EcrosTechWiki | ISO15693 / Air (RF) Interface

This page introduces the radio frequency (RF) "operating field" generated by the tag reader (VCD) in an ISO 15693 RFID system. Tags (VICCs), having no built-in power source, must scavenge / harvest energy from this field. This is discussed here. For how the field is used by the reader to send information to tags, see at first page RF, Reader to Tag. For how tags use the field to send information back to the reader, see at first page RF, Tag to Reader.

Frequency and Power Transfer

The ISO/IEC standard requires that the tag reader generate an RF operating field at a frequency of 13.56 MHz ±7 kHz (that is, somewhere in the range 13.553 to 13.567 MHz). We are expected to already know, or figure out for ourselves, that this is primarily a magnetic field, but we will be coming back to this. Tags must "operate as intended" when the magnetic field intensity is anywhere in the range 0.15 to 5.0 A/m (amps per meter) r.m.s. (root mean square). Unsurprisingly, the tag reader is expected to produce a field intensity within this range, but the manufacturer is allowed to specify the "operating volume" where this requirement is met (i.e. where minimally conforming tags will be read). The tag reader is prohibited from generating a field intensity of greater than 12 A/m in "any possible [tag] position". This is the maximum intensity that, according to the standard, a tag is required to survive without damage. (None of this, of course, prevents manufacturers from making tags that operate in weaker magnetic fields or are able to survive exposure to stronger fields; it just sets the limits for conformance to the standard.)

Induction, NOT Radiation

The free-space wavelength of a 13.56 MHz RF signal is 22.124 meters. To efficiently radiate electrical energy, an antenna must be of a physical size not too much below half a wavelength. For this frequency, that's about as long as a school bus. But ISO 15693 RFID systems are not as big as school buses (take my word for it). Do they use small and therefore inefficient antennas? No, the truth is that they don't use antennas at all. The tag reader does not actually radiate a significant amount of RF energy. Instead, the RF field reaches the tag by electromagnetic induction. The mechanism by which the tag is powered by the reader is exactly the same as in inductive charging, except that only a very small portion of the total energy in the field is captured by each tag. Although the term "antenna" is widely used to refer to the structure that couples the RF field between reader and tag, it is best to keep in mind that this is completely inaccurate and the structure is actually a magnetic coupling loop.