The ABCs of RFID
There are a myriad of acronyms out there related radio frequency identification (RFID) and near-field communications (NFC) technology – UHF, LF, RF, UHF, NFC, LF. What do they all mean and how exactly do designers use these types of NFC/RF technology and specifications in application design? A great example of NFC/RF technology in use everyday is your automobile keyless entry system. As the NFC/RF technology area continues to grow in innovations and sheer volume, it will be important to select the correct specifications and technology for your application. There is no one RF technology that is best-suited for every application, and some markets even combine two NFC/RF technologies to get the desired result. Also, as NFC/RF is making more of an impact in new applications, highly integrated devices are becoming available to decrease the time to market for designers. We’ll make sense of the NFC/RF “alphabet soup,” decoding the key selection criteria for your NFC/RF designs.
Getting Started: Defining NFC/RF Technologies
So, you want to design a system that offers NFC/RF technology, but you have no idea where to begin. Let’s start by defining the four basic types of NFC/RF technology available today:
1. LF Passive: Uses magnetic coupling to transfer power and data.
2. HF Passive: Also uses magnetic coupling to transfer power and data.
3. UHF Passive: E-field coupling/limited magnetic coupling.
4. UHF Active: E-field coupling.
There are four NFC/RF standards that relate to each other in terms of radio frequency and read range. You can see these in Figure 1. Depending on the read range and frequency your application requires, you’ll need to refer to the diagram below to ensure you choose the correct type.
Tag reading: A key difference in NFC/RF Passive tag reading is a key differentiator for NFC/RF technology. Passive tags do not require batteries or a power source to operate. When a tag is within range of an NFC/RF reader, a magnetic or e-field coupling transmits data to the reader. This is done through a loop antenna tuned to the center frequency of the field. In contrast, active tags have access to a power source, enabling them to operate over greater distances.
Tags operate in two modes:
1. Full Duplex (FDX, FDX-B): An FDX-based system has data and energy traveling at the same time. This differentiation is applied for technology in the LF 125kHz to 135kHz range but not for HF or UHF passive technologies as not used here. Figure 2 shows the reader and tag signals during a transmission. Both the reader and tag transmit at the same time. This is because for the tag to be powered so it can transmit data, the reader must be transmitting energy, as well. A full duplex (FDX) based system can support data traveling in both directions at the same time, but he reader must employ complex decoding algorithms to separate the tag signal from the reader signal and ambient noise.
1. Half Duplex (HDX): The reader or tag is transmitting, but not both (Figure 3). TI’s HDX-based tags have an integrated charge capacitor in the transponder. When the reader first connects, it charges the capacitor. When the reader stops transmitting, the transponder is then powered by the charged capacitor and is able to transmit the requested data to the reader TI’s half duplex (HDX) tags integrate a charge capacitor that allows tags to be charged by a reader. Then, they transmit the requested data while the reader is not transmitting, which results in a significant performance improvement (approximately 1.5 times the read range) with higher reliability and lower cost than FDX readers. HDX readers can also use simpler decoding techniques. This is because in an FDX transmission, the reader must separate the tag signal from its own transmission and any ambient noise. With an HDX-based tag, because the tag does not have to compete with the reader’s transmission, the reader can use simpler decoding techniques. The result is higher reliability, greater range, and lower reader cost.
Where does near-field communications (NFC) fit? Near Field Communications (NFC) is an established technology. In fact, ABI Research reports that 345 million NFC-enabled devices were shipped in 2013, and more than double this number are anticipated for 2014. Applications for NFC include consumer electronics (printers, tablets, routers, cameras, wireless audio), automotive (infotainment system pairing, access control, wireless and battery-less sensors), medical (diagnostics, fitness, bio med patches), smart grid (eMeters, flow meters, home automation gateways), and retail (point of sale, product authentication).
