A detailed explanation of the two components of RFID technology

Radio Frequency Identification (RFID) technology is a contactless automatic identification technology. Its basic principle is to use radio frequency signals and their spatial coupling and transmission characteristics to automatically identify objects to be identified by stationary or moving objects.

An RFID system generally consists of two parts: the electronic tag and the reader. The electronic tag and reader achieve spatial (contactless) coupling of RF signals through coupling elements. Within the coupling channel, energy transfer and data exchange are realized based on timing relationships. There are two types of RF signal coupling between the reader and the electronic tag.

1) Inductive coupling: coupling is achieved through a high-frequency alternating magnetic field in space, based on the law of electromagnetic induction.

(2) Electromagnetic backscatter coupling: In the radar principle model, the emitted electromagnetic waves reflect off the target upon contact and carry back the target information, based on the spatial propagation laws of electromagnetic waves.

Inductive coupling is generally suitable for short-range RFID systems operating at medium and low frequencies. Typical operating frequencies are: 125kHz, 225kHz, and 13.56MHz. The recognition range is less than 1m, with a typical working distance of 10cm~20cm. The electromagnetic backscatter coupling method is generally suitable for long-distance RFID systems operating at high frequency and microwave levels. Typical operating frequencies include: 433MHz, 868MHz, 915MHz, 2.45GHz, and 5.8GHz. The recognition range is greater than 1m, with a typical range of 3m~10m.

Electronic tags, also known as radio frequency tags, transponders, or data carriers, attach electronic tags to the item to be identified and are the true data carriers of the RFID system. Generally, electronic tags consist of a tag antenna and a dedicated tag chip.

Depending on the power supply method, electronic tags can be divided into active tags and passive tags. Active tags have built-in batteries, while passive tags do not.

A reader, also known as a readout device, can be a read/write or read-only device. When an item to be identified with an electronic tag passes through its readout range, it automatically and non-contactlessly extracts the agreed identification information from the tag, thereby achieving automatic identification or automatic collection of item identification information.

Acoustic surface wave device technology

Surface acoustic waves (SAW) are mechanical waves propagating on the surface of piezoelectric crystals, with a sound speed of only one hundred-thousandth of the electromagnetic wave speed, resulting in very little propagation attenuation. SAW devices are made using microelectronic technology on piezoelectric substrates to manufacture fork-shaped electroacoustic transducers and reflector couplers. By utilizing the piezoelectric effect of the substrate material, electrical signals are converted into acoustic signals by input to an inter-finger transducer (IDT), which is confined to the substrate surface and then restored to electrical signals. This enables the electrical-acoustic-electrical conversion process, completing the electrical signal processing and producing electronic devices for various applications. Surface acoustic wave devices manufactured using advanced microelectronic processing technology feature advantages such as compact size, light weight, high reliability, good consistency, multifunctionality, and flexible design. They have been widely used in communications, television, remote control, and alarm systems, with hundreds of millions of mobile phones and televisions employing multiple surface acoustic wave filters. With the rapid development of processing technology, SAW devices now cover operating frequencies of 10MHz~2.5GHz, making them indispensable key components in the modern information industry.

Working principle of SAW passive electronic tags

SAW passive electronic tags use reflection modulation to transmit tag information to the reader.

A SAW tag consists of a cross-finger transducer and several reflectors, with two bus lines of the transducer connected to the antenna of the electronic tag. The reader's antenna periodically sends high-frequency query pulses. Within the reception range of the electronic tag antenna, the received high-frequency pulses are converted into acoustic surface waves by the intercanted transducer and propagate on the crystal surface. The reflector group partially reflects the incident surface waves and returns them to the cross-finger transducer, which then converts the reflected sound pulse string into a high-frequency electrical pulse string. If the reflector group is designed according to a specific pattern so that the reflected signal represents the specified encoding information, then the reader receives a reflective high-frequency electrical pulse string with the specific code of that item. Through demodulation and processing, automatic identification is achieved.

Due to the slow propagation speed of surface acoustic waves, the effective reflected pulse train only returns to the reader after a delay of several microseconds. During this delay, interference reflections from around the reader are attenuated and do not interfere with the effective signal of the acoustic surface wave electronic tag.

SAW passive electronic tag applications

Electronic tags using surface acoustic wave technology began in the late 1980s, and in recent years, research on acoustic surface wave tags has become a hot topic. Surface acoustic wave electronic tags represent a new system achievement applying modern electronics, acoustics, semiconductor planar process technology, radar, and signal processing technologies. They are a new type of contactless automatic identification technology distinct from IC chip recognition.

Since SAW devices operate in the RF band, they are passive and highly resistant to electromagnetic interference. Therefore, electronic tags implemented with SAW technology have certain unique advantages and serve as a supplement to integrated circuit technology. Its main features are:

1. Large and reliable reading range, up to several meters;

2. Can be used on metal and liquid products;

3. Simple matching between tag chips and antennas, low manufacturing costs;

4. Not only can it recognize stationary objects, but it can also recognize high-speed moving objects at speeds up to 300 km/h;

5. Can be used in harsh environments such as temperature differences (-100°C~300°C) and strong electromagnetic interference.

Electronic tag technology has a wide range of applications, including logistics management, road and bridge toll collection, public transportation, access control, anti-counterfeiting, health and safety monitoring and identification of farms, supermarket theft prevention and collection, airline baggage sorting, parcel tracking, factory assembly line control and tracking, equipment and asset management, sports competitions, and more.

SAW tags are also suitable for measuring changes in parameters such as pressure, stress, torsion, acceleration, and temperature, such as hot shaft positioning in railway infrared shaft temperature detection systems, rail scales, overload detection systems, and automotive tire pressure.

The OIS-W acoustic surface wave tag produced by Baumer Ident GmbH in Germany and the SOFIS acoustic surface wave system by Siemens have been successfully used in the automatic crossing toll collection system in Oslo, Norway, and the train station arrival positioning system in Munich, Germany. In the United States, companies such as Identtronix, i-Ray, and RF-SAW also produce commercial SAW electronic tags.