GB2326529A

GB2326529A - Tag interrogation field system

Info

Publication number: GB2326529A
Application number: GB9711384A
Authority: GB
Inventors: Bernard John Regan
Original assignee: Identec Ltd
Current assignee: Identec Ltd
Priority date: 1997-06-04
Filing date: 1997-06-04
Publication date: 1998-12-23
Anticipated expiration: 2017-06-04
Also published as: GB9711384D0; GB2326529B

Description

Radio Frequency Antenna This invention relates to the field of radio frequency tagging and more particularly to an antenna for use in tagging systems.

Radio frequency tagging systems typically consist of a reader (interrogator unit) and a tag (transponder). The reader transmits data and/or power to the tag, causing the tag to respond by transmitting data back to the reader. In some radio frequency tagging systems, the tag responds by taking energy out of the electromagnetic field created by the reader, rather than transmitting a signal. This is considered to be an equivalent process, and everything described herein applies equally to this. The transmission process relies on the coupling between the antennas of the reader and tag.

Particularly in the inductive band, where the distances involved are well below one wavelength, these antennas are made up of coils of wire. These coils may be air cored, or they may be wound on a ferrite or similar material. A limitation of such coils is that there is an optimum orientation, and away from this optimum orientation the coupling falls off according to the cosine of the angle between the optimum orientation and the actual orientation.

It is well known that this orientation dependence can be reduced, but not eliminated, by connected the reader to two coils essentially arranged orthogonally. The currents in these two coils have a phase shift between them, preferably of 90 degrees. The tag will respond if it picks up a signal from either coil. This would not be true if there were no phase shift, as the fields generated by the two coils might cancel each other out. Indeed two coils with no phase shift can be seen to be equivalent to having just one coil at an intermediate orientation. With the phase shift, no cancellation can occur. The return channel from tag to reader normally uses the same two coils, and the circuit must be arranged to ensure that there are also different phase shifts applied to the return signals, and the ideal is again for the difference between the phase shifts to be 90 degrees.

One known method of achieving these phase shifts is to have the two coils driven by separate amplifiers. The requisite phase shift can be applied before amplification. The disadvantage of this is that there is the additional cost of more amplifiers, and more connections between the reader electronics and its antenna which may be some distance away.

It is desirable to have only two wires to the antenna.

For instance, Hirano et al (M. Hirano, M. Takeuchi, T.

Tomoda and K. Nakano; IEEE Transactions on Industrial Electronics, Vol. 35, No. 2, May 1988) describe configurations which require only 2 wire from the reader to its antenna, where that antenna consists of two coils, and there is a phase shift between the currents in the two coils. The circuits which they describe are not particularly efficient in terms of the current through the coil related to the current taken from the reader's power supply.

In a "series tuned circuit", there is an inductance in series with a capacitance. The circuit has some resistive components, although sometimes a resistor is also placed in series. The Q factor of the tuned circuit is defined as the impedance of either the inductance or the capacitor, divided by the series resistance. The same current flows through each component of the circuit. The voltage across the inductance is approximately Q times the voltage applied. A measure of the efficiency of this circuit is the VA product of the inductance divided by the VA product of the whole circuit, where the VA product is the voltage multiplied by the current. The VA product of the inductance is approximately Q times the VA product of the circuit as a whole. In a "parallel tuned circuit", the inductance is placed in parallel with the capacitor. The voltage across the tuned circuit is the same as the voltage across the inductance, but due to circulating currents the current through the inductance is Q times the current taken by the whole circuit.

When such tuned circuits are used as the antenna of a tag reader, it is known that the parallel circuit can have some advantages. These arise when data is to be transmitted to the tag from the reader. Having a highly tuned circuit affects the modulation waveform. The rise-time of the modulated waveform is Q/r cycles. If Q is too high, the effect on the modulation becomes unacceptable. This formula applies for the series tuned circuit, but it does not apply to the parallel tuned circuit in the same way. This is because the parallel tuned circuit takes more current at frequencies other than the resonant frequency.

Tag readers that use the parallel tuned circuit will have a series resistor, whose purpose is to limit the current when the circuit is not properly tuned. It is this resistor which determines the rise-time for any modulation waveform, not the inherent Q of the parallel tuned circuit. Where the tuned circuit is designed to have no more than a predetermined risetime, the parallel tuned circuit can be several times more efficient than a series tuned circuit.

