How to Select a Correct RFID Tag – Passive vs. Active

RFID tags come in many different designs, shapes, and sizes. A tag is designed for a particular application or a set of applications. Tags may be classified under four categories, depending on how the tags obtain power, the frequency at which they operate, the protocol used, and the various functionalities implemented on the tags. Let’s discuss tag types based on their power source.

A tag requires power to process signals received from the interrogator and to send the data encoded signals back to the interrogator. Signals sent back may be reflected signals or signals generated by the tag. Depending on how tags obtain the power and how they use that power, tags are classified as:
• Passive tags
• Semi-passive tags
• Active tags

A passive tag does not have its own power source; it has no battery onboard. The tag obtains power from radio waves received from the interrogator. The amount of power thus received is very small, just enough to energize its IC. Therefore, passive tag functionalities are limited. Due to a lack of enough power, it cannot support an active transmitter to communicate with the interrogator. The good thing about the lack of transmitter, however, is that passive tags do not contribute to radio noise. To communicate with the interrogator, passive tags operating at low and high frequencies use inductive coupling, while those operating above a high frequency range use radiative coupling. (See Dig Deep – Inductive and Backscatter Coupling [link to page]). Inductively coupled tags have a read range of a few inches to about 2 feet, while radiatively coupled tags have read range of around 20 feet (sometimes more).

In RFID applications, passive RFID tags are used often. They are often embedded into adhesive labels, which are easy and quick to attach, or sometimes into object themselves. Because of their low cost, passive tags are well suited in applications for which tags are not reusable. For example, in a supply chain, a case of goods may have passive tags attached to it; when the items are removed from the case and the case is discarded, the tags are also discarded. This makes economic sense due to the low cost of the passive tag.
The following table lists some of advantages and disadvantages of passive tags.

Semi-passive tags are also called semi-active, battery-assisted passive (BAP), or battery-assisted tags (BATs). This tag has an onboard battery to power its IC, but, like a passive tag, it does not have an active transmitter. It uses backscatter to communicate with the interrogator. It modulates the reflection of the waves from the interrogator and requires an interrogator to send data. Since no transmitter is present, it also does not contribute to radio noise. This type of tag is used because it can provide a longer read range than passive tag and can accommodate environmental sensors onboard. The sensor on the tag helps record the environmental experience of the object to which the tag is attached.

The second reason to have a battery on the tag is to attach an environmental sensor to the tag. Environmental sensors require reliable continuous power to operate, and they require a higher power level than the tag IC. The passive tag cannot have reliable and continuous power because it gets power only when it is in the IZ, which is not the case most of the time. In addition, when it gets the power inside the IZ, it is very limited—not enough to power an onboard sensor. Therefore, if a battery is added to the tag, the sensors and the tag IC can use that power, which is available all the time. The sensors on the tag can collect data and transmit that data along with the identifying number when the tag is interrogated. This is a valuable prospect, because, for example, it allows you to collect the temperature experience of the object automatically as the object moves around during its lifecycle. Therefore, when a case of temperature-sensitive drugs is transported, you can collect a record of how long the case was subjected to the temperatures beyond certain set limits and dynamically calculate its expiry date or its potency. Frozen food storage conditions can be easily monitored using this type of tag. Temperature, pressure, relative humidity, acceleration, vibration, motion, altitude, and chemical sensors can all be placed on a tag.

How do you know when a battery is drained? The interrogator can never detect a tag that does not respond. If the object with the tag is sitting outdoors in below-freezing temperatures, the battery may not work and the tag will not respond. Batteries on tags have a typical maximum shelf life of 2 to 7 years, but when tags are used and interrogated frequently, the batteries can quickly be depleted.
The following table lists some of advantages and disadvantages of semi-passive tags.

You may also ask, “Why not put an active transmitter on semi-passive tags?” An active transmitter requires a lot more electronics on the tags as well as a lager power source. This would make the tags bigger, heavier, and costlier. If you need to acquire data only when the tags are in the IZ, why pay for the extra functionality?

Omni-ID Semi-passive RFID Tags

An active tag has an onboard power source, usually a battery, and an active transmitter. The IC of this tag may contain more processing power to implement additional functionalities such as data manipulation. This tag uses the battery to power its IC and transmitter. It does not need emitted power or radio signals from the interrogator to transmit its data. Actually, it does not even need an interrogator. An active tag may be set to broadcast (beacon) its data at a preset time, periodically, or on occurrence of certain event. Its typical read range is 300 to 750 feet. The read range depends on the battery power and type of transmitter on the tag. An active tag, like a semi-passive tag, may have onboard sensors or external sensors connected to it. With more processing power, the tag may collect data from the sensors and locally process the data before broadcasting. Active tags are often used by real time location systems (RTLS).

The advantages and disadvantages of active tags are similar to those of semi-passive tags and are listed in the following table:

Identec Solutions Active RFID tag with Temperature Sensor