Design of MF RC500 Matching Circuits and Antennas

ANTENNA MATCHING CIRCUIT FOR DIRECTLY MATCHED ANTENNAS
It is recommended to do the design of a directly matched antenna step by step. Firstly, the antenna coil has to be designed. The antenna itself is a low ohm device. To connect this antenna coil to the MF RC500 a matching circuit is required. Starting with the estimation of the antennas equivalent circuit and the calculation of the quality factor, the recommended values for the capacitors of the matching circuit will result.

Determination of the Antenna’s Equivalent Circuit:
The reader antenna coil can be described with the equivalent circuit shown in the left part of Figure 3-4. The recommended antenna design for directly matched antennas should have a grounded centre tap in the antenna coil. This centre tap is implemented to improve the EMC behaviour of the antenna. The coil itself can be described by the inductances La and Lb, the resistances Ra and Rb to describe ohmic losses and capacitive losses described by the parallel capacitances Ca and Cb. Anyhow it is not recommended to calculate the components of this equivalent circuit because of coupling effects between La and Lb.

Instead of the complete model, it is recommended to use the model shown in the right part of Figure 3-4. The complete antenna coil between the connectors Tx11 and Tx12 can be described by Lant, the complete ohmic losses by Rant. The coil capacitance Cant describes losses between both windings and between the connectors.

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Figure 3-4. Equivalent circuit of an antenna coil for a directly matched antenna

It is recommended, to measure the antennas equivalent circuit with an impedance analyser. Connect the antenna loop (when using shielded antennas connect the shield to ground) and measure the shown equivalent circuit. The value for the coil’s capacitance Cant can be neglected for the calculation of the quality factor and the tuning of the antenna.
Note: If an impedance analyser is not available, use as starting value the calculated values for the inductance and resistance. To estimate these values formulas are given in Annex A. The operating frequency of MIFARE® is 13.56 MHz. At this frequency the ohmic skin effect losses can not be neglected, that is why it is not correct to use only the DC resistance of the coil. Please use the estimation in Annex A to find a start value for the impedance Rant. It is recommended to check the complete design later by measuring the quality factor. If necessary, this starting value has to be changed and the complete tuning procedure has then to be done once again.

Quality factor
For the following part it is presumed that the values for the antenna’s inductance LANT and resistance RANT are known. It is recommended to measure LANT and RANT with an impedance analyser. If the estimation is done using the formula to calculate the values, keep in mind that these values are only starting values and might have to be changed after verification of the Q-factor.
The antenna’s quality factor is an important characteristic for the correct tuning of the antenna and the achievable performance. The quality factor of the antenna is defined as:

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Depending on the geometrical conditions of the antenna Q has usually a value in the range of 50…100. This value has to be reduced for a proper data transmission. As mentioned in chapter 2.3.2 the baudrate of MIFARE® is 105.9 kHz/sec and the data transmitted from the RWD to the card are Miller-coded with a pulse length of T=3µs. Using the definitions for the bandwidth B Mifare DESFire Smart Cards For Access Control systems,Mifare DESFire EV1 2K Offset Printing Cards,NXP MF3ICD21 Cards

And the definition for the time – bandwidth product  B × T 1

 The required Q- factor can be calculated as Mifare DESFire EV1 2K Offset Printing Cards,Mifare DESFire 2K Card,HF 13.56MHz Mifare DESFire 2K Contactless Cards

Due to tolerances and temperature dependencies of the components, it is recommended to use a value of 35 for the Q-factor. To reduce the original Q-factor it is required to implement an additional external resistance REXT as shown in Figure 3-5. The value of REXT can be calculated by

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As mentioned above, it is recommended to use a centre tap to design an antenna coil for a directly matched antenna. Therefore, the result for the external resistances has to be split up into two equal parts. The complete circuit to reduce the antenna’s quality factor is shown in Figure 3-5.

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Figure 3-5. External resistance to reduce the antenna’s quality factor

Impedance Matching for directly matched antennas
To design a matching circuit for a directly matched antenna it is recommended to use the circuit shown in Figure 3-6. The values for the capacitors Cs and Cp depend on the antenna itself and the environmental
influences.

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Figure 3-6. Complete Matching circuit

It is recommended to use the values for the capacitors shown in Table 4 as starting values for the tuning procedure. To tune an antenna to an optimum the procedure  for directly matched antennas has to be followed. The start values depend on the antenna’s inductance.

 

LANT [µH] CS [pF] CP1 [pF] CP2 [pF]
0.8 27 270 330
0.9 27 270 270
1.0 27 220 270
1.1 27 180 || 22 220
1.2 27 180 180 || 22
1.3 27 180 180
1.4 27 150 180
1.5 27 150 150
1.6 27 120 || 10 150
1.7 27 120 150
1.8 27 120 120

This table assumes a stray capacitance of 15 pF of the antenna coil. The capacitors Cs and Cp should have a NP0 dielectric with a tolerance of +/- 2%. Actual values of the antenna inductance and capacitance depend on various parameters.
–antenna construction (Type of PCB)
–thickness of conductor
–distance between the turns
–shielding layer
–metal or ferrite in the near environment
Due to these influences the values for Cp have to be optimised with the actual design.

Table 4.Starting Values for the antenna matching circuit