One of NFC’s primary features is that it can be used to eliminate complicated wireless set up in consumer devices. This is achieved through “tapping,” where devices are able to quickly pair without requiring complex configuration by users, enabling devices to be used with convenience. NFC is being used to enable pairing of Bluetooth and Wi-Fi devices without requiring manual setup by users.
NFC specifications are based on HF NFC/RF ISO and ECMA standards. These specifications are in place to enable seamless interoperability between vendors to help drive adoption of NFC-based equipment and applications. NFC is based on 13.56 MHz, HF passive RFID/contactless card technology and provides a bi-directional link between devices. Short range transactions up to 10 cm are supported, depending upon reader, as well as tag antenna geometry and reader output power. Some applications that use NFC include public transportation ticketing and electronic payments, as well as a connection handover of an alternative carrier and as service interface. NFC offers three operating modes:
1. Peer-to-peer: Either device can initiate communications, enabling simple sharing of data, such as between two smartphones.
2. Reader/writer: The NFC device can read data from, and write data to, NFC /RFID and contactless smartcards.
3. Card emulation: This mode enables an NFC device to behave like a contactless smartcard. A single NFC device can emulate more than one card – ideal for touch-to-pay applications.
Which NFC/RF Technology to Choose for Your Design
There are several key factors to consider when selecting an NFC/RF technology. Note that the priority of these factors depends upon the application. For example, reliability will be tantamount for a medical application while a point-of-sale application requires robust security. Here are a few things to consider when selecting your NFC/RF technology:
- Cost is often one of the most important factors in design, but it can also be difficult to evaluate. Because the variation in cost between tags, readers, range and sensitivity, designers should consider total system cost when evaluating technologies.
- Reliability is another huge factor where range and use in various environments must be considered – can the NFC/RF technology chosen do its job consistently and accurately?
- Security factors in how secure the data must remain that is passing from the tag to the reader.
- Compliancy considers whether the technologies you’ve chosen for your design meet the standards from HF NFC/RF ISO and ECMA.
- Read/Write range and speed are important when factoring in reliability and the needs in your design.
- Multi-tag capabilities are important if a number of tags need to be read at many reading points – you will need to ensure your ratio or tags to readers is considered.
- Environment is key for evaluating the needs for your application. In fact, environment will likely influence the other factors considered. Does the tag need to be read through water or metal, or is it facing extreme temperatures?
Other factors that are maybe not as important, but must be considered include:
- Consistency weighs the ability of the NFC/RF implementation to reliably read tags as required by the application. Factors such as read distance/range and data rate/speed need to be matched to the requirements of the environment.
- Antenna impacts both read/write range and footprint. LF tags offer high flexibility as the antenna can be designed to include a ferrite core. A tag the size of only a few millimeters can achieve inches of range. UHF tags face restrictions around antennas and cannot be wound a round a ferrite core. They need a larger footprint to deliver good read range.
- Battery-powered range and integrated processing capabilities of a tag can also be increased by selecting an active tag over a passive tag. Passive tags are powered by the transmitted energy received and have a limited range of up to 30 feet in good operating conditions. Active tags have a battery-powered transmitter that enables them to achieve up to miles of range.
- Absorption takes into account the presence of metal, which can potentially block or reflect RF signals, significantly impacting range and reliability. A tag behind a metal door may not be able to be read. Similar issues can arise with water as water absorbs the UHF RF signal, blocking it.
Based on the 4 basic technologies, there is a trend towards technology convergence, which combine e.g. two RFID technologies such as low frequency (LF) and active ultra-high frequency (UHF) with a Microcontroller and a sensor. Furthermore, as RFID gains inroads into new applications, highly integrated devices are becoming available to accelerate time to market and lower cost, such as RF technology integrated with wireless charging and energy harvesting capabilities.
Okay, now we’ve said our NFC/RF ABCs. Won’t you design with me?
For more information on technologies, reference designs, tools and software solutions for NFC/RF, please visit www.ti.com/rfid.