Tags and Readers: How RFID works and how to design your first RFID tag antenna
Radio Frequency Identification (RFID) has been around for over 50 years and over the past decade, it has finally been cheap enough to be mass-produced and placed in everyday items. This provides some new opportunities for electronic designers who want to understand RF and antenna design. While RFID implements the same functionality as a barcode or magnetic strip on a credit card, it has some unique use cases worth learning and designing. In this blog, we will cover how RFID works and how to create your own RFID tag antenna circuit.
RFID Working principle
RFID System consists of three parts: a scanning antenna, an RFID tag that contains all the information of the product, and a reader that decodes and interprets the data on the tag. These three parts are suitable for a process, where:
- data is first stored in the RFID tag in read-only or read-write format. Tags are either battery-powered or passive.
- When the tag enters the range of the scanned antenna, the electromagnetic (EM) energy triggers the tag to start sending data in the form of radio waves.
- These radio waves are received by the antenna and sent to a reader, which decodes these radio waves into digital information.
tag communicates with the reader to share data. (Picture source)
RFID tag is divided into one of two types: active and passive:
tag usually contains antennas and microchip . (Picture source)
Active RFID tag
Active RFID tag uses internal battery power to power its circuit and send radio signals. Due to their battery power, active tags can be broadcasted at higher frequencies from 805 to 950 MHz. This allows them to scan from as far as 100 feet.
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passive RFID tag
Passive RFID tag has no battery power and relies on the electromagnetic coupling of the antenna to supply power. This makes them shorter reading distances, up to 20 feet. However, because they have fewer components in their circuits, they are also cheaper to produce and use once. This is the kind of tag you can find in consumer-grade products.
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All RFID tags are classified by their generation and category. The current Gen-2 RFID tag was adopted in December 2004, with multiple advantages over Gen-1, including:
- interoperability with all RFID systems, the first global standard
- faster read speed than Gen-1
- Advanced anti-collision protocol provides precise performance
- Improve security and privacy
- Physical deployment of multiple readers is easier
RFID tag categories range from classes 0 to classes 5 and define available features.Course details include:
Class | Functional | Functional |
0 Level | UHFI Read-only, pre-programmed passive tag | |
1 level | Ultra-high frequency or high frequency, write once, multiple reads (WORM) | |
2 level | Passive read and write tag | |
3 Level | with passive or active read and write tags for sensors for recording temperature, pressure and motion. | |
Class 4 | Active read and write tags with integrated transmitters, which can communicate with other tags and readers. | |
5 level | can power other tags and communicate with external devices other than readers. |
RFID Pros and cons
RFID Wireless technology brings huge benefits to the logistics, inventory control and retail industries, but that does not mean it is a perfect technology. However, RFID has some obvious advantages over barcodes and other manual scanning systems, including:
- Scan multiple items simultaneously . Compared to an optical scanner that can only process one item at a time, an RFID system can scan all incoming items at once.
- reduces costs . The cost of manufacturing active and passive RFID tags has dropped significantly, making them available for single-use consumer products.
- Reading and writing ability . The barcode can only be written once, but the RFID tag can be updated multiple times according to the needs of Class 2 or above tags.
- No vision problem . Unlike optical scanners, there is no line of sight requirement when scanning items. This makes RFID an ideal choice for efficient industrial environments. The disadvantages of
RFID are divided into two categories, technical issues related to RFID implementation and security/privacy use.
- RFID can be disrupted . People who use the correct frequency on the electromagnetic spectrum may interfere with the RFID system. You can also continuously read the battery-powered RFID tag until it is powered off.
- card reader collision problem . The system needs to be carefully set up to avoid any conflicting problems with multiple signals overlapping. This has been improved through Gen-2’s improved anti-conflict protocol.
- can read from a distance. Most RFID systems are designed to work within small distances. However, with high gain antenna , you can read tags from over 100 feet away without anyone knowing it.
- can be read without permission . When we started embedding RFID into everyday items like clothing and consumer goods, we were also at the mercy of shops scanning every item we carried to prevent theft. Is this a privacy issue, or is it just a convenience cost?
