WO2009108106A1

WO2009108106A1 - Method and device for adjusting sensitivity

Info

Publication number: WO2009108106A1
Application number: PCT/SE2009/000116
Authority: WO
Inventors: Urban Bilstrup; Per-Arne Wiberg; Emil Nilsson
Original assignee: Leptonradio Ab
Priority date: 2008-02-29
Filing date: 2009-03-01
Publication date: 2009-09-03
Also published as: SE532443C2; SE0800494L; EP2260437A1; EP2260437A4

Abstract

A batery powered electronic device and a method of operating the device. The device receives a first signal having a first frequency and a predetermined signal strength and retransmits a signal after a latency period. The device comprises an oscillation circuit operating in a sub-threshold area in a meta-stable, first mode of operation. When the first signal is received, the oscillation circuit is trigged and passes to an active mode in a second mode of operation, wherein the circuit oscillates. The circuit includes means for changing the sensitivity in the first mode of operation. The oscillations in the second mode may be used as a local oscillator in a heterodyne receiver. The device may have an identity, which is used for the purpose of the system, which may be an RFID system.

Description

TITLE: METHOD AND DEVICE FOR ADJUSTING SENSITIVITY

AREA OF INVENTION The present invention relates to a device for wireless operation and a method for operating the device. The device may be an identification member in a system, such as a wireless article surveillance system. The method relates to adjusting a sensitivity of the device.

BACKGROUND OF INVENTION

Wirelessly operated devices have been used for a long time. Such devices may be powered by batteries. However, batteries have limited capacity and lifetime. In order for the batteries to last long, the devices may be designed for as small power consumption as possible. An article surveillance system uses several identification tags attached to the articles included in the system. Readers including transmitters are arranged to detect the presence and identity of such tags within the coverage area of the reader.

An intrusion alarm system uses several sensors arranged to detect unauthorized persons adjacent to the sensor. A central system is alerted if a sensor is trigged. A temperature monitoring system of a building comprises several temperature sensors arranged to detect the temperature adjacent the sensor. A central computer polls the sensors intermittently for receiving temperature values. The sensors may as well detect humidity and other data, such as mains power consumption or light conditions (night or day light). Such systems have in common that a plurality of battery-powered devices are included in the system. The devices may be operated intermittently or continuously for delivering data to a central system. Other systems than those described above may also benefit from the present invention.

The device may be "sleeping" for a long time, such as in article surveillance systems, wherein an identification tag is attached to an article during manufacture and may reside on the article during transport and storage during months and years, until used by the system at the time of selling the article.

Thus, there is a need for a battery-powered device having a very low power drain during a "sleep" mode and which can be activated at an arbitrary time. US 5970398 discloses a system comprising an antenna circuit configured for use in a radio frequency data communications device. An antenna is constructed and arranged to transfer electromagnetic waves, the electromagnetic waves corresponding to_. a signal carried by the antenna and generated from a signal source. A Schottky diode is electrically coupled in serial relation with the antenna, and in operation the signal is applied serially across the antenna and the diode in direct relation with electromagnetic waves transferred by the antenna. A bias current supply is also electrically coupled to the Schottky diode and is configurable to deliver a desired bias current across the current. The diode is responsive to the bias current to realize a desired diode impedance such that a desired impedance matched/mis-matched is provided between impedance of the diode and impedance of the antenna when the signal is applied across the antenna circuit, which selectively tunes the antenna circuit by imparting a desired power transfer therein.

US 2007/0013481 discloses a tag, which has first and second modes of operation, and uses substantially less battery power in the first mode. In the first mode, the tag is responsive to receipt of a first wireless signal from a remote location with a first transmission range for shifting to the second mode. In the second mode, the tag transmits a second wireless signal with a second transmission range. In one configuration, the second transmission range is greater than or equal to the first transmission range. In a different configuration, the tag periodically checks for the first wireless signal during the first mode at points in time spaced by a time interval. The second transmission range is less than the first transmission range by a difference that is greater than or equal to the time interval multiplied by a speed of movement of the tag toward the remote location.

WO 01/67625 discloses a transponder for amplification of a received signal to a signal for retransmission, wherein a quenched oscillator is incorporated as an amplifying element. The oscillator is preferably of the superregenerative type and exhibits negative resistance for the received signal.

WO 2008/026988 discloses a receiver circuit having low power consumption. The receiver circuit is activated at receipt of a RF signal with a specific frequency and with a sufficient amplitude for activating the receiver circuit.

However, during the life time of a identification tag, the receiver of the tag may be activated by spurious RF sources in the neighbourhood of the tag. When the receiver is activated, it draws current until it recognizes that the activation was not be a proper activation signal, whereupon it is shut down. If the tag is unintensionally activated too many times, the battery may run out of power.

DISCLOSURE OF INVENTION

Accordingly, the present invention seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the art singly or in any combination.

