D3.7 A Structured Collection on Information and Literature on Technological and Usability Aspects of Radio Frequency Identification (RFID)

Untitled POWER SUPPLY

When considering power supply, we typically only consider the supply for the tag, since the reader is most often either powered from mains supply, or from standard batteries since size is rarely much of an issue.

In the majority of RFID applications it is desirable to have small and low cost tags. Additionally, the tag should have a long expected lifetime, perhaps of many years, with no maintenance required. These factors rather prohibit the use of batteries within the tags since this goes against all of these design criteria. Equally, in different applications, the tag may be required to perform some intensive processing, and thus requires significant power to do this. In these cases, batteries may be the only option. 

Figure : An example of a passive RFID tag, shown next to a UK two pence piece (roughly the size of a two Euro coin) for scale

As such, two variants of RFID tags have evolved. The tag is known as a passive transponder if it is unable to function without the reader since the reader supplies power to it. If the tag has its own power supply such as a battery, then it is an active transponder. Figure 3 shows an example of a passive tag. It is completely enclosed in glass, and has no serviceable components. Notably, the top section appears as a red-brown colour, and is in fact a tight coil of wire which is an integral part of its power and communication system – essentially acting as an aerial.

RFID systems that utilise frequencies between approximately 100 kHz and 30 MHz operate using inductive coupling, whereas microwave based systems in the frequency range 2.45–5.8 GHz are coupled using electromagnetic fields. However, it should be noted that the ranges achievable at higher frequencies are suitable for data transfer, but not for supplying power from the reader, and as such these typically utilise active devices. Conversely, almost all inductively coupled system utilise passive tags. 

1. 1. 1. Data rate

Typically, the data quantity a tag holds is in the region of a few bytes to a few kilobytes (sometimes referred to as n-bit). However, some tags only operate using 1 bit – that is the reader can only tell if a tag is there or not, and nothing else. This is useful in applications such as shop security (Electronic Article Surveillance (EAS)) where you want an alarm to sound if a tag passes through the door regardless of what the tagged item is.

Some n-bit tags are programmable, that is the data that they contain can be changed by the ‘reader’. Systems that have this functionality typically use Induction Coupling as their means of communicating between reader and tag, and most IC systems utilise passive tags. Simpler programmable tags contain simple logic (known as a state machine) which can control read/write access or to perform fairly complex sequences as well as holding ‘state variables’. More complex varieties use a microprocessor which allows some degree of complex operations to be performed, and is ultimately more flexible than the state machine solution. 

1. 1. 1. Operation type

The operation type of an RFID system is dependant on the application. Essentially, the RFID system can operate based on one of two basic protocols: Full (or half) duplex (FDX / HDX) or sequentially (SEQ).  

During FDX / HDX the tag transponder sends its data when the RFID reader is asking for it. In the case of passive tags it is during the communication of the data request that power it is actually supplied, i.e. power is actually drawn from the communication signal itself. The difference between full and half duplex is simply that during full duplex, both the reader and tag and send data at the same time, whereas with half duplex, only one can send data at any one time. In either case, the reader continuously supplies power to the tag. 

The SEQ protocol however requires the reader to briefly turn off the supply of power to the tag, during which time the tag sends its data. The reasoning behind the choice of operation type is essentially one of power coupling, that is, the way in which energy is transferred to the tag. Because full duplex systems can send data bi-directionally while power is being supplied to the tag, the tag is continuously using power. This means that the internal circuitry of the tag and its aerial have to be power matched in order to use power optimally. Unfortunately if they are matched, only half the potential source voltage (i.e. supply voltage to the tag) can be achieved, thus there is a trade off between power and voltage matching. This is not the case in SEQ systems, and as such provides distinct power advantages.