US engineers have reduced the size of radio-frequency identification tags by 25% and made them cheaper by eliminating the need for the tags to convert AC to DC power.

Traditionally, passive radio-frequency identification (RFID) technology uses a “reader” to transmit a radio signal that is picked up by the tag. The AC of the radio signal is converted by the tag into DC in order to power its internal circuits.

Those circuits control the signal that is bounced back to the reader. Passive RFID technology is used in everything from parking passes to merchandise and asset tracking. For example, passive RFID is the technology that tells a traffic barrier to lift when you wave a parking pass in front of the scanner.

Researchers at North Carolina State University have successfully redesigned the RFID tag’s circuits to operate directly off of AC power by incorporating additional transistors into the circuits. The circuits share a few transistors that enable them to operate correctly using an AC power source.

Paul Franzon, a professor of electrical and computer engineering at NC State and lead author of a paper on the work, said: “By eliminating the hardware that is used to convert the AC signal to DC for powering the circuit, we are able to make the RFID tag much smaller and less expensive.”

“We’re currently looking for industry partners to help us bring this technology into the marketplace.”

Tags made using the new design are called “RF-only logic” RFID tags, and the prototypes have less range than conventional, passive RFID tags. However, Franzon and his team have plans to develop new RF-only logic tags that they anticipate will have similar range to conventional tags.

The RFID tag is implemented in a 0.13 µm CMOS technology. The tag dimensions are 0.6 mm × 0.3 mm. The paper, “Design of a Rectifier-Free UHF Gen-2 Compatible RFID Tag using RF-Only Logic,” was presented May 5 at the IEEE RFID 2016 conference in Orlando, Florida.

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