According to one aspect of the invention, there is provided a method of operating a battery powered electronic device, comprising: initially operating the electronic device in a meta-stable, first mode of operation; operating the electronic device in an oscillating, second mode of operation, wherein the electronic device oscillates at a predetermined oscillation frequency; transferring the electronic device from said first mode of operation to said second mode of operation by means of a bias circuit, which is triggered by a signal having a predetermined signal frequency and power level and being wirelessly received by an antenna of the electronic device; transferring the bias circuit from a first sensitivity mode, in which the bias circuit has a first sensitivity, to a second sensitivity mode, in which the bias circuit has a second sensitivity, which is larger than said first sensitivity, by means of a wireless signal.

In an embodiment, the method may comprise transferring said bias circuit from said first sensitivity mode to said second sensitivity mode upon receipt of a wireless transfer signal by said antenna. Transferring said bias circuit from said first sensitivity mode to said second sensitivity mode may take place by radiofrequency, microwave, infrared, laser, radiactive radiation or ultrasound means.

In another embodiment, the electronic device may be used in a heterodyne receiver as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier.

In a further embodiment, the method may further comprise terminating the second mode of operation and transferring said device to said meta-stable, first mode of operation by means of a switch circuit.

In another aspect, there is provided a battery powered electronic device, comprising: an amplification member and a filter member connected to form an oscillation circuit; a bias circuit for maintaining the oscillation circuit in an inactive, meta-stabile, first mode of operation, having low power dissipation; an input circuit comprising an antenna for receiving a signal for said amplification member for transferring said device into a second mode of operation, wherein the device oscillates at a predetermined oscillation frequency; wherein said bias circuit comprises a first sensitivity mode, in which the bias circuit has a first sensitivity, and a second sensitivity mode, in which the bias circuit has a second sensitivity, which is larger than said first sensitivity.

In an embodiment, a device may be arrangend for transferring said bias circuit from said first sensitivity mode to said second sensitivity mode upon receipt of a wireless transfer signal. The transfer signal may be a radiofrequency, microwave, infrared, laser, radiactive radiation or ultrasound signal.

In another embodiment, the device may be included in a heterodyne receiver, wherein said electronic device is used as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier. In a further embodiment, the device may comprise a device for terminating the second mode of operation and transferring said device to said meta-stable, first mode of operation by means of a switch circuit, for example after a predetermined time period. In a further aspect, there is provided a method of operating a battery powered electronic device, comprising: initially operating the device in a meta-stable, first mode of operation; operating the electronic device in an oscillating, second mode of operation, wherein the electronic device oscillates at a predetermined oscillation frequency; transferring the device from said first mode of operation to said second mode of operation by means of a signal having a predetermined signal frequency and power level and being wirelessly received by an antenna of the electronic device; using said electronic device in a heterodyne receiver as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier. In a still further aspect, there is provided a battery powered electronic device, comprising: an amplification member and a filter member connected to form an oscillation circuit; a bias circuit for maintaining the oscillation circuit in an inactive, meta-stabile, first mode of operation, having low power dissipation; an input circuit comprising an antenna for receiving a signal for said amplification member for transferring said device into a second mode of operation, wherein the device oscillates at a predetermined oscillation frequency; and a heterodyne receiver wherein said device is used as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier.

BRIEF DESCRIPTION OF DRAWINGS

Further objects, features and advantages of the invention will appear from the following detailed description of embodiments of the invention, reference being made to the accompanying drawings, in which:

Fig 1 is a schematic diagram of an RFID system in which the invention may be used. Fig. 2 is a schematic diagram of a previously known identification tag used in the

RFID system of Fig. 1.

Fig. 3 is a schematic diagram of an embodiment of a device of the invention.

Fig. 4 is a detailed schematic diagram of an embodiment in which the receiver circuit is used in a heterodyne receiver. Fig. 5 is a detailed schematic diagram of a circuit according to a further embodiment.

Fig. 6 is a portion of the diagram of Fig. 5 in which the bias circuit is adjustable. Fig. 7 is an amplification diagram of the non-linear amplifier of Fig. 5. Fig. 8 is a diagram of the band-pass property of a band-pass filter of Fig. 5. Fig. 9 is a schematic diagram of a device including a "wake-up" feature. Fig. 10 is a schematic diagram of a device comprising several receiver circuits.

Fig. 11 is a detailed schematic diagram similar to Fig. 5 of another embodiment. Fig. 12 is a diagram similar to Fig. 11 with a different bias circuit. Fig. 13 is a detailed schematic diagram of a circuit according to a further embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS The embodiments described below disclose the best mode and enables a skilled person to carry out the invention. The different features of the embodiments can be combined in other manners than described below. The invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The invention is only limited by the appended patent claims.

Fig. 1 is a schematic view showing an RFID system 10 comprising a reader (R) 11 and several tags (T) 12, 13, 14, 15, 16. Although five tags are shown, such systems may comprise a great number of tags, some of which are within the range of a specific reader 11. The reader 11 comprises a wireless transceiver (transmitter/receiver), which transmits a radio signal having predetermined frequencies and listens for response signals from the tags.

