RFID tagging technology is successfully used among us. Still, we don’t notice them always and do not think about them as they are not noticeable because we do not have to take any actions to transfer or read any information from RFID tags. But probably everyone notices in big supermarkets, book stores two parallel frame antennas and thicker labels on products you buy, or maybe you have an RFID entrance card at your work. So let’s see how this technology works.
RFID is a so-called Radio Frequency Identification system that consists of two main parts: transmitter and receiver. The labels, access cards, and even passports in some countries have RFID transponders integrated. These transponders are called tags.
Each tag has information programmed in it. A reader can read this information via the Radio Frequency link. Tags can be very miniature, like thicker paper sheets. The simplest ones have only an antenna and diode. These tags only reflect signals incoming from transmitters. These are tags that are used in supermarkets and other places for securing goods from stealing. If the signal is reflected, an alarm is triggered. When the seller scans the tag, it is electrically destroyed to trigger an alarm.
RFID also has another advantage comparing to other widely used labeling technologies like bar codes. RFID tags can carry much more data than bar codes. As bar code is limited to like 20 characters, the RFID tag can easily 512bits and more. Data of RFID tags can be rewritten many times. So it is easy to track shipping, assign properties, or give additional security access to personal of the office.
Another advantage of using RFID tags is that they do not need to be in “line of sight” to be read. Or there can be read multiple tags within the range.
There can be three types of RFID transponders: Passive, Active, and semi-passive.
Passive RFID tags
Transponders (tags) of passive RFID systems don’t have a power supply. This is why they are called passive. Passive tags are powered by the electromagnetic field generated by a reader antenna. Reader antenna has to transmit enough power to provide enough energy to tag to transmit back data. Because of this, the reading distance is minimal – up to several centimeters.
Well, some of the implementations may reach several meters. Passive tags are most commonly used because they are cheap. They can last indefinitely long as there is no need for a power supply. They are small in size, allowing them to integrate almost in every environment, starting with wrists, necklaces, cards, and stickers.
Passive tags simply consist of single IC and antena coil which is usually flat.
Passive tags are operating below 100MHz frequencies (the most common frequencies are 125kHz – 134.2kHz and 13.56MHz), and a magnetic field carries the main transfer energy. The magnetic field generated voltage in the coil is used as a power supply as a data signal. There are also HF passive tags that operate at 900MHz and 2.45GHz. These tags have dipole antenna (1/8 wavelength)construction. With these tags, the distance may reach more than 3 meters. High-frequency tags require more expensive manufacturing processing with more precise electronics, but they can support up to 2Mb/s data stream.
Active RFID tags
Active RFID tags may provide all advantages of the RFID system because tags are fully powered transmitters. They don’t have to be activated by an antenna reader. Active RFID topics may be extensive because there are many areas where and how they can be used. In some cases, tags may not need a reader antenna because tags in some particular cases can be configured to interact with each other. Active transponders can communicate in very long ranges up to several hundred kilometers. The main disadvantages of active tags may be the relatively big size and production price compared to passive ones.
Semi-Passive RFID tags
Semi-Passive tags are more similar to passive transponders than active ones. A battery or so-called battery-assisted tags power these tags. Still, radio transmission depends on antenna activity as the data processor had its own power, so all received power can transmit back the signal, which is stronger than the passive transducer. This allows increasing communication distance with quite a cheap solution.
Semi-passive RFID tags augment the reader antenna’s energy, but they are not constantly beaming signals as active tags do. Semi-passive RFID tags use a process to generate a tag response similar to that of passive tags. Semi-passive tags differ from passive in that semi-passive tags possess an internal battery for the tag’s circuitry, which allows the tag to complete other functions such as monitoring environmental conditions (temperature, shock) the tag signal range.
Let’s take an example of passive transponder data transmission, which sends out data at a frequency of 134.2kHz. The reader or so-called interrogator sends out a 134.2kHz pulse, which lasts for 50ms, which is used to power the passive tag and start up the transponder controller. Received energy is stored in the capacitor inside the transponder.
After this 50ms pulse ends, the transponder transmits data back over a period of 20ms. After tag transmitting is over, the capacitor discharges and resets for the next readings.
Passive Tag transmission is usually performed by switching low resistance across the antenna coil. The change of voltage across the antenna generates radio frequency – which is referred to as backscattering. Data is read from tags ROM memory and transmitted serially by switching coil load switch. The coil switch is driven by a carrier clock source, modulated with data from serial ROM memory data.
On another end, the reader removes(demodulates) the carrier signal, filters and cleans up the signal with Schmitt trigger. Then signal enters the digital section, where the CRC code is checked and decoded.