RFID tag and reader design development project-complete report
RFID Proximity Security System
Before we start with actual circuit design, it is neccessary to understand the principals behind the technology that this project has set out to harness; passive RFID communications. Passive RFID tags work in such a way that they are actually powered by an external signal, which, in most cases is the carrier signal from the tag reader circuit. These tags are fairly simple and are comprised of merely an L-C antenna (similar to the one shown in the block diagram below) and the circuitry neccessary to modulate this carrier signal once powered on. The reader and tag communicate using magnetic coupling since their respective antennas can sense changes in magnetic field, which is observed as a change in voltage in the reader circuit.
The Cornell ID cards we use in this project were developed by HID®; specifically the HID DuoProx II cards. These are useful because they have both embedded RFID as will as a magnetic strip, while much of campus is starting to switch over to proximity entry systems, many current systems (including the dining halls) are still swipe-operated. From looking at their website, it was difficult to determine much information about the card’s operation, asside from the fact that it operates at a 125 KHz carrier frequency and it could have a tag size anywhere between 26 and 128 bits long.
After many hours of research we discovered that the modulation type used in the cards is Frequency Shift Keying (FSK), one of the more common ways used in RFID. FSK modulates the signal by essencially multiplying a lower amplitude, lower frequency signal with the carrier signal, creating an AM-like effect; the lower frequency enveloping the carrier frequency. To switch between a “1” and a “0”, the tag switches the modulating frequency. The two frequencies used by our cards were 12.5 KHz (125 KHz/10) and 15.625 KHz (125 KHz/8), which correspond to 1 and 0 respectively. The modulation produces an effect that looks similar to the figure below: