Principle of RFID ( Radio Frequency Identification ) in Internet of Everything

Principle of RFID

• Consider a coil made of copper wire through which alternating current is flowing. The coil offers impedance to the source and a voltage develops across its terminals. It is possible to increase the voltage by connecting a capacitor in parallel with the coil. Let us call this the “primary” coil. Now we bring in another coil, called the “secondary” coil, close to the first. Due to electromagnetic induction, voltage appears across the terminals of the secondary coil. The amplitude of the voltage depends on the size, shape, location and orientation of the secondary coil. If we connect a resistor (also known as a load) across the terminals of the secondary coil, current flows through it. The strength of the current flowing through the secondary coil depends on the load. The interesting phenomenon is that the current flowing in the secondary coil induces a voltage back into the primary coil, which is proportional to its strength. The induced voltage, also known as back emf (electromotive force), can easily be sensed by using suitable electronics. Therefore, by observing the voltage on the primary, it is possible to estimate what is connected to the secondary coil.

• The system is excited by a sinusoidal source. Capacitors are connected across both primary and secondary coils, forming a parallel resonant circuit. A load resistance is also connected in parallel with the secondary coil. 
  
• As the frequency of the source increases, the voltage also increases, reaches a maximum, and then decreases. The frequency corresponding to the maximum voltage is known as the resonant frequency. Now, if the load resistance is changed, the voltage at the primary corresponding to the resonant frequency drops sharply.
  
• It is possible to change (or modulate) the voltage at the primary by changing the load connected to the secondary. This is known as “load modulation”.
  
• The primary coil can be thought of as the reader of the RFID system, and the secondary coil as the transponder or tag. The tag can convey any message back to the reader using RF signals, by simply changing the load connected to its terminals. This could be achieved by switching in a load to represent a logical state 1 and taking off the load to represent a logical state 0. Using load modulation, a tag is able to communicate with the reader and transfer its identity without actually using a transmitter. The identity information is stored in a memory chip located on the tag. A processor (also known as the state machine) reads this information and modulates the load by operating a switch. Two more ingredients are required to operate the entire system: power and clock.
  
• Load modulation is the principle used to establish communication between the reader and the tag operating in the LF and HF bands.
  
• When the electromagnetic energy falls on the antenna attached to the tag, it backscatters a portion of the energy. The amount of backscattered energy depends on the load connected to the tag antenna. Therefore, by modulating the load according to the data, it is possible to change the strength of the backscattered signal from the antenna. The backscattered signal is sensed by the reader and is able to extract the information carried by it.