Design of MF RC500 Matching Circuits and Antennas

ENERGY TRANSMISSION
The energy transmission between the reader antenna and the passive MIFARE® card is based on the transformer principle. At reader side an antenna coil is required as well as a card coil implemented in the MIFARE® card. Figure 2-3 shows the basic principle and the equivalent electronic circuitry. The figure’s left part describes the antennas and the energy transmission basically.

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Figure 2-3. Transformer Model

The current I in the RWD antenna coil generates a magnetic flux. Parts of this flux flow through the card coil and induce a voltage in the card coil itself. This voltage is rectified and the card IC is activated when the operating voltage is reached. The induced voltage will vary within the distance between reader antenna and the MIFARE® card. Due to that voltage variation, the achievable operating distance is limited by the transferred power. The right part shows the equivalent electrical circuitry, the transformer model. In detail, the energy transmission is described in Annex A of this document.

DATA TRANSMISSION RWD   CARD
To transfer data from the reader to the card, MIFARE® uses a half-duplex communication structure. The reader talks first and starts the communication. The data transmission from the reader to the card is done using a 100 % ASK pulse-pause modulation according to ISO14443 Type A. Figure 2-4 show a typical signal shape.

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Figure 2-4. Data Transmission RWD — Card, typical signal shape

Due to the quality factor Q of the antenna the transmitted signal deforms to the shape shown in Figure 2-5. This shape can be used to measure the tuning of the antenna. The theoretical background to calculate the antenna quality factor Q and the procedure to calculate the components of the matching circuitry will be described in chapter 3. As mentioned before, the MIFARE® card is passive. To communicate between reader and card, energy has to be transmitted to the card. Therefore, MIFARE® uses an optimised coding to provide a constant level of energy independently from the data transmitted to the card. This is the modified Miller code, which is used to transmit data from the reader to the card. Figure 2-6 describes the Miller coding in detail.

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Figure 2-6. Data Transmision RWD — Card, Miller Coding

The data rate of MIFARE® is 105.9KHz, so the length of a bit frame is 9.44μs. A pulse in the Miller coding has a length of 3μs.
logical ‘1’ is expressed with a pulse in the middle of the bit frame.
Two possibilities are given to code a logical ‘0’. This coding depends on the previous bit:
If the previous bit was a ‘0’, the following ‘0’ is expressed with a pulse of 3μs at in the first half of the next bit frame.
If the previous bit was a ‘1’, the following ‘0’ is expressed without a pulse in the next bit frame.

DATA TRANSMISSION CARD à RWD
Subcarrier Load Modulation Principle
The data transmission from the card back to the RWD is done using the principle of load modulation shown in Figure 2-7. The card is designed as a resonance circuitry and consumes energy generated by the reader.
This energy consumption has a reactive effect as a voltage drop on RWD side. This effect can be used to transfer data from the card back to the reader by changing a load or a resistance in the card IC.

DATA TRANSMISSION CARD à RWD
Subcarrier Load Modulation Principle
The data transmission from the card back to the RWD is done using the principle of load modulation shown in Figure 2-7. The card is designed as a resonance circuitry and consumes energy generated by the reader.
This energy consumption has a reactive effect as a voltage drop on RWD side. This effect can be used to transfer data from the card back to the reader by changing a load or a resistance in the card IC.

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Figure 2-7. Subcarrier Load Modulation Principle

The MIFARE® reader antenna should be tuned to a resonance frequency fR of 13.56 MHz. In fact, the resonance circuit generates voltages at the reader antenna several times higher than the supply voltage. Due to the small coupling factor between the RWD and card antenna the card responds is about 60 dB weaker than the voltage generated by the reader. To detect such a signal, requires a well designed receiving circuit. Instead of using a direct load modulation, MIFARE® uses a sub-carrier frequency fSUB to modulate the data. The result of this sub-carrier modulation is the generation of side-bands at ±fSUB around the the carrier
frequency of 13,56 MHz. The sub-carrier load modulation allows an easy and robust detection of the received signal.

The MIFARE® RF interface uses a Manchester coding for the data in the base-band before the sub-carrier modulation is done. Figure 2-8 shows the typical data coding and the sub-carrier load modulation in the time
domain. Firstly, the data are internally coded to the Manchester coding. The data rate of MIFARE® for the communication from the card to the reader is 105.9 kHz and the same as for the communication between
reader and card, so the length of a bit frame is 9.44μs. The Manchester code uses rising and falling edges to code the data.

A logical ‘1’ is expressed with a falling edge in the middle of the bit frame.
A logical ‘0’ is expressed with a rising edge in the middle of the bit frame.
The MIFARE® card IC generates the sub-carrier frequency fSUB = fR/16 = 847.5 kHz. The time TO expresses the pulse length of the operating frequency, TO=1/fR=74ns .The Manchester coded data is modulated to the
sub-carrier frequency. Finally, the sub-carrier load modulation is done.

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Figure 2-8. Principle of Data Coding Cardà RWD, time domain

Thus, the sub-carrier load modulation generates two side-bands in the frequency domain; an upper at 14.41 MHz and a lower one at 12.71 MHz. Figure 2-9 shows the spectral domain of the signal. On the one hand the side-bands of the data coding are shown, on the other hand the side-bands of the carrier frequency to the operating frequency are shown.

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Figure 2-9. Data Coding Cardà RWD, Frequency Domain