WO2004088583A1

WO2004088583A1 - A radio frequency identification tag

Info

Publication number: WO2004088583A1
Application number: PCT/GB2004/000231
Authority: WO
Inventors: John Domokos
Original assignee: Roke Manor Research Limited
Priority date: 2003-04-01
Filing date: 2004-01-23
Publication date: 2004-10-14

Abstract

A radio frequency identification tag (11), the tag comprising a transponder (3) directly mounted to a dielectric antenna (12) at a point on the dielectric antenna at which the impedance is substantially equal to the input impedance of the transponder, such that an optimum power match is achieved.

Description

A RADIO FREQUENCY IDENTIFICATION TAG

This invention relates to a radio frequency identification tag. Radio frequency identification devices (RFID) are widely used in industry and commerce. A basic RFID system consists of two parts: a reader and a tag. The reader is a transceiver that controls the system and the data acquisition. The tag is a transponder that is electronically programmed with unique information and typically, is attached to goods or carried by people. RFID tags are categorised as either active or passive. Active tags are powered by an internal battery. Passive tags obtain their energy externally, from the RF field of the reader. Programmable passive RFID tags consist of three parts. The first is an antenna

(or coil) that collects the RF field from the reader to provide electrical power for the tag and also to provide communication with the tag; the second part is a chip comprising a rectifier to supply DC power for the tag from the RF field, a memory to hold digital information and an air interface unit, which formats the digital information from the memory and sends it back to the reader via the antenna; and the third is a transformer, which transforms the RF signal from the antenna to a higher voltage that is required for the rectifier to operate efficiently. The transformer may be located with the antenna or implemented on the chip.

RFID tags operate in four different frequency bands. Early systems were developed to operate at either 125 KHz or 13.5MHz using coils and magnetic fields for communication. Capacitors and printed inductors can easily realise the required voltage transformation for these systems. These systems operate at relatively short range at low data rates. More recently, systems have been designed to operate at 900MHz or 2.45 GHz and communicate via electro-magnetic waves using dipole antennas. The advantage of these higher frequencies is that the data rate between the reader and the tag is higher, therefore larger number of tags can be read simultaneously and/or more information can be exchanged within a certain time interval.

At high frequencies, the most commonly used antenna is the dipole or some of its derivatives. The typical impedance of these antennas is in the order of 50 to 200 ohms. The RF rectifier used in current integrated circuits exhibits an input impedance of 5K to 3 OK ohm under typical DC load conditions. It is well known that the antenna impedance needs to be matched to the rectifier impedance to achieve optimum power transfer and therefore to maximise the reading range. This means that the voltage transformation ratio of the transformer ideally should be in the order of 5-24 (i.e. the square root of the impedance transformation). However, it is very difficult to match to large RF impedances without incurring electrical losses, or increasing the size and cost of the circuitry considerably. Furthermore, realisation of the high impedance levels is only feasible in narrow band, so this makes the tag more susceptible to proximity effects. In current devices these problems cause poor matching and inefficient RF to DC conversion. Similarly, by reducing the size of the conventional antenna, which is desirable for most applications, the radiation efficiency of the tag drops rapidly. As a result of these difficulties, a compromise is made between transformation ratio, antenna size and sensitivity of the tag. This problem does not generally arise with active tags because they are powered by an internal battery, so do not need a high transformation ratio to produce a high DC voltage. In accordance with a first aspect of the present invention, a radio frequency identification tag comprises a transponder directly mounted to a dielectric antenna at a point on the dielectric antenna at which the impedance is substantially equal to the input impedance of the transponder, such that an optimum power match is achieved.

By mounting the transponder of the radio frequency identification tag directly to a dielectric antenna, the need for a transformer to match the power from the antenna to the transponder is removed and a smaller, less complex tag can be manufactured, saving space and cost.

Preferably, the dielectric antenna comprises a high dielectric constant material, at least one surface of which has been metallised; and the transponder is mounted to the metallised surface.

Preferably, the antenna is one of an open-circuit patch antenna and a short- circuited patch antenna.

Preferably, the tag is a programmable passive tag.

Preferably, the transponder is a radio frequency application specific integrated circuit (RF-ASIC). In accordance with a second aspect of the present invention a radio frequency identification system comprises a radio frequency identification tag according to the first aspect and a reader.

