North of Austin is a piece of Old Texas where two railroad lines and a 120-year-old lime factory converge. During a recent train-spotting stop, I found a curious little box trackside and recognized it as an asset-tracking badge that had somehow gotten detached from a passing railcar. That meant electronics within--and a teardown waiting to happen.

A search identified the find as an AT5110 transportation tag from Amtech (now part of Transcore Inc.). The RFID tag uses a "passive backscatter" technology to allow a unique identifier to be picked off as a train car passes interrogators (readers) along the line. Data sheets for the AT5110 list an operating frequency of 915 MHz and an "indefinite" service lifetime.

Getting the thing open proved a challenge. Because of the harsh operating environment, the outer enclosure must provide a water- and dustproof seal around the electronics, which nonetheless take a beating from temperature and vibration extremes. A session with a carbide blade and precision saw was needed to crack open the ultrasonically welded two-piece plastic shell.

Inside, a single printed-circuit board holds fairly straightforward electronics. More than half the length of the board is dedicated to a patch antenna used in RF interactions with readers. Beyond the patch antenna is a pair of microstrip circuits--pc board traces, really. The balance of the board is sparsely populated with passive devices, diodes, transistors and a single IC from Atmel. Either a custom device or a private-labeled version of Atmel's long-running RFID chip series, the IC handles circuit operation, storage and identification tasks.

No battery is employed; energy to run circuitry comes from the outside. A portion of the RF signal received in the tag as it passes the interrogator beam is converted to dc power, providing the burst of energy needed for the Atmel chip to wake up and perform its primary function of tag identi- fication. The backscatter feature means the system does not use a "ping" and "response" technique; rather, the Atmel part encodes ID data into a signal that's reflected back to the reader. No active transmissions take place in the tag, just a twiddling of the RF patch antenna reflective properties.

Taking note of the filter and matching network (possibly a misnomer) that separate the circuits and the patch antenna, there are two distinct sections. Closest to the antenna is a loop that splits the signal at the antenna end and rejoins itself. A right-angle microstrip emanates from the junction, and circuits tap in at the end of this stub. At 915-MHz operation, the loop legs and stub are precisely quarter- and eighth-wavelengths long, respectively, once adjustments are made to account for the circuit board dielectric. Sorting out the role of the microstrip circuits falls outside the scope of this article, but an impedance transformer and/or filter function is assumed.

The microstrip network lets the Atmel part control a shunt load at the end of the structure and affect the backscatter modulation. A short or open circuit applied to the terminus of the matching network causes a shift in antenna reflectivity such that the tag's identifying bit stream can be encoded, based on a clock derived from the reader beam itself. The tag holds a 120 bits of ID data in a section of E²PROM on the 1.4-mm² Atmel chip. The circular array of pads on the board corresponds to a pluggable hole in the tag enclosure for loading data.

The AT5110 helps the rail industry figure out whose cars are where, and when, without much human intervention. Consider the scope of goods moving globally, and the need for asset tracking seems clear. RFID is big and bound to get bigger. So next time you're stuck at a rail crossing, try and spot the tags. It'll help pass the time. n

By David Carey, president of Portelligent (Austin, Texas), which produces teardown reports and related industry research on wireless, mobile and personal electronics (www.teardown.com).

    See related chart