In recent years, the automatic identification method has seen rapid development across various fields such as service, goods sales, logistics distribution, commerce, manufacturing, and material circulation. Among these, radio frequency identification (RFID) technology has advanced significantly and is now recognized as an independent interdisciplinary field. It encompasses areas like high-frequency technology, semiconductor technology, electromagnetic compatibility, data security, telecommunications, and manufacturing. As a critical component of RFID systems, the antenna directly influences the system’s overall performance.
The principle of an RFID system typically involves a reader (PCD) and a transponder (PICC). A typical reader includes a high-frequency module (transmitter and receiver), a control unit, and a coupling element connected to the transponder. The transponder serves as the true data carrier in the system and consists of a coupling element and a microelectronic chip. Unlike the reader, it does not have its own power supply but instead receives RF power from the reader within its response range. The energy required for operation, including clock pulses and data, is transmitted non-contact through the coupling unit. Therefore, the antenna, which acts as the coupling element, plays a crucial role in determining the communication distance and data transmission reliability.
This article focuses on the U2270B base station chip to discuss the design of RFID antennas. In an RFID system, there are two LC circuits: one formed by the base station coil and a connecting capacitor (LRCR), and another formed by the transponder coil and a connecting capacitor (LTCT). For a single-coil system, both LC circuits must be tuned to the same resonant frequency. If the frequencies do not match, zero modulation occurs, which can degrade system performance. Once the system is designed, the antenna's inductance becomes fixed, so adjusting the capacitance in the loop is the only way to change the resonant frequency.
The reader’s antenna forms a series resonant circuit consisting of an inductor, capacitor, and resistor. Its characteristics are defined by the resonant frequency (fo) and the Q factor. Fo is determined by the antenna’s inductance and capacitance, calculated using the formula:
$$ f_o = \frac{1}{2\pi\sqrt{LC}} $$
For the U2270B, the operating frequency is usually 125 kHz. The relationship between the Q factor (QR) and the bandwidth (B) is given by $ B = \frac{f_o}{Q_R} $. A higher QR increases the reader antenna voltage, improving energy delivery to the transponder, but reduces the antenna bandwidth, potentially causing issues when the transponder frequency shifts.
The coupling factor represents the interaction between the reader’s magnetic field coil and the transponder’s coil. It depends on the system’s structural parameters and directly affects the reading distance. Optimizing this factor enhances both energy transfer and signal transmission. To determine the coupling factor, Temic provides a test response coil (TTC) and associated circuitry. The measurement process involves placing the TTC in the transponder’s position and measuring the induced voltage.
The coupling factor is influenced by mechanical dimensions such as coil diameter, reading distance, and coil alignment. To maximize the coupling factor, the reading distance should be minimized, and the reader and transponder coils should be aligned parallelly. The optimal antenna radius is approximately equal to the reading distance.
To meet actual frequency tolerance, the allowable frequency offset must be considered. The total allowable frequency offset increases with a higher coupling factor and decreases with higher inductance. For the U2270B, the maximum antenna current is limited to 400 mA, which restricts the minimum inductance value. After determining the inductance, the capacitance can be calculated based on the operating frequency.
An example of antenna design involves setting the allowable frequency offset for the reader and transponder, choosing the appropriate coil radius, calculating the coupling factor, and determining the number of turns and capacitance values accordingly. This process ensures that the system operates efficiently under real-world conditions.
In conclusion, this paper outlines the general steps for designing RFID antennas using the U2270B chip. While external factors may introduce additional challenges, the insights provided here aim to inspire further research and development in RFID technology.
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