The object of this invention is to provide an antenna which consists of two coils, whose currents have a phase shift between them, but with the efficiency of the parallel tuned circuit. It is also necessary to ensure that the response signal from the tag has a similar phase difference applied.

In accordance with the present invention, there is provided an antenna which comprises two coils arranged such that their magnetic fields are inclined to each other, said coils being included in respective parallel tuned circuits which have different resonant frequencies, one above and the other below the transmitted frequency, such that there is a phase difference between the currents in the two tuned circuits.

Preferably the two coils are arranged such that their magnetic fields are inclined at greater than 60 degrees to each other: most preferably, the coils are arranged such that their magnetic fields are substantially orthogonal to each other.

One arrangement, which provides a flat geometry suited to tags, comprises a first coil wound in the form of a flat frame, and a ferrite-wound coil positioned in the centre of this. More than two coils can be provided, but they do not add to the number of possible tag orientations that can be detected. Two coils provide fields in two axes, and the third spatial axis can be covered using time-multiplexing, where coils in different axes are operational at different times.

For maximum efficiency, the phase difference between the currents in the two coils should be less than 90 degrees.

Preferably the antenna includes means for modifying the characteristics of the tuned circuits in the receive mode, to maintain the phase difference between the received signals.

The modification thus effected may change the resonant frequencies of the tuned circuits to take account of the fact that the received frequency is different from the transmitted frequency. The characteristic - modifying means may comprise a switching element for bringing additional components into effect: preferably this switching element is a MOSFET.

Preferably the switching element is controlled by a signal derived from a signal supplied to the antenna.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawing, the single figure of which is a circuit diagram of an antenna for a radio frequency tagging system.

The circuit shown in the drawing comprises two parallel tuned circuits, 10 and 11, which are placed in series. The inductances, 13 and 15, of these two tuned circuits are the antenna coils, and are arranged so that their magnetic fields are substantially orthogonal to each other. It is preferred that the coupling factor between these two coils is low, which would normally be the case if their fields are orthogonal. The two tuned circuits have tuning capacitors, 12 and 14, which determine their resonant frequencies. Series resistors, 16 and 17, complete the circuit. It is not necessary to have two separate resistors, but it is often found to be advantageous to retain a level of symmetry.

The antenna circuit is driven at points 21 and 22.

Only one of these points need be driven, with the other held at a fixed d.c. potential, but it is also often convenient to drive these two points in antiphase to give better matching of impedances. This is the version that will be described, but everything that is said applies equally to other drive configurations.

The two tuned circuits are not tuned to the transmitter frequency (f), but are slightly offset. One tuned circuit is resonant at slightly below the transmitter frequency, while the other is resonant at slightly above the transmitter frequency.

This takes advantage of the fact that when a tuned circuit is driven at a frequency other than its resonant frequency its impedance is not purely resistive. When the resonant frequency is f +(f/2Q) the real part of the impedance equals the imaginary part. When tuned circuit 10 is resonant at f+f/2Q, and tuned circuit 11 is resonant at f-f/2Q, the currents between them will have a 90 degrees phase difference as desired.

For maximum efficiency, the phase difference should be less than 90 degrees. At a phase difference of 90 degrees, the voltage across each tuned circuit is rather more than half the total applied voltage, due to the phase shifts. As the phase difference is reduced the voltage across each inductance drops, but the current passing through the whole circuit drops more rapidly. A phase difference of below 90 degrees is acceptable.

A tag in the worst case orientation will detect a magnetic field which depends on the sine of the phase difference. One possible criterion for the optimum phase difference is when the efficiency times the sine of phase difference is maximised.

This can be shown to occur when the phase difference is just over 70 degrees. In am embodiment where the drive voltages at 21 and 22 are equal and opposite, so that they have a phase shift of 180 degrees, the point between the two tuned circuits 23 has a voltage which is about 70% of that at either 21 or 22, and has a 90 degree phase shift from them. This is a useful way of ensuring that the circuit is properly set up.

When the coupling between the two coils 13 and 15 is not zero, so that they have a small mutual inductance, the resonant frequencies of the two tuned circuits are affected.

The transmission from the reader is still optimised by adjusting the actual resonant frequencies, and ensuring that the point 23 between the two tuned circuits has the waveform described above.

When the tag is transmitting, and the reader is in receive mode, there will be a voltage generated between points 21 and 22. For the best performance the phase of this voltage should change, depending on whether the tag is coupled to coil 13 or coil 15. If there is a 90 degree phase shift between the voltage generated when the tag couples to coil 13, and the voltage generated when the tap couples to coil 15, there will always be a voltage of similar magnitude if the tag is in an orientation that is coupled to both coils. Cancellation of the signals never occurs.