An example of a radio frequency identification (RFID) tag and system according to the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a block diagram of a conventional RFID tag; Figure 2 shows an example of an RFID tag according to the present invention; and,

Figure 3 shows how radiation resistance of a dielectric patch antenna for use with an RFID tag according to the present invention varies with distance along the y- axis of the antenna.

In Fig. 1 a typical RFID tag is shown. A dipole antenna 1, for example 600mm long and operating at 2. 5GHz, picks up an RF field from a reader (not shown) and an RF voltage is developed across terminals c,d. The dipole impedance at c,d is typically 70 ohm, hence the RF voltage is low. By contrast, an input rectifier 2 of a unit 3 containing the detector part of the RFID tag requires a high voltage and has a relatively high input impedance level, of the order of 5K ohm, so it is necessary to transform the input voltage up, with a transformer 4, to get a required power match. The rectifier 2 rectifies the voltage supplied to a memory and air interface unit 5 of the detector.

A typical example of a voltage doubler of the type shown in Fig. 1, uses an IBM 5DM BICMOS processor, with an input impedance of the order of 5.8Kohm at nominal DC load of 2V, 50μA. This means that a voltage transformation ratio of 9:1 is required to perfectly match the reader to the antenna 1. Such a high transformation ratio cannot be realised efficiently with small and low cost components, hence considerable power is lost in the devices used in practice, due to mismatch and circuit losses, hi one embodiment of this invention, a programmable passive RFID tag comprises a dielectric antenna and an RF-ASIC chip. The chip may further comprise a rectifier and a memory and air interface unit, with the RF-ASIC chip being mounted on the dielectric antenna and directly connected to the antenna at a point where the optimum power match is achieved between the antenna and the rectifier. The present invention provides an efficient passive RFID tag that is small in size compared to the wavelength of operation. In this example, the RFID tag comprises a substrate made of substantially high dielectric constant material, at least one surface of the substrate suitably metallised to form a resonant dielectric antenna, and the RF-ASIC chip is mounted onto the substrate and directly connected to the high impedance and high voltage point of the metallised surface of the dielectric antenna.

Fig. 2 shows an example of the present invention, in which the tag is provided with a short circuited microstrip patch antenna with a dielectric substrate. The dielectric substrate 11 supports the patch antenna 12 and has a metallised backplate 13. The detector unit 3, for example a CMOS chip, is mounted directly onto the dielectric substrate 11 and directly connected to an edge 16 of the patch antenna 12. The metallised back plate 13 of the ceramic substrate is routed 14 to the top of the substrate 11. Due to the properties of the dielectric antenna, the field strength across edges la, lb of the antenna is very high and therefore a large voltage is developed for the rectifier input. The required impedance level of 5.8Kohm can now be implemented by appropriately dimensioning the antenna width and thickness. This means that an optimum power match and better sensitivity can be achieved with the new arrangement. Although the patch antenna can be left open-circuit, further size reduction is made possible by using a shorted patch antenna, which is shown in this example. For this case, at another edge 17 of the patch 12, a short circuit to ground metallisation 18 is provided, so that the patch is grounded and the overall length required for the same frequency is approximately halved. This is achieved at the expense of some deterioration of efficiency and bandwidth. A further advantage of the present invention is that by using a material with a high dielectric constant, the overall size of the antenna is reduced. For example, the length of the dipole antenna in free space in Fig. 1 is around 600mm at 2.45GHz. By contrast, at the same frequency, the length required for a dielectric antenna as shown in Fig. 2 is only 100mm. In both cases, the radiation efficiency is around 70%-80%. Typically, the RFID tag comprises the metallised dielectric substrate arranged to form its antenna and a transponder or detector unit which comprises a rectifier to rectify the RF signal delivered by the antenna and to power up the tag, a memory to store digital information and an air interface unit to format and transmit the stored information via the antenna to the reader, although the detail of the transponder is not shown in the diagram of Fig. 2.

Furthermore, the rectifier, the memory and the air interface unit can be implemented in an RF-ASIC chip, the RF-ASIC chip being mounted on the metallised dielectric substrate and connected directly to the antenna at the point where substantially optimum power match is achieved. The antenna may be an open circuit patch antenna or a short-circuited patch antenna

To illustrate the trade-off between size and range, the conventional high frequency RFID tags use a credit card size antenna at 2.4 GHz and achieve a maximum reading range of 30cm with a 4W reader. This implies an overall efficiency of 5-20%. Smaller tags only operate from much shorter distance and with an on-chip antenna only a few mm reading range can be achieved.

