RFID Tag
Passive
Passive RFID tags have no internal power supply. The minute electrical current is induced in the antenna by the
incoming radio frequency signal,it provides just enough power for the CMOS integrated circuit in the tag to power up
and transmit a response. Most passive tags signal by backscattering the carrier wave from the reader. This means that
the antenna has to be designed to both collect power from the incoming signal and also to transmit the outbound
backscatter signal. The response of a passive RFID tag is not necessarily just an ID number; the tag chip can contain
non-volatile, possibly writable EEPROM for storing data.
Passive tags have practical read distances ranging from about 10 cm (4 in.) (ISO 14443) up to a few meters (Electronic
Product Code (EPC) and ISO 18000-6), depending on the chosen radio frequency and antenna design/size. Due to
their simplicity in design they are also suitable for manufacture with a printing process for the antennas. The lack of
an onboard power supply means that the device can be quite small: commercially available products exist that can be
embedded in a sticker, or under the skin in the case of low frequency RFID tags.
Active
Unlike passive RFID tags, active RFID tags have their own internal power source, which is used to power the
integrated circuits
and broadcast the signal to the reader. Active tags are typically much more reliable (i.e. fewer errors) than passive tags
due to the ability for active tags to conduct a “session” with a reader. Active tags, due to their onboard power supply,
also transmit at higher power levels than passive tags, allowing them to be more effective in “RF challenged”
environments like water (including humans/cattle, which are mostly water), metal (shipping containers, vehicles), or at
longer distances, generating strong responses from weak requests (as opposed to passive tags, which work the other
way around). In turn, they are generally bigger and more expensive to manufacture, and their potential shelf life is
much shorter.
Many active tags today have practical ranges of hundreds of meters, and a battery life of up to 10 years. Some active
RFID tags include sensors such as temperature logging which have been used to monitor the temperature of perishable
goods like fresh produce or certain pharmaceutical products. Other sensors that have been married with active RFID
include humidity, shock/vibration, light, radiation, temperature, and atmospherics like ethylene. Active tags typically
have much longer range (approximately 500 m/1500 feet) and larger memories than passive tags, as well as the ability
to store additional information sent by the transceiver. The United States Department of Defense has successfully used
active tags to reduce logistics costs and improve supply chain visibility for more than 15 years.
Semi-passive
Semi-passive tags are similar to active tags in that they have their own power source, but the battery only powers the
microchip and does not broadcast a signal. The RF energy is reflected back to the reader like a passive tag. An
alternative use for the battery is to store energy from the reader to emit a response in the future, usually by means of
backscattering.
The battery-assisted receive circuitry of semi-passive tags lead to greater sensitivity than passive tags, typically 100
times more. The enhanced sensitivity can be leveraged as increased range (by a factor 10) and/or as enhanced read
reliability (by one standard deviation).
The enhanced sensitivity of semi-passive tags place higher demands on the reader, because an already weak signal is
backscattered to the reader. For passive tags, the reader-to-tag link usually fails first. For semi-passive tags, the reverse
(tag-to-reader) link usually fails first.
Semi-passive tags have three main advantages 1) Greater sensitivity than passive tags 2) Better battery life than active
tags. 3) Can perform active functions (such as temperature logging) under its own power, even when no reader is
present.