Design of MF RC500 Matching Circuits and Antennas
This application note is intended to support RF-related design–in of the MF RC500 MIFARE® reader IC. The aim is to provide the required understanding of the MIFARE® RF interface (ISO 14443A) to design application specific antennas and matching circuits to achieve the best performance for a communication with a contactless MIFARE® card. This paper shall give a background on the system’s RF part and an overview on the procedure how to design and tune antennas for standard applications. Two different antenna and matching concepts are explained in detail as well as examples for the antenna design itself. Furthermore, the complete tuning procedure is described. As part of the Annex, the interested reader will find a detailed theoretical description of the RF interface.

Block Diagram
The MF RC500 is member of a new family of highly integrated reader ICs for contactless communication based on 13.56 MHz. The MF RC500 supports all layers of ISO 14443. Figure 2-1 shows a simplified block diagram.

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Figure 2-1. Simplified MF RC500 Block Diagram

The MF RC500 fulfils the following functions:
–The parallel μ-Controller interface detects automatically the connected 8 bit parallel interface.
–The data processing part performs the parallel to serial conversion of the data. It supports the framing generation check, the CRC/Parity generation and check as well as the bit coding and processing. All layers of ISO14443-A are supported, as the MF RC500 operates in full transparent mode.
–The status and control part allows the configuration of the device to environmental influences to achieve the best performance for each application.
–The Crypto1 stream cipher unit is implemented to support communication to MIFARE® CLASSIC products.
–A secure non-volatile key memory is included to store Crypto 1 key-sets.
–The analog part includes two internal bridge driver outputs to achieve an operating distance up to 100mm depending on the antenna coil and the environmental influences. Furthermore, the internal receiving part allows the receiving and decoding of data without external filtering.

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Figure 2-2. System Configuration

Both concepts have to fulfil different requirements to achieve an optimum in performance. Designing these components is subject of the next chapters.

The MIFARE® RF Interface

The MIFARE® technology describes an ISO 14443-Type A compliant RF interface for a communication between a reader and a contactless card.

Table 1 gives a short overview on the MIFARE® RF interface. Basically, the MIFARE® RF interface follows the transformer principle. The MIFARE® card is passive with no onboard battery. Thus, an energy transmission is required for a communication between a reader module and a card as well as a possibility to transmit data in both directions.

Table 1. Overview MIFARE® RF interface

Energy transmission Transformer principle; MIFARE® card is passive
Operating frequency 13.56 MHz
Communication structure Half duplex, reader talks first
Data rate 105.9 kHz
Data transmission RWD • Card Card • RWD Both directions 100 % ASK, Miller Coded subcarrier load modulation, subcarrier frequency 847.5 kHz, Manchester Coded

The following parts describe the fundamentals of the MIFARE® RF interface starting with the basic energy transmission. Finally, the data transmission and the used data coding in both directions will be shown.