Specifications

5.1. INTRODUCTION
enter the reading field.
Passive devices typically offer limited user-programmable data storage
capacity of less than 128 bytes, and often none at all. This is the c ase for
several reasons, but most importantly because of power restrictions. Passive
tags do not have on-board power supplies and most of the power collected is
used for transmission rather than memory reading or writing.
Active RFID
Active tags have an on-board power source, usually in the form of a battery.
This means that the tags have a continuous power source rather than the
passive tags which are only powered while in the presence of the reader’s field.
The primary function of an active reader’s RF emission is to request data
from the tag rather than actually charging the device. The reader’s received
signal strength can thus be easily improved by using a more powerful power
supply in the tag. The range of these devices can easily extend to 100m.
Active devices offer a further advantage of being able to read multiple tags
in the same field quickly as all tags remain self-powered during handshaking.
Thousands of tags can be read from a single reader at high speeds. Readers
can thus power down more often and still guarantee that the tags will be
detected. Additionally, data handling is more intelligent, with tags able to
store upwards of 128 kilobytes each. However, active tags have the drawback
of requiring their own power supply - either in the form of a replaceable
battery or a disposable tag which needs to be replaced once the on-board
supply has been exhausted.
The operational differences be tween active and passive systems are illustrated
in Table 5.1. Hybrid active-passive schemes are also available.
5.1.3 Operating Frequency
A further distinction between RFID systems is often made based on the
system’s operating frequency (high or low). Low frequency devices (less
than 500kHz) make use of inductive coupling to transfer data. This limits
range, but allows for propagation through many mediums, including water
24