1 Introduction
Radio Frequency Identification (RFID) is an automatic identification technology that uses non-contact, two-way communication to identify and exchange data through radio frequency bands. RFID systems can be classified into active and passive types based on the data modulation method of the radio frequency card. Active RFID systems are particularly suitable for applications such as supply chain management, military logistics, and real-time positioning systems due to their advantages like real-time data transmission, large data capacity, fast read/write speed, and long-distance readability. However, in the past, the application of active RFID was limited by factors such as the large size, high power consumption, and short battery life of active RF cards. In recent years, advancements in micro-integrated circuit technology and micro-machining manufacturing have led to the development of micro-intelligent RF cards. Breakthroughs in low-power IC technology have also made it possible to create small, low-power active RFID cards, significantly expanding their potential applications.
Taking the nRF905 RF chip as an example, this paper presents the design of an active RFID system operating in the microwave frequency band and provides a detailed communication module design scheme for the system.
2 RFID System Overview
An RFID system typically consists of two main components: a reader and a radio frequency card (transponder). The reader can be either a read-only or read-write device, and usually includes a radio frequency module for transmitting and receiving signals, a control unit, and a coupling component such as an inductor coil or antenna. It also features a communication interface to transmit data to a host computer. The RF card is attached to the object being identified and serves as the actual data carrier in the RFID system. A typical RF card comprises a coupling element and a microelectronic chip, as shown in Figure 1.
Figure 1: Schematic diagram of an active readable and writable RFID system
RFID systems are categorized based on their operating frequency into low-frequency, intermediate-frequency, and microwave systems. Microwave RFID systems commonly operate at frequencies such as 433 MHz, 869 MHz, 915 MHz, 2.145 GHz, and 5.18 GHz. These systems rely on electromagnetic coupling and are ideal for applications requiring long-range communication and high-speed data transfer. Additionally, based on the data modulation method, RFID systems can be divided into active and passive types. Passive systems rely on backscattering to transmit data, while active systems use internal power sources to actively send data to the reader. Active RFID cards are more reliable and capable of longer communication distances due to their built-in batteries.
3 Active RFID System Design
The integration of RF chips into RFID systems has enabled the miniaturization, modularity, and intelligence of RFID products. This leads to lower costs, longer read ranges, better scalability, and significant progress in the development and application of RFID systems, especially active ones. The following section presents a design of an active RFID system using the nRF905 RF chip as an example.
3.1 Introduction to the nRF905 Wireless Transceiver Chip
The nRF905 is a high-performance single-chip wireless transceiver developed by NordicVLSI. It operates in the ISM (Industrial, Scientific, and Medical) frequency bands at 433 MHz, 868 MHz, and 915 MHz. With a low operating voltage ranging from 1.9 V to 3.6 V, it integrates functions such as frequency synthesis, RF transmission and reception, modulation and demodulation, and multi-channel switching. The chip employs GFSK modulation and demodulation technology with strong anti-interference capabilities, achieving a maximum data rate of 100 kbps. Its differential antenna interface makes it easy to connect to low-cost PCB loop antennas or single-ended antennas.
The nRF905 is available in a compact 32-pin QFN package (5 × 5 mm), featuring minimal external components, stable operation, and extremely low power consumption. It includes on-chip hardware Manchester coding, standby, and power-down modes, and communicates with a microcontroller via SPI (Serial Peripheral Interface), making it ideal for low-cost, low-power applications where performance and integration are critical.
As shown in Table 1, the nRF905 has two operational modes and two power-saving modes, controlled by three pins: PWRUP, TRXCE, and TXEN. Thanks to ShockBurst technology, data can be processed at a low speed in the microcontroller and transmitted at high speed through the nRF905, with long idle periods in between. This reduces memory usage, simplifies programming, and saves microcontroller resources.
The nRF905 is configured via the SPI interface and has five internal configuration registers. These include the status register, RF configuration register, transmit address register, data register, and receive data register. In power-down mode, the nRF905 consumes only 2.15 μA, and its register contents remain unchanged. It's important to note that the SPI interface can only be activated during standby or power-down mode, which should be considered when designing the communication protocol.
3.2 Hardware Design of the Active RFID System
Traditionally, RFID system hardware designs relied on discrete components, leading to high design complexity, low integration, increased costs, and longer development cycles. Designs based on CPLDs or DSP chips often required complex software for communication modules and were relatively expensive. In contrast, the active RFID system designed using the nRF905 leverages the high integration, low power consumption, and stable frequency of the RF chip, eliminating the need for Manchester coding and simplifying the communication protocol. This significantly reduces design costs and shortens the development cycle. Additionally, the hardware is easier to debug and scalable. The block diagram of the hardware design is shown in Figure 2.
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