Transponders

This invention relates to transponders, and particularly to a transponder which is adapted to receive and subsequently transmit electromagnetic wave energy and is in the form of a tag or other planar element which may be flexible.

The device, hereinafter called for convenience "tag" is particularly although not exclusively intended for use in a system for inhibiting theft. Such a system may comprise a transmitter, which may be disposed at or near the doorway of a shop, and a receiver disposed likewise. The transmitter may radiate at appropriate intervals an electromagnetic signal constituted by a short pulse of high frequency energy, for example at a frequency between 400 and 700 MHz. The radiation pattern of the transmitter or receiver may be appropriately confined so that if a tag, which would normally be applied to goods for sale but removed at the point of sale, passes into a particular region, a signal radiated by the transmitter will be received by the tag and re-transmitted to the receiver in order to provide an alarm.

For efficacy of the system, the signal re-transmitted by the tag must be distinguishable from the original signal. It is the object of the present invention to provide a tag which can receive and transmit electromagnetic wave energy and provide, between reception and transmission a significant delay so that at a receiver a signal re-transmitted from the tag may be distinguished from any signal emanating from an original transmitter.

According to the invention, a tag which is capable of reception and re-transmission of electromagnetic wave energy and is adapted to convert received wave energy into acoustic wave energy and to re-convert this wave energy into electromagnetic wave energy, includes piezoelectric material which can sustain the acoustic wave energy in the form of a surface acoustic wave for a significant time subsequent to the reception of the electromagnetic wave energy.

There follows a description of the construction and manner of operation of preferred embodiments of the invention.

In preferred embodiments of the invention a tag includes piezoelectric material which is subjected by an appropriate transducer to an oscillatory electrical signal, which induces in the material a mechanical oscillation, namely an acoustic wave which travels along the surface of the piezoelectric member, which may be a crystal or an amorphous or polycrystalline layer on a substrate, to another transducer and is re-converted to an electrical signal.

The re-transmitted signal is similar to, although smaller than, the original signal, and endures at a detectable intensity after cessation of the original transmitted signal and attenuation of its random reflections.

A longer delay between reception and re-transmission may be achieved by the piezoelectric member comprising a resonator, preferably a surface-acoustic-wave resonator (SAW), the high quality factor of acoustic wave resonators providing longer duration of the re-transmitted signal after the transmission of the original signal has ceased. The narrow bandwidth to which such a resonator responds efficiently enables the bandwidth of the transmitter and receiver to be limited. Fabrication of SAW resonators lends itself to large scale production of small devices, e.g. up to a few millimeters square, with a relatively uniform centre frequency e.g..+-.0.5 MHz in several hundred MHz.

A preferred embodiment of the invention is now described, by way of example, with reference to the accompanying drawings in which:

FIG. 1 illustrates schematically the transmitted and corresponding re-transmitted signal;

FIG. 2 illustrates a plan view of a flexible tag according to the invention, and

FIG. 3 illustrates a cross-section along the line x--x of the tag shown in FIG. 2.

Referring to FIG. 1, diagram A illustrates a signal pulse from a transmitter of duration tp, diagram B shows the re-transmitted signal from a tag such as illustrated in FIG. 2. After time tp, when the transmitter signal ceases, the amplitude of the re-transmitted signal progressively decreases; during time T random reflections of the transmitted signal may be present, and so the tag signal is not sampled until the end of the interval T by which time the transient reflections have been attenuated below the detection threshold. Sampling of the re-transmitted signal takes place during time t.sub.s, which is the interval in which the re-transmitted signal along is sustained above the threshold detection amplitude I. The total decay time (T+t.sub.s) for a tag incorporating a quartz SAW resonator may be of the order of several microseconds; this compares favourably with a printed inductance-capacitance resonant circuit in which the decay time may be of the order of 100 nanoseconds and the time periods T and t.sub.s not separable.

The tag shown in FIG. 2 comprises a flexible substrate I made, for example, of a polyester film such as `MYLAR`, on which two metal antennae 2 and 3 are disposed, e.g. by screen printing, or by selective etching of a metallic (preferably copper) layer on the film, and which are connected to a surface acoustic wave resonator 4 at the centre of the tag. It is possible to use one or more antennae, the purpose of these being to assist efficient reception and re-transmission of the signal. Typically the long dimension of the antennae is equal to or less than .lambda.4 of the transmitted frequency.

A quartz crystal may be used for the resonator, or alternatively a thin layer of piezoelectric material such as zinc oxide disposed on a substrate. Such a thin layer may be deposited by sputtering, either directly on to the flexible substrate 1, or on to a rigid substrate such as glass which is embedded in or attached to the centre of the flexible substrate 1. The layer thickness may be 2 to 3 .mu.m, although a thicker layer of 20 to 30 .mu.m may be used.

FIG. 3 shows a sectional view of the tag illustrated in FIG. 2, in which the SAW resonator 4 consists of a zinc oxide layer 5 deposited on a glass substrate 6. The antennae are conductively bonded to the transducer layer 5 by, for example, ultrasonic or thermal compression bonding. A recess 7 in the substrate allows the surface of the device to remain free to promote efficient surface wave propagation, and the entire tag may be encapsulated by a flexible coating.

In a system where several tags are used, such as in a pilfering protection system, it is necessary for a range of frequencies to be transmitted; this may be achieved by sweeping the transmitter through the frequency range in which the resonant frequencies of all the tags will lie. An exemplary specification for such a system is as follows:

Transmitter power: 100 MW

Transmitter frequency: 400 MHz

Transmitter's amplitude modulation: by a 15 KHz square wave (requiring a band width of .+-.1/2 MHz)

Transmitter's frequency modulation: 10 KHz per millisecond

Transmitter interrogater tags over a frequency range determined by the centre frequency spread occurring in bulk manufacture of the tags, for example .+-.0.5 MHz

Tag's Q (quality factor): 10,000 at 400 MHz

The FM bandwidth nominally .+-.0.5 MHz, giving a total bandwidth of .+-.1 MHz and a total search time of 75 milliseconds.