820Ω – 5%  
R2 2.7kΩ – 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.

W FULL RANGE SOLUTION
To be able to connect a 50W coaxial cable to the MF RC500 an impedance transformation has to be done. This impedance transformation should fulfil three requirements:
–Implementation of an EMC- Filter
–Impedance transformation between the low output impedance of the MF RC500 and 50 Ω.
–The MF RC500 has symmetrical output drivers TX1 and TX2. To be able to connect a coaxial cable an unsymmetrical potential to Ground has to be generated.
A way to design a circuit fulfilling these requirements is to use a transformer or a Balun1 to generate a ground unsymmetrical potential. Figure 3-7 shows one typical realisation using a Balun. The EMC filter based on L0 and C0 has the same structure as mentioned in the design hints for directly matched antennas. The combination of the components L0, C0 and C1 have the structure of a T- filter. This filter transforms the output driver resistance to the 50Ω resistance of the coaxial cable. The balun B1 should have a 1:1 ratio and should match to 50W. The capacitance C2b is just optional2. The small unsymmetrical behaviour of the balun can be reduced trimming that tuning capacitance to the maximum output voltage at the antenna.
Important Note: The bridge output drivers of the MF RC 500 are low ohmic devices. To achieve the best performance a matching of 30W between Tx1 and Tx2 should be used.
The easiest way to calculate the needed impedance transformation is to use a smith chart.

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Figure 3-7. Full Range solution: 50 Ω impedance transformation

Using a transformer or a balun is a way to generate a to ground unsymmetrical potential. The balun concept demonstrates how to reach with a few external components the full operating distance with a 50Ω match. The basic functions of the balun and the calculation for impedance networks can be found in standard literature. The result of that calculation will give starting values for a tuning procedure to find the best solution. To provide the functionality of the EMC filter a compromise between a matching to 50 Ω and the filtering has to be found. Table 6 shows the results of the tuning procedure. The optional tuning capacitance should be used to find the best result for the actual design.
Table 6. Type50-1.Values for the impedance transformation

Components Value Remark
C1 82pF – 2% NP0 material
C2a 69 pF– 2% NP0 material
C2b 0-30 pF  
B1 1:1 Transformer e.g. Coilcraft 1812WBT-3

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