GB2331611A

GB2331611A - Recovering data from a contactless data carrier

Info

Publication number: GB2331611A
Application number: GB9724657A
Authority: GB
Inventors: Hans Staufer
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1997-11-22
Filing date: 1997-11-22
Publication date: 1999-05-26
Also published as: GB9724657D0; GB2331611A9

Abstract

A system for recovering data from a contactless data carrier has a reader device 100 to transmit a signal over an air interface and a contactless data carrier 10 to receive the signal over the air interface. The contactless data carrier 10 provides selective absorption of the signal in dependence upon the data to be recovered. The reader device 100 includes a standing wave detector circuit 130, which is arranged to detect the selective absorption of the modulated carrier signal by measuring a phase parameter thereof.

Description

2331611 SYSM4, READER AND METHOD FOR RECOVERING DATA FROM A CONTACTLESS

DATA CARRIER

Field of the Invention

This invention relates to data recovery from contactless data carriers.

Background of the Invention

Contactless, data carriers such as smart cards are commonly used in many applications. Typically data is stored in non-volatile memory in a smart card, and this can be recovered by a base station reader via an inductive coupling.

In many smart card applications the card itself is arranged to not contain any intrinsic power. Power is transferred from the base station reader to the card by a carrier signal via the inductive coupling. The carrier signal may also be periodically modulated, in order to send data to the card. The smart card transmits data back to the base station reader by repeatedly switching an internal resistor (or capacitor), which changes the tuning of a loop antenna in the card. This provides a signal disturbance in the inductive coupling (known as a back-scattered signal). This back- scattered signal can be sensed by the base station reader.

A problem with the above arrangement is that the magnitude of the backscattered signal is very small in comparison to the carrier signal, and is difficult to detect.

A known method to improve the detection of the back-scattered signal is to use filtering techniques to mask the carrier signal, and to enhance a portion (usually the lower sideband third-order portion) of the backscattered signal.

However, the strength of the carrier signal has to be very high to ensure that the back-scattered signal can be detected. Therefore detection of the back-scattered signal and hence the derivation of the data from the card is difficult. Furthermore the filters used in the filtering techniques must exhibit good temperature stability, to ensure good data derivation from the back-scattered signal over large ambient temperature ranges.

This invention seeks to provide a reader for a data carrier which mitigates the above mentioned disadvantages.

Summajy of the Invention

According to a first aspect of the present invention there is provided a system for recovering data from a contactless data carrier, comprising: a reader device arranged to transmit a signal over an air interface; a contactless data carrier arranged to receive the signal over the air IL5 interface, the contactless data carrier being arranged to provide selective absorption of the signal in dependence upon the data to be recovered therefrom; wherein the reader device includes a standing wave detector circuit, arranged to detect the selective absorption of the modulated carrier signal by measuring a phase parameter thereof.

According to a second aspect of the present invention there is provided a reader device for recovering data from a contactless data carrier, the reader device arranged to transmit a modulated carrier signal over an air interface, the contactless data carrier being arranged to provide selective absorption of the modulated carrier signal in dependence upon the data to be recovered therefrom; wherein the reader device includes a standing wave detector circuit, arranged to detect the selective absorption of the modulated carrier signal by measuring a phase parameter thereof.

Preferably the standing wave detector circuit comprises a bridge splitter circuit and a phase detection circuit.

According to a third aspect of the present invention there is provided a method for recovering data from a contactless data carrier, the method comprising the steps of-- transmitting a modulated carrier signal over an air interface; the contactless data carrier selectively absorbing the modulated carrier signal in dependence upon the data to be recovered therefrom; measuring a phase parameter of the modulated carrier signal, thereby detecting the selective absorption.

In this way data may be recovered from a contactless data carrier, the data having an improved signal-to-noise ratio, and the data recovery being improved when operating over a large distance. These improvements also mean that a reduced field strength, and hence reduced power consumption by the reader device is possible.

Brief Description of the Drawings

An exemplary embodiment of the invention will now be described with reference to the drawing in which:

FIG. 1 shows a system incorporating a contactless data carrier reader in accordance with the invention.

FIG. 2 shows a circuit forming part of the contactless data carrier reader of 20 FIG. 1.

Detailed Description of a Preferred Embodiment

Referring to FIGA, there is shown a system including a contactless data carrier 10 and a base station reader device 100, arranged to communicate over an air interface 200. The reader 100 comprises a power amplifier 110, data input terminal 115, a reference oscillator 120, a standing wave detector circuit 130 and a loop antenna 140. The data input terminal 115 is arranged for receiving data to be transmitted to the data carrier 10. The power amplifier 110 is coupled to receive the data from the input terminal 115, and to receive a carrier frequency signal from the reference oscillator, and is arranged for modulating the carrier frequency signal by the data to be transmitted. The loop antenna 140 is coupled to receive the modulated carrier frequency signal, for propagating the signal over the air interface 200.

The standing wave detector circuit 130 is coupled to the reference oscillator 120 and to the loop antenna 140, and will be further described below.

The contactless data carrier 10 may be a smart card or other similar device, and has a loop antenna 20 and a tuning capacitor 25 coupled to tune the loop antenna 20. A microcontroller 30 of the data carrier 10 is coupled to receive a power supply voltage VDD induced in the loop antenna 20, and provided by a diode 35. A storage capacitor 45 is coupled between the positive voltage path and a ground terminal, for storing surplus charge provided by the power supply voltage VDD.

