Proximity Cards - Introduction

Introduction | Keying | Decoding | Encoding

The proximity cards that we examined are an example of a passive RFID system. By definition, a passive RFID system includes a reader (sometimes called an interrogator) and a tag (or proximity card). The tag consists of an antenna (a coil) and a chip that can do basic modulation based on data stored in its non-volitile memory. The tags we examined also contained a discrete capacitor to tune the antenna to the proper frequency. The pictures below show the innards of one of these cards.

Note from the image at the right that the capacitor was broken during the process of removing the coil and chip assembly from it's plastic housing (this was accomplished by dissolving the plastic in chloroform). The black square is the chip with the data memory and modulation circuitry. Interestingly enough, this proxcard tag still works, even with the broken capacitor.

The system is considered passive because the tag has no power source of its own. Instead, the reader generates a varying electromagnetic field that couples energy into a coil in the tag (in the same way that energy is transferred from one winding of a transformer to another). This provides enough power to turn on a chip, which opens and shorts the coil to modulate the electromagnetic field.