Theoretically the basic antenna consisting of the two tuned circuits also gives a similar phase difference for the received signals. In practice the phase difference is often less, for instance if there is some coupling between the coils 13 and 15. The additional circuits, 30 and 40, make it possible to modify the characteristics of the two tuned circuits in receive mode. In transmit mode the switches 32 and 42 are open circuit, and circuits 30 and 40 have no effect.

In receive mode, the switches 32 and 42 are closed. Coil 31 is wound on the same former as coil 13, so they have a relatively high mutual inductance. When switch 32 is closed, the resistor 33 and the capacitor 34 modify the characteristics of tuned circuit 10. Resistor 33 reduces the Q of the tuned circuit 10, and capacitor 34 lowers the resonant frequency of tuned circuit 10. The values of resistor 33 and capacitor 34 are chosen to ensure that the real and imaginary parts of the impedance of tuned circuit 10 are approximately equal. In the same way coils 15 and 41 have a mutual inductance, and when switch 42 is closed the resistor 43 and inductance 44 reduce the Q of tuned circuit 11 and increase is resonant frequency.

Ideally inductance 44 should not couple in to any of the other coils, for instance by winding it on a toroid of ferrite.

It will be seen that the characteristics of tuned circuits 10 and 11 can be adjusted over quite a large range.

The desired phase difference can still be achieved even if there is significant coupling between coils 13 and 15. If the frequency of the received signal is not the same as the frequency of the transmitted signal, the tuned circuits 10 and 11 can be retuned accordingly. For instance if the received frequency is below the transmitted frequency, inductance 44 would be replaced by a capacitor.

The additional circuits 30 and 40 are not always necessary, but it will be seen that there are many cases where they will improve performance.

It will be appreciated that by reducing the Q of the tuned circuits, the amplitude of the voltage generated by the magnetic field from the tag will also be reduced. This is not really a great concern, as the signal to noise ratio is not compromised. The dominant noise source is electromagnetic radiation picked up by the coils, and reducing the amplitude of both signal and noise by the same amount does not affect the signal to noise ratio.

The switches 32 and 42 are preferably constructed using MOSFET transistors, although other types of switch could be used. If MOSFET transistors are used it is preferred to place a large capacitor in series with the MOSFET transistor. For instance an electrolytic capacitor can be used. Without this large capacitor, the voltage across the transistor will go outside the operating range of the transistor.

One of the advantages of the invention is that the number of wires between the reader and its antenna is minimised. It is preferred that if the additional circuits 30 and 40 are deemed to be desirable or necessary, the switches 32 and 42 should be controlled by the voltage that appears between wires 21 and 22. The techniques for doing this are well established, and will vary according to the nature of the transmitted signals. For instance where the transmitted signal is turned off in receive mode, the switches will be arranged to be open circuit whenever the transmitted signal is detected.

It is an advantage of the MOSFET transistors used in the preferred embodiment that their power requirements are small enough not to affect the overall efficiency.

Claims

1) An antenna which comprises two coils arranged such that, in use, their magnetic fields are inclined to each other, said coils being included in respective parallel tuned circuits which have different resonant frequencies, one above and the other below the transmitted frequency, such that there is a phase difference between the currents in the two tuned circuits.

2) An antenna as claimed in claim 1, in which the two coils are arranged such that, in use, their magnetic fields are inclined at greater than 600 to each other.

3) An antenna as claimed in claim 2, in which the two coils are arranged such that, in use, their magnetic fields are substantially orthogonal to each other.

4) An antenna as claimed in claim 2 or 3, in which one coil is wound in the form of a flat frame and the second coil comprises a ferrite-wound coil positioned in the centre of the first coil.

5) An antenna as claimed in any preceding claim, arranged such that the phase difference between the currents in the two coils is less than 90".

6) An antenna as claimed in any preceding claim, further comprising means for modifying the characteristics of the tuned circuits in a receive mode of the antenna, to maintain a phase difference between the received signal.

7) An antenna as claimed in claim 6, in which said characteristic-modifying means comprises switching means for completing a circuit through additional components.

8) An antenna as claimed in claim 7, in which said switching means comprises a MOSFET.

9) An antenna as claimed in claim 7 or 8, in which said switching means is controlled by a signal derived from a signal supplied to the antenna circuit which includes said tuned circuits.