TransCore Amtech IT2200, Инструкция по установке, обслуживанию и ремонту

Страница: 35 / 204

Theory of Operation
2-11
More conductive materials, such as water (especially salty water), not only attenuate
the RF signal, but can also reflect a portion of the RF energy from the surface of the
material, resulting in poor system performance. The amount of reflection produced is
related to the dissimilarity in electromagnetic impedances of adjacent materials.
Multipath and Signal Diffraction
Many fundamental properties of radio waves influence the RFID system. Radio waves
are reflected from dielectric (nonconducting) as well as metallic surfaces. These RF
reflections can allow tags outside the antenna's line of sight to be read, and can cause
multipath effects as well.
Multipath occurs when two or more favorable radio paths exist between the tag and
antenna. Multipath can cause high fields (extended reading range areas) and low fields
(null regions), which must be taken into account in system configuration.
Signal diffraction can also affect system performance. Diffraction is the bending of
radio waves around an obstacle. The combined effects of signal diffraction and
multipath reflections can allow tags to be read under seemingly impossible
circumstances.
Frequency and Power
All RFID systems must operate within national and international laws and guidelines
with respect to frequency and power. The common frequency bands available are near
888 MHz, 915 MHz, and 2450 MHz. (The authorized frequency bands in the United
States are 902 to 904 MHz and 909.75 to 921.75 MHz.)
Tag Lifetime
The housing and internal electronics of the IT2200-series tags, excluding the battery,
have an operational life of at least 10 years.
Tag Read/Write Accuracy Rate
By using the specified equipment and system configuration outlined in this chapter, a
tag read/write accuracy rate greater than 99.97% has been achieved with the IT2200
Reader System with Multimode Capability.
Amtech
®
RF Technology
The Amtech
®
system operates on the principle of modulated backscatter. The system
sends an unmodulated RF signal toward the tag, which acts as a field disturbance
device. The tag signal is varied in a coded fashion so that the tag reflects a coded
signal back toward the antenna at the same frequency transmitted by the RF module.
Modulated backscatter technology is similar to systems used in air-traffic control and
police radar systems, as well as in marine sonar systems.
To provide enhanced lane-to-lane selectivity and the ability to isolate and operate in
noisy RF environments, the IT2200 Reader System with Multimode Capability uses
backscatter technology for tag-to-reader communications. In this system, the reader