A data and clock recovery circuit 50 is coupled to the loop antenna 20, and is arranged for recovering the data transmitted in the modulated carrier frequency signal over the air interface 200, in order to provide the data to the microcontroller 30.

The microcontroller 30 is further arranged to provide a data output 55, which is used to selectively switch a resistor 60 via a switch 40. In this way the resistor 60 is selectively coupled to the loop antenna 20, such that the input impedance of the loop antenna 20 is switchable between two values.

In operation, data to be transmitted to the contactless data carrier 10 is received at the data input terminal 115. The power amplifier 110 modulates the carrier &equency signal by this data, and the modulated signal is transmitted via the loop antenna 140. When the data has been transmitted, the power amplifier 110 continues to provide an unmodulated carrier frequency signal, which continues to provide power for the contactless data carrier 10.

The loop antenna 20 of the contactless data carrier 10 receives the carrier signal by induction. The power supply voltage VDD is provided to the microcontroller 30 and the data is demodulated and provided to the microcontroller 30 via the data and clock recovery circuit 50.

The contactless data carrier 10 is arranged to transfer response data back to the base station by using the switched resistor 60 to vary the impedance, and hence the tuning of the loop antenna 20. With the switched resistor 60 in a first position, a first impedance is presented, and in a second position, a second impedance is presented.

In this way the unmodulated carrier signal is selectively absorbed by the loop antenna 20, and a standing wave is thereby generated, which emanates from the loop antenna 20. The standing wave is known as a back scattered signal, and contains the response data. The loop antenna 140 of the base station reader 100 experiences an induced current caused by the back-scattered signal, and in this way the response data is transferred from the contactless data carrier 10 to the base station reader 100. In a prior art arrangement, a filter circuit of the base station reader 100 is used to isolate and enhance the back-scattered signal.

The standing wave detector circuit 130 comprises a reflection bridge splitter circuit 140, a phase detector circuit 150, a phase shifter circuit 160 and an RC network 170.

The reflection bridge splitter circuit 140 is shown in greater detail in FIG. 2.

This circuit is effectively a transformer, having a first winding 141 coupled between the power amplifier 110 and the loop antenna 140, a second winding 146, which has an output containing the back-scattered signal and a phase balance variable resistor 145, which is coupled between the first winding 141 and the second winding 146. The back-scattered signal is provided to the phase detector circuit 150.

The phase detector circuit 150 has an output coupled to provide a demodulated back-scattered signal. This is achieved by virtue of a feedback loop formed with the RC network 170 and the phase shifter circuit 160. The phase shifter circuit 160 is coupled to receive the carrier frequency from the reference oscillator 120, and this is used, in conjunction with the feedback arrangement of the RC network 170, to demodulate the back-scattered signal to reproduce the response data.

In this way the response data is recovered from the back-scattered signal by the base station reader 100. The above arrangement achieves a significantly improved signal-to-noise ratio when compared with a prior art arrangement, and exhibits improved performance when the air interface involves a large distance. These improvements also mean that a reduced field strength, and hence reduced power consumption by the power amplifier 110 is possible.

It will be appreciated by a person skilled in the art that alternate embodiments to the one described above are possible. The loop antennae 140 and 20 could be replaced by other media suitable for contactless data transfer, such as infra-red apparatus.

It will be appreciated by a person skilled in the art that alternate embodiments to the one described above are possible. For example, the contactless data carrier 10 could be arranged to provide response data as soon as the carrier frequency signal has provided VDD to power the carrier 10. In other words, no data need be transmitted by the reader 100. Furthermore, the precise arrangement of the standing wave detector circuit 130 may be varied from that described above.

Claims

1. A system for recovering data from a contactless data carrier, 5 comprising:

reader device arranged to transmit a signal over an air interface; contactless data carrier arranged to receive the signal over the air interface, the contactless data carrier being arranged to provide selective absorption of the signal in dependence upon the data to be recovered therefrom; wherein the reader device includes a standing wave detector circuit, arranged to detect the selective absorption of the modulated carrier signal by measuring a phase parameter thereof.

2. A reader device for recovering data from a contactless data carrier, the reader device arranged to transmit a modulated carrier signal over an air interface, the contactless data carrier being arranged to provide selective absorption of the modulated carrier signal in dependence upon the data to be recovered therefrom; wherein the reader device includes a standing wave detector circuit, arranged to detect the selective absorption of the modulated carrier signal by measuring a phase parameter thereof.

3. The system or reader device of claim 1 or claim 2 wherein the standing wave detector circuit comprises a bridge splitter circuit and a phase detection circuit.

4. A method for recovering data from a contactless data carrier, the method comprising the steps of.- transmitting a modulated carrier signal over an air interface; the contactless data carrier selectively absorbing the modulated carrier signal in dependence upon the data to be recovered therefrom; measuring a phase parameter of the modulated carrier signal, thereby detecting the selective absorption.

5. A system substantially as hereinbefore described and with reference to the drawings.

8_

6. A reader substantially as hereinbefore described and with reference to the drawings.

7. A method substantially as hereinbefore described and with reference to the drawings.