Existing dielectric resonator antennas are used in communication equipment such as handsets, computer peripherals with radio interfaces or GPS terminals. These antennas are essentially patch antennas printed on high dielectric ceramic substrates such as Titania (TiO₂) , BaTi0₃ or other compositions. The properties of these antennas include their small size, high efficiency and high stability. The dimensions of the patch are reduced, as compared with those possible in air, by the square root of the dielectric constant, with typical dielectric constants, ε for ceramic substrates ranging between 20-90. The substrates have extremely low loss, the tan δ is typically 0.00001. Furthermore, due to size reduction, the conductive losses are also reduced and the overall Q factor is high. Typically 70%-80% radiation efficiency is achieved. Despite the narrow bandwidth, the resonant frequency of the antenna remains stable against proximity effects. This is due to the high dielectric constant of the substrate.

A less well known property of the patch antenna is that the signal can be tapped off at different impedance levels along the length without affecting the bandwidth or the efficiency of the antenna, provided that the signal is terminated with the matched impedance. For example, in the case of a λ/2 patch, the impedance varies from zero well into the patch to a maximum towards the edge of the patch; at each point the efficiency and the bandwidth of the antenna is the same and the impedance is purely resistive at the resonant frequency. The radiation resistance at the edge of the patch is largely unaffected by the dielectric constant of the substrate material for a given width. This means that substrates with high dielectric constants can be used, achieving considerable size reduction without lowering the impedance at the edge of the patch. Optimising the width and the thickness of the antenna, a very high impedance, even several K ohm, can be realised efficiently at the edge of the patch. While the high impedance is usually a disadvantage in ordinary RF applications, it is a distinct advantage in RFID application when interfacing with the input rectifier of the chip.

The rectifier circuits used at the input of the RFID chips exhibit impedance levels of typically 5Kohm to 30Kohm. The unique property of the dielectric patch antenna is that it can provide a high voltage and high impedance, at an accessible point. The required voltage transformation is therefore readily achieved without circuit loss of a transformer and without the size penalty and undesirable proximity effects associated with high RF transformation ratios.

The radiation conductance, G of the antenna at the edge of a substrate strip (y = 0) as shown in Fig. 2 is

where

- ^2π

W = width of the substrate λ₀ = wavelength in free space h = height of the substrate (i.e. thickness)

The radiation resistance {R=\IG) on the surface of the antenna in the y direction varies by

where y = distance from one edge of the substrate in the longitudinal direction L = length of the substrate

The design of a typical dielectric patch antenna and the interface with an RFID chip is illustrated by the following example. Assume that the tag operates at 2.4GHz and the dielectric constant, ε of the antenna is 30. The length of the antenna is reduced by approximately -jε , therefore the quarter wave length is 5.7mm. If we choose a square geometry (W=5.7mm) with a substrate thickness of 0.6 mm, the radiation resistance (1/G), at the edge of the strip as shown in figure 3 (y=0) according to equation (1) is 2.6K ohm.

The radiation resistance along the y direction can be tapped off at an arbitrary level using equation (2). The radiation resistance variation in this example is plotted in Fig. 3. If the RFID tag has an input impedance of 1.5K ohm for example, for optimum power match the chip should be mounted at ~2.5mmm from the edge of the substrate as shown in Fig. 3. It should be noted that this antenna is capable of interfacing directly with very high impedance such as the impedance of a rectifier circuit. For example, at the edge of the substrate the impedance level is 2.6K ohm. This means that the antenna is capable of producing high voltage levels at this point and therefore the antenna can directly drive the input rectifier of the chip without the need for transformation.

Claims

1. A radio frequency identification tag, the tag comprising a transponder directly mounted to a dielectric antenna at a point on the dielectric antenna at which the impedance is substantially equal to the input impedance of the transponder, such that an optimum power match is achieved.

2. A tag according to claim 1 , wherein the dielectric antenna comprises a high dielectric constant material, at least one surface of which has been metallised; and the transponder is mounted to the metallised surface.

3. A tag according to claim 1 or claim 2, wherein the antenna is one of an open- circuit patch antenna and a short-circuited patch antenna.

4. A tag according to any preceding claim, wherein the tag is a programmable passive tag.

5. A tag according to any preceding claim, wherein the transponder is a radio frequency application specific integrated circuit (RF-ASIC).

6. A radio frequency identification system comprising a radio frequency identification tag according to any preceding claim and a reader.

7. A radio frequency identification tag as hereinbefore described with reference to the accompanying drawings.