What are the different taxonomies of RFID tags?

The common way of categorizing RFID tags is based on how they utilize power for communication between the reader and the tag. They can also be classified based on operating frequency and absence/presence of chips. This first part of the FAQ will cover the passive, semi-passive, and active RFID tags categorized based on power.

Types of RFID tags based on power source

Figure 1 illustrates how the passive, semi-passive, and active RFID tags communicate with the reader, considering the power availability of the tags. Each of these tags is explained briefly with examples as follows.

Figure 1. Classification of RFID tags based on power source and capabilities of the tags. (Image: ScienceDirect)

Passive RFID tags

Today, passive RFID tags are the most common type of RFID technology. They are used for various identification and tracking tasks in many different businesses. These small devices, about 4×4 mm², do not have a power source inside them, hence the name “passive.” They only work when they enter the electromagnetic field that an RFID reader creates.

The simplicity of passive tags is what makes them so useful. When a reader sends out a carrier signal, the passive tag picks up this energy and returns the same signal by backscattering it. This clever communication eliminates the need for batteries, making small, cheap, and long-lasting tags.

Despite being small, passive RFID tags store information that is useful for more than just recognition. Even though they can only hold lower data bits (32, 64, 128) and do not have much computer power, they can store unique identifiers similar to Universal Product Codes (UPCs) but have extra information that makes them more useful.

The antenna design of passive tags varies based on the specific application requirements. These tags can operate across high-frequency (HF) and ultra-high frequency (UHF) ranges, enabling technologies like NFC (Near Field Communication) in smartphones. However, this versatility comes with certain constraints — particularly regarding power consumption when integrating sensors or analog-to-digital converters.

Figure 2. A passive RFID tag from Molex operating at UHF of 920 MHz. (Image: Mouser)

An example of a passive RFID tag is shown in Figure 2 from Molex with 96 or 128-bit memory sizes. This product is meant for outdoor unit applications such as container tracking, agriculture use cases, and construction. It can operate from -50°C to +85°C with IP67 or IP68 ratings to withstand the harsh conditions.

Semi-passive RFID tags

Semi-passive RFID tags are a key part of RFID technology because they combine features from both passive and active RFID tag designs. These tags have a full system design that includes an antenna, matching network, tag IC, sensors, and, most importantly, an outside power source, a battery, or an energy harvesting system.

Passive RFID tags need to get all their power from reader signals, but semi-passive RFID tags can power their sensors and circuits. This power arrangement has a lot of benefits, especially when it comes to contact range. Passive RFID tags can only communicate over short distances because they collect energy. On the other hand, semi-passive RFID tags can communicate over long distances because their electronics do not depend on the reader’s power field.

This architecture enables more sophisticated applications requiring low-power microcontrollers and advanced sensors. When designing semi-passive RFID systems, engineers should carefully select tag ICs based on sensitivity, sensor interface compatibility, and data logging needs. Many implementations connect the tag IC to a microcontroller through standard SPI or I2C communication protocols, creating a flexible platform for sensing applications (Figure 3).

Figure 3. A semi-passive RFID tag Monza X Dura utilizing the I2C Bus for communication purposes. (Image: Impinj Inc)

Memory capacity in semi-passive RFID tags significantly exceeds passive counterparts, with many ICs offering internal storage ranging from 128 bits to over 8 kilobits. This expanded memory facilitates extended data logging capabilities essential for environmental monitoring and supply chain applications.

In semi-passive RFID systems, while the tag benefits from battery-powered operation, the reader’s sensitivity in detecting the backscattered signal can become the limiting factor in the overall system range.

Active RFID tags

Active RFID tags are the most advanced type of RFID technology. They are different from other types because they have a built-in power source. In contrast to passive and semi-passive tags, these have batteries inside that constantly power their internal parts.

This constant power source changes how active tags communicate with each other. Active tags do not have to wait for a reader’s signal to turn them on; they can communicate independently anytime. This autonomy lets them send and receive data at much higher rates, making them perfect for users who need to monitor things in real time and send and receive data quickly.

Figure 4. An active RFID tag from Savi operating at 433.92 MHz is suitable for tracking shipping containers and other conveyances. (Image: Savi)

Active RFID systems usually operate at either 433 MHz or 2.45 GHz frequencies. The 433 MHz band, such as the one developed by Savi (Figure 4), is often preferred for challenging environments as its longer wavelength penetrates materials like water and metal more effectively. This frequency selection, combined with their self-powered nature, allows active tags to achieve better read ranges of approximately 100 meters — far exceeding passive systems.

Some trade-offs go along with this increased potential. Because they need batteries, active RFID tags are much bigger than passive RFID tags, which makes them less useful for labeling small items. Because they are so complicated inside, they cost anywhere from $20 to over $100 per tag.

Despite these problems, active RFID tags are still the second most popular RFID technology after passive systems. They offer unmatched performance for real-time location tracking, environmental sensing, and long-range communication, which makes up for their higher cost and larger size.

Summary

The FAQ covered how RFID tags are classified based on the usage of power sources by the RFID tags. The passive tags are popular because they meet the requirements of many real-world applications most cost-effectively compared to the other two types.

Active tags are helpful when the reading range needs to be extended, and communication cannot be compromised for critical safety applications. Semi-passive tags play the middle ground when better communication performance than passive tags is needed, but the power source does not always need to be active.

The next FAQ will cover how RFID tags can also be classified based on operating frequency and the absence/presence of chips.