The tag may be of different configurations depending on the system.

The tag may be a passive device having selective absorption of energy, for example at said predetermined frequency. The reader 11 may sense the absorption and determine a property of the tag. The passive device needs no power supply.

A more versatile, active tag comprises a receiver and a transmitter. The receiver is a low power receiver, which (intermittently) listens to radio signals transmitted by any reader. The tag may be activated at regular intervals. When the receiver of a tag senses a radio signal, for example in a predetermined frequency interval, a power source is connected to the transmitter of the tag for sending a reply. The reply may be an acknowledgment signal and/or may include some information, such as the identity of the tag and/or further data. The internal power source may be supplemented by power obtained from the radio signal transmitted by the reader. Such a tag needs an internal power source, but the power consumption may be very low in stand-by mode. However, the receiver is activated at regular or controlled intervals even if there is no reader within reach, thereby consuming power.

The tag may have an identity and may respond to the reader only if the identity matches a signal transmitted by the reader. Such an identity may be a series of frequencies transmitted by the reader. The reader may be arranged to transmit signals at different frequencies in a frequency band, for example frequencies fO to f79 included in a free band in the 2,4 GHz area that is open to low power appliances. Such frequency band can include 80 bands with a frequency distance of 1 MHz, compare the Bluetooth® standard. The reader may for example transmit the frequency series of fl, f5, f33, f2 and the tag having the identity vector (1, 5, 33, 2) will be activated and send a reply if this specific tag is within the reach area of the reader.

The tag is produced in great quantities and should be as inexpensive as possible. Thus, there is a demand for a design that is as simple as possible. Moreover, the power consumption should be minimized so that small batteries can be used. The battery should last over the total life cycle of the product, which can be, for example, 5 years or more.

A receiver of the tag has according to prior art been constructed as a heterodyne receiver, as shown in Fig. 2. The prior art tag 20 comprises an antenna 21 connected to a broadband amplifier 22 having an output that is connected to a mixer 23. The signal from the antenna is mixed with a signal having a predetermined frequency from an oscillator 24, such as a voltage controlled oscillator (VCO). The difference signal from the mixer 23 is fed to a narrow-band, intermediate frequency (IF) amplifier 25 and the amplified signal is delivered to an output terminal, which is connected to some evaluation circuit, not shown. Thus, the frequency of the VCO 24 determines which antenna signal frequency that is amplified by the IF amplifier 25. However, the power consumption of such a receiver is relatively large. Activating the receiver only during short time periods with long intervals reduces the total power consumption of the tag 20. However, if the time interval is too long, the tag may risk missing a reader signal.

Fig. 3 shows an embodiment of an active tag comprising a non-linear amplifying member. The tag 30 comprises an antenna 31 connected to a band-pass filter 32, which may be an inductor connected in series with a capacitor. An amplifier 33 is connected across the band-pass filter. The amplifier 33 may be a threshold amplifier, in which the signal has to be above a specific threshold value in order to be amplified. When the signal from the antenna 31 is above said threshold value, after filtration by the filter 32, the signal is amplified by the amplifier 33 and fed back to the input of the filter 32 as a still larger signal. The amplifier 33 is connected with positive feedback and the amplifier 33 will enter into oscillation within a short time period, called a latency period, and with a frequency determined by the filter 32.

The oscillations produce a signal that is broadcasted by the antenna 31. The reader 11 may sense this broadcasted signal as a response. The signal produced by the amplifier 33 is additionally emitted at an output 38 via a diode 37 or transistor to a control circuit of the tag

30. At the same time, the output signal controls a switch 34 in the feedback loop of the amplifier 33 via a control line 35. When the oscillation signal is sufficiently large, the switch 34 is opened and connects a resistor 36 into the feedback loop, which causes the oscillations to cease. Thus, the tag 30 according to Fig. 3 produces an oscillation signal when the antenna

31 receives a signal having a specific frequency and sufficient signal strength. This oscillation signal starts after a latency period and continues until interrupted by the switch 34. Thus, the reader 11 may transmit a signal for a short time period and then listen for signals having the same or similar frequency. If such a signal is sensed after the reader signal has ceased, this indicates that a tag 30, which is responsive to substantially that frequency is present within the range of the reader 11.

Fig. 5 is a more detailed schematic diagram of an amplifier 60 with voltage dependent amplification, a varamplifier. The amplifier comprises one FET transistor 61 four inductors 62, 63, 64, 65, two variable capacitors 66, 67, two isolation capacitors 68, 69 and a resistor 70 connected to form an oscillator. A positive feed voltage is connected to a voltage terminal 71 at one end of inductor 63. A bias voltage source 72 is connected to the gate of the transistor 61 via the resistor 70 and the inductor 62. An antenna signal is connected to an antenna terminal 73. A diode 74 or transistor provides an output signal at an output terminal 75 to a control circuit 76. A reset switch 77 is controlled by the control circuit via control line 78.

The circuit is essentially a modified Colpitt oscillator circuit, but other known circuit designