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Passive RFID Tags

Written By: 

Shurvi Sisodiya

The passive RFID tags do not have any power source and hence they have indistinct operational life span. The power needed for functioning is taken from the reader when the tag comes in the vicinity of the reader. They are available in a variety of sizes ranging from sizes which can fit into adhesive label. The passive RFID is basically made up of three parts: Antenna which is responsible for capturing energy and transferring the tag ID, Semiconductor chip appended to the antenna and an encapsulation which maintains the tag integrity. The encapsulation protects the antenna and chip from harsh environmental conditions. These encapsulations can be made up of small glass vial or from a laminar plastic substrate with adhesive on one side so that it can be easily attached to the goods.

 

Unlike an active RFID tag, passive RFID don’t have their own source of power and therefore the tag reader is responsible for powering the communication with the tag. Power can be transferred in two different ways. The first one is magnetic induction method and second is electromagnetic wave transfer method by using the EM properties related with the RF antenna i.e. the near field and the far field. The transfer of power ranges from 10µW to 1mW depending on the type of tag. So these kinds of tags are used in the cases and in items where the tags are not used again and the cost of the tag is also not important. The operating frequency ranges of Passive tags are 128 KHz, 13.6 MHz, 915 MHz, or 2.45 GHz.
 
In the Near field technique the reader passes a large amount of a.c. current through the reading coil due to which an alternating magnetic field is created in the nearby region. If a tag is placed in this region of magnetic field, then alternating voltage will appear across it. This voltage is rectified and coupled to the capacitor and a pool of charge gathers, which can be used to power the tag chip.
 

Diagrammatical Figure Displaying Near Field Technique in passive RFID Tags

Fig. 1: Diagrammatical Figure Displaying Near Field Technique in passive RFID Tags 

 
 
In the far field technique, the tag captures EM waves transmitted from the dipole antenna which is attached to the reader. The small dipole antenna receives this energy in the form of alternating potential difference that appears across the arms of the dipole. After the rectification it is linked to the capacitor which results in accumulation of energy in order to supply power to the tags.
 

Diagrammatical Presentation of Far Field Technique in passive RFID Tags

Fig. 2: Diagrammatical Presentation of Far Field Technique in Passive RFID Tags 

 
There can be one more method to transmit the signal from the tag that is when passive RFID tag stores the energy of the signal from the reader in an onboard capacitor. The tag uses the energy of the capacitor when it is fully charged.
 
The passive tags can be used in forming the identification cards for building access, credit cards, identity cards, bus fares, on the purchasable items etc where just a small tag of size as small as quarter is required to fulfill the needs and the reader reads the information from the tags and supply us the information of the items that are stored in the tags.