Tag Construction

As we discussed earlier, RFID tags can have a different construction depending on their functions and application. Two components are common to all tags: a chip-also called a microchip, an integrated circuit (IC), or an application-specific integrated circuit (ASIC)-and an antenna. These tag parts can be manufactured and attached in many ways; each method has its advantages and disadvantages that affect tag performance.

The Chip, or IC

The widespread acceptance of the EPC architecture is based on the ability to mass-produce a 5-cent RFID tag. This concept was originally driven by the fact that supply-chain applications require you to tag so many items that tag cost would be paramount in determining the business case outcome. Although the emergence of many new asset-tracking solutions and the use of RFID on higher-value goods can challenge this assumption, there is no doubt that lower cost is a benefit to business cases of all kinds. To achieve low-cost RFID tags, it is critical that material and manufacturing costs are minimized. This translates directly into small chip sizes and memory. With this as the primary design criterion, the IC offers bare-bones logical functionality:

• On-Chip Memory Is Limited Gen 2 tags are required to store at least a 96-bit EPC number accompanied by a 16-bit cyclic redundancy check (CRC) for error checking. This number is then used to point to additional data about the product stored in a networked database. This is not enough memory to provide details other than the most basic product identification and serial number. For companies looking to track lot codes and expiration dates, this information must be stored externally.

• Tags Must Respond in Collision-Free Channels When multiple tags are in the field simultaneously, they must talk in turn to prevent data collision at the receiver. Generation 2 EPC tags utilize a slotted ALOHA algorithm to prevent collisions. The characteristics of these protocols have a significant impact on the rate at which tag data can be collected.

The IC also holds responsibility for converting RF energy into usable electric power and for modulating the backscatter signal. Design parameters related to power requirements include the following:

• Small Onboard Chip Memory Reduces Power Needs Because storage requirements have been minimized, the power required to read EPC tags is very low, on the order of microwatts (1 × 10E^-6 W).

• The Efficiency of the Power Circuitry The IC receives energy for the tag antenna in the form of oscillating current at the frequency of the reader transmission. This current must be down-converted and rectified by using circuitry tuned to a specific frequency. The precision of these components and how well they are matched determines power conversion efficiency.

• The Impedance Match of the Chip and the Antenna Impedance can best be described as opposition to the flow of current in an electrical system. To illustrate, think of water flowing through an open pipe vs. through a pipe that is clogged and leaky. If an impedance mismatch exists between the chip and the tag antenna, power will be reflected away from the chip and unavailable for use on the tag.

• The Ability of the Chip to Alter the Impedance of the Antenna Backscatter modulation (or simply reflecting usable signals back to the receiver from the tag) occurs as the IC alters the impedance of the tag antenna at specific time intervals. The chip's ability to change the impedance abruptly and in sync with the reader determines signal clarity and strength.

Tag Antenna Production

There are three main methods of manufacturing a passive tag antenna: etching, stamping, and printing.

The etching process starts with a sheet made of three layers: a substrate, metal film (usually copper), and a photopolymer layer. A special mask "burns" an image of a future antenna into the photopolymer layer. The sheet is then washed by a chemical solution that dissolves all metal around the burned image. The burned photopolymer is removed from the image by another chemical solution to expose the etched metal antenna. The antenna then goes through a special process to prevent oxidation of the metal.

This method produces high-quality metal antennas with great conductivity. However, it has a main disadvantage: producing chemical waste. Etching uses chemical solutions for dissolving the remaining metal as well as the photopolymer layer, and these require special handling.

The stamping method uses a metal foil (usually aluminum) and a "cookie cutter" in the shape of an antenna, which is attached to a roller. When the roller stamps out the antenna in the foil, the unused foil is removed and potentially reused. This method produces high-quality antennas that are also relatively cheap (because of the reused aluminum), and also eliminates the need for any chemicals and reduces waste.

Antennas also can be printed using conductive inks. This is a very fast method and does not require sheet metal. Conductive inks are made of liquid containing solvents, binders, and very fine metal particles. Ink is applied through a mesh screen with a cut-out opening in the shape of an antenna. The rest of the mesh is sealed off. This method produces an antenna of comparable conductivity to pure metal antennas via a high-speed manufacturing process. There are concerns regarding printed antennas' durability as compared to pure metal antennas, but there is absolutely no waste.

Chip Attachment

When handling RFID tags, you probably noticed that not only do they have various shapes of antennas, but also some of them have a different chip attachment. There are two primary methods of affixing the chip to the antenna. A chip can be connected to an antenna by using a strap or it can be directly attached.

When using a strap technique, the chip is first attached to a strap. The strap is made of conductive material and is then connected to an antenna. An advantage of this method is the ability to handle very small chips and enables a high-volume production compared to direct attachment. However, a possible disadvantage can occur if the strap is not made of highly flexible material. (This is the reason to avoid using tags manufactured a couple of years ago.) If the tag is bent, the strap could interrupt the connections to the antenna or totally fall off, which would render the tag useless. This used to happen in early stages of the strap attachment technique, but today this problem has been eliminated. Another potential disadvantage is that the strap creates a slight "bump" on a tag, which makes it prone to damage during handling of a tagged product.

Note 

There are many methods for placing the chips onto the straps; some innovative designs not yet put into practice are trying to increase throughput and reduce cost. One of them, developed and patented by Alien Technology, is called Fluidic Self Assembly (FSA). This approach uses chips floating in a liquid over a base that has cut-out gaps in the shape of the chips. After all gaps receive a chip, the base is then sealed and cut into straps. A similar technology called PICA uses vibration and was patented by Symbol Technologies.

The direct attachment approach uses a robotic hand with a high-precision vacuum nozzle that places the chip onto an antenna. The chip has to be flipped over to allow for attachment to the antenna, and for this reason this method is sometimes called "flip-chip." When the chip is placed onto the antenna, it is then bonded with it by using pressure and heat to cure the electrically conductive epoxy on the chip's surface.

Modulation: Why It Matters

This would be a good time to take a deep breath, stretch your medulla oblongata, and get ready for some heavy but important technical talk. The EPC Generation 2 specification provides unprecedented levels of flexibility for tuning readers in accordance with customized business processes designed to fit with existing operations. But beware: selecting the wrong modulation scheme is analogous to driving an Indy 500 race car, when you should be nestled securely in an Abrams battle tank. Do you need a fast exchange between readers and tags to enable the use case that integrates seamlessly with your existing business processes, or a robust interface impenetrable to sources of RF interference that would otherwise rain destruction on your RFID system?

The modulation flexibility found in the Gen 2 protocol makes RFID more customizable to your application, translating into superior performance potential. This improvement will enable applications that improve operational efficiencies, saving end users time and money.

Selecting a modulation scheme determines how information is communicated on an RF wave. The Gen 2 protocol allows readers to use multiple modulation schemes in both the reader-to-tag link and vice versa. What you need to understand is how a Gen 2 UHF tag responds to a query command issued by a reader.

Gen 2-compliant tags have two built-in modulation operations, FM0 baseband and Miller subcarrier, as mentioned earlier. The reader tells the tag which scheme should be employed.

When FM0 is activated, the tag is instructed to use the same frequency as the reader to communicate its EPC information. This exchange is very quick, but problematic because the reader's signal is a million times stronger than the tag's and can drown it out. Alternatively, Miller subcarrier modulation moves the tag's response away from the reader's strong carrier signal, making the reader less susceptible to interference due to its own signaling.

The option of multiple modulations is available to enable superior reader optimization. In cases where many readers (a dense-reader environment) or other strong sources of interference are present, the Miller subcarrier modulation scheme should be employed. The trade-off is that Miller subcarrier data rates are significantly slower than FM0 by a factor of two, four, or eight. At the very least, understanding the key differences between the two will help you design a better system, or at least provide some fodder for cocktail parties when you want to bring a conversation to a screeching halt-just start talking about the various Gen 2 RFID modulation schemes.

Q Factor

Without getting into my penchant for James Bond movies, suffice it to say that Q is one of my favorite names. Q in this case is for Quality. The Q factor is often used as a classification of the quality or efficiency of a resonant circuit, which also applies to RFID tags.

Tags are tuned to operate at a certain frequency and respond to frequencies that are the closest to their own frequency. The higher the Q factor, the higher the amount of energy that could be transferred between the reader and a tag. The most efficient energy transfers (with the highest Q value) take place when the frequency is very close to the tag's resonant frequency. This means that a high Q factor comes with the slight disadvantage of a smaller bandwidth. In other words, an HF tag will have a higher Q factor than a UHF tag (13.56 MHz bandwidth vs. the 902–928 MHz wider spectrum). I know this may be a bit confusing, but if you remember that you can have either high Q or high bandwidth, but not both, you will have the basic concept.