RFID Use Case
Due to its low manufacturing cost, RFID uses are spreading across a variety of industries and environments. Some of the most common applications we see today include:
Retail and Inventory Control
RFID tags are posted on retail stores' clothing and other consumer products. This allows employees to scan items and track inventory levels without having to process barcodes.
Retail giants like Walmart are integrating RFID into their retail supply chains. (Picture source)
subway pass and toll station.
RFID tags are usually added to toll stations and subway passes. This allows commuters to swipe their cards and automatically deduct fares.
Many toll stations are now implementing RFID technology.(Picture source)
Anti-theft
High-priced luxury goods like custom guitars are now embedded in RFID tags. This inventory is then shared with local law enforcement, dealers and repair shops to help prevent theft.
The small dot under the truss rod of the Gibson guitar is an RFID chip. (Image source)
These are just a small part of RFID's many uses. In the future, this technology will prove to be the main disruptor of the retail shopping experience in concepts such as Amazon Go. Some other use cases for RFID include:
- Human and Animal Tracking
- NFC Contactless Payment
- Travel Document
- Medical Data Management
- Track containers and railway vehicles
Design your first RFID antenna tag circuit
When designing RFID systems, engineers usually use existing tag microchips (such as STMicroelectronics ST25) with custom antennas. In this section, we will cover some practical tips on how to handle this antenna design process, as well as more details in the application notes.
Understand your application frequency
Before designing a custom antenna, you need to understand the frequency of the application. Almost all RFID systems operate on two lower bands, 13.56 MHz or 125 KHz, both of which are suitable for almost every country. In the 13.56 MHz frequency band, your application can be divided into:
- Remote (LR) . These products have tuning frequencies between 13.6 MHz and 13.7 MHz.
- Standard Short Range (SR) . These products have tuning frequencies between 13.6 MHz and 13.9 MHz.
- SR product used as transportation ticket converts between 14.5 MHz and 15 MHz.
To determine the frequency of the application, you also need to consider the material and environment of the device. For example, an RFID tag that uses adhesive to bond paper tags together reduces the antenna frequency by about 300 KHz. This reduction makes it necessary to tune the antenna to 13.9 MHz instead of the specified 13.6 MHz.
Reference equivalent circuit model
Many antenna design application notes will contain an equivalent circuit model that you can follow when designing yourself. The following diagram shows how the chip and antenna are combined together in a typical circuit:
(image source)
chip
- chip circuit takes R chip as reference and represents the current consumption in parallel with the capacitor C tun
- C tun defines the internal tuning capacitor and internal parasitic parameters of the chip.
antenna
- ant . _ Determine stray capacitance
- Ant . _ Determine the resistance loss
- ant. _ determines the self-inductance
determines the inductance of the antenna coil
At 13.56 MHz, your antenna design will be round, spiral or square. What shape you use will depend on your design requirements.Here are three formulas that can be used to quickly determine the inductance for a specific antenna type:
circular antenna inductance
r is the average coil radius R 0 is the wire diameter in millimeters N is the number of turns 0 = 4π x 10 -7 H/mL Helical antenna inductance
d is the average coil diameter c is the thickness of winding , and N is the number of turns 0 in micrometers N is the number of turns 0 = 4π x 10 -7 H/mL Square antenna inductance in Henry
- d is the average coil diameter
- d = (d out + d in ) / 2 in millimeters, where d out = outer diameter, d in = inner diameter
- p = (d out – d in ) / (d out + d in ) in millimeters
- K1 and K2 Depending on your antenna layout, please refer to the following table
layout | K1 | K1 | K2 |
Square | 2.34 | 2.75 | |
hexagon | 2.33 | 3.82 | |
Octagonal | 2.25 | 3.55 |
Read the full application instructions
The details we introduced above are just design customization RFID The surface of the tag antenna . Be sure to read the full application instructions for STMicroelectronics to learn how to design a 13.56 MHz antenna for ST25 RFID tags.
Wireless Magic
RFID Use radio waves to read and capture information stored on tags that can be attached to various objects. From high-priced guitars to pets and containers, RFID has almost unlimited uses. This technology is expected to replace standard barcodes and magnetic stripes in the future, but what will it look like in the future?
