Micore Reader IC Family; Directly Matched Antenna Design

How to design an antenna for the MICOREcontactless reader IC family. (The MIFORE reader IC family includes the MF RC500, MF RC530, MF RC531, SL RC 400 and the CL RC 632. The antenna design and matching is the same for all of these ICs.)

Mifare, ICode, Micore, RC400, RC500, RC530, RC531, RC632, RFIDReader, Directly Matched Antenna Design .

For a good overview the Micore Antenna Principle section shows the blocks of each reader antenna design, how these blocks fit to Micore, and also the complete schematic of a Micore Antenna.

Because there are many parameters that influence the overall performance of an antenna, a basic RF knowledge is needed to design an antenna that takes all these parameters into account. Although this (specific) knowledge is important for understanding the background, a complete antenna design can be done without RF-specific knowledge . Therefore the complete design of a directly matched antenna is divided into two design levels:

  1. The basic parameter design is dedicated to those, who are not really familiar with RF design and its tools. Although a certain number of requirements and parameters are frozen, the design will yield an optimized antenna. A guideline, the required formulas, and a tool to support the design is described and provided. Depending on some simplifications this design might not be completely compliant to the ISO/IEC14443, even though it fully satisfies the Mifare and I-Code requirements.
  2. The full parameter design is dedicated to those, who want to cover all the specific cases of various design requirements to make a perfect design for the application. This design describes how to build an antenna, which is fully ISO/IEC14443 compliant (including the higher bit rates). The complete antenna design covers all the open parameters, some background information for the design, and the general behaviour of such an antenna.

In section 5 some useful antenna design hints are given.

Appendix 6.1contains a checklist to help optimizing and finishing an antenna design for a proximity reader based on the Micore. It also contains a collection of relevant formulas and an antenna design example.

—Card“ in this document means a contactless smart card according to the ISO14443 (or MIFARE) or a contactless tag / label according to the ISO15693 (or I-Code).

 Micore antenna principle

The Micore is a single reader IC family designed to achieve operating distances up to 100mm without external amplifiers. The design rules and parameters are basically the same for ISO14443, Mifare®, ISO15693 and I-Code®, i.e. the same antenna can be used to communicate with all products mentioned. A complete antenna design always includes the antenna coil and resonance circuit design, the matching of the antenna circuit, the receiving circuitry and the EMC filtering. 

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Fig 1. Block diagram of the complete directly matched antenna circuitry, 500 ohm

Although some of theses blocks may contain only a few passive components, it is important to consider all these blocks and all their functionality to guarantee the proper working of the complete antenna.

The overall functionality can be separated into three basic functions:

  1. Transmit power: The radiated magnetic field has to be maximized considering the radiation and datasheet limits, especially the limits for the radiation of the harmonics (up to 1GHz).
  2. Transmit data: The 10% or 100% ASK modulated data signal has to be transmitted in such way, that every card is able to receive it. The signal shape and timing (i.e. the Q-factor) has to be considered.
  3. Receive data: The card‘s answer has to be delivered to the receive input of the Micore considering the datasheet limits.

Basically this complete antenna circuitry consists of 8 capacitors, 2 inductors, 2 or 4 resistors and the symmetrical antenna coil (La + Lb) as shown in Fig 2.

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Fig 2. Schematic of a complete directly matched antenna

Table 1: Components for directly matched antenna
Abbreviation Explanation
Rext External resistor to adjust the quality factor. Please also consider the power consumption.
C0, C1… C3 Typically 0402, 0603 or 0805 SMD parts with low tolerance (< ±2%).
  NPO is required. The voltage limit has to be considered.
C4 XR7 capacitor.
L0 Typically a small inductance with high Q for general applications. The frequency range and the maximum allowed current have to be considered. This inductance should be magnetically shielded.

Remark: Plan on placing two capacitors, C1a and C2a in parallel to achieve the required matching values. The tuning procedure most likely will return matching values, which cannot be realized with a single capacitor.