RFID Tags, Tagging, & Smart Labels

- overview or tutorial about the basics of RFID tags or RFID transponders including RFID smart tags or RFID smart labels and Radio Frequency Identification tagging.

RFID tags and RFID tagging techniques are essential to the whole operation of the Radio Frequency Identification system. In general the RFID tags or RFID transponders as they are sometimes called, are small low cost items that can be attached to items that need tracking or other forms of data collection to be performed.

The RFID tags that many are aware of are the small plastic elements that are added to many goods in shops to prevent goods from being stolen. However tags take a variety of forms and RFID tagging can be accomplished in a variety of ways.

RFID tag elements

RFID, Radio Frequency Identification tags are made to be as simple as possible and they contain comparatively little in the way of electronics. Fundamentally they comprise two main elements:

  • Electronics circuitry:   The electronics within an RFID tag are kept to the minimum to ensure that cost are minimised and power levels are kept as low as possible.
  • Antenna:   The antenna within the RFID tag is the element that takes the largest amount of space. It must be able to operate satisfactorily at the frequency of operation. With wavelengths being smaller at higher frequencies (especially UHF and microwave), this makes antennas for these frequencies much more efficient.

RFID tag types

RFID tags or RFID transponders can take a variety of forms. There are three main categories into which they fall:

  • Passive:   Passive RFID tags are by far the most common. They do not contain any power and receive this from the RFID reader. This is sufficient to power any device in the RFID tag and reply with the required data.

    The so called RFID smart tags, or RFID smart labels are all passive.
  • Semi-passive:   This form of RFID tag uses a battery to supply the internal operation of the tag, but relies on the RFID reader to supply the power to transmit the signal to the reader.
  • Active:   An active RFID tag is one in which battery power is used to supply power to the electronics. This enables greater distances to be achieved as the tag is not dependent upon the received power to provide a reflected signal, and the control and processing circuits can be more sophisticated as in the case of the semi-passive RFID tag.

Advantages and disadvantages of active and passive RFID tags

When choosing an active or passive tag, it is necessary to determine its requirements and whether an active RFID tag or a passive RFID tag is needed.

Some of the advantages and disadvantages of active and passive RFID tags are tabulated below:

Passive RFID tags:

Advantages Disadvantages
  • A RFID passive tag by definition does not have a battery and their life is almost indefinite.
  • Passive RFID tags are simpler to manufacture than active RFID tags and therefore cost considerably less.
  • As passive RFID tags are generally simpler, and they do not contain a battery they can be made much smaller than active ones.
  • The range of passive RFID tags is limited - typically a metre or so.
  • As power in RFID passive tags is very limited, sensors are rarely fitted as these will require additional battery power.
  • The fact that the tag is passive means that it remains operable for possibly years after it was initially used - this can have personal privacy implications unless it is deactivated.

Active RFID tags:

Advantages Disadvantages
  • The range of active RFID tags can be many metres - often 30 metres or more.
  • As battery power is required, this may allow much greater functionality to be incorporated.
  • An active RFID tag cannot function without battery power. This limits its lifetime, or requires maintenance.
  • RFID active tags are much more expensive to manufacture.
  • An active RFID tags will be physically larger than a passive one and this may limit its use.
  • Battery end of life can result in loss of functionality while the battery is down. This can be seen as a failure within the system

Read only and read-write RFID tags

RFID tags may be able to either perform as a read only RFID tag, or they may be a read-write Radio Frequency Identification tag. In view of the cost of manufacturing different types against the quantities made and the differences between the two, most RFID tags today are the read-write variety, and for applications where only a read function is required, the write ability is not used.

Read only radio frequency identification tags are typically programmed either in the factory. Data included will be a unique identifier and other specified data that cannot be changed.

Read-write RFID tags normally contain an area where data cannot be altered - this is often a segregated secure read-only area in the memory. Again this will include a unique identifier, and other data that may be required. The writeable area can then be used to contain data that may be required. For example if the RFID tag is used with a container, it can contain details of the container contents, etc. This area of memory within the RFID tag can be re-written many times.

RFID tag storage and processing

One important area and function of the RFID tag is the area that handles the information storage and processing. RFID tags range vastly in their capabilities as some do not have their own power, relying on the received signals to provide any power and this limits their abilities. Other RFID tags with their own battery power are able to carry out far more sophisticated tasks.

There are several types of RFID tag that may be used:

  • One-bit EAS RFID tags:   EAS (Electronic Article Surveillance) tags are commonly found in shops and stores to prevent theft. EAS tags are often termed "1 bit" tags. The reason for this is simply that they are only designed to communicate one bit of information, i.e. their presence. They are widely used in anti-theft measures in shops and stores. If the RFID tag is present and active, then it means that the item has not been through the checkout. If they have been passed through the checkout the RFID tag is either deactivated or removed.

    Because of their use, EAS tags are used in their millions and possibly the most widely used form of RFID tag. They do not have any memory or other chips as these would make them too expensive. Coupling used for these tags is generally inductive or backscatter. The tags simply consist of a resonant circuit, and the reader is able to detect their presence. A further point to note about EAS tags is that the readers have to sweep across a small frequency band, because the manufacturing tolerances of these RFID tags is such that there is a spread in the resonant frequencies of the different tags.
  • RFID smart labels :   Smart labels are simple RFID tags that are embedded in a an adhesive paper label. The advantage of this form of tag is that they can be used by RFID and barcode readers as well as having the option for human readable characters. They can be used in areas where the end product may enter one of a number of scenarios where the form of reader is not known - for example retail outlets a product may be shipped to may have either a barcode reader or an RFID reader, and outlets will have different options. Therefore to cover all eventualities a combined RFID and barcode tag is printed.
  • SAW RFID tag:   SAW - Surface Acoustic Wave tags form a half way house between the very basic 1-bit RFID tags and the more advanced tags that are available. The SAW RFID tags operate in the microwave region using backscatter techniques, and although they do not have a processor, they can be encoded at point of manufacture with a number. This number is limited by the technology but may be up to 32 or 64 bits.
  • Smart card tags:   Smart card tags are different to smart labels. Advanced smart card tags are used for many applications, and in particular where secure communications is required, for example for transactions involving finance. These cards may have complicated processors on board along with sufficient memory. When using these cards there is a balance to be made between functionality and cost - this needs to be taken at the outset of the design and needs to be carefully balanced.

Although RFID tags may appear to be the more straightforward or simple element within an RFID system, this may not be the case as considerable ingenuity and careful design is required to ensure the RFID tags perform correctly while being capable of being manufactured to a very low cost and within constraints of size, weight, form factor and also reliability. While most RFID tags are very cheap to manufacture, this hides the design behind them.

By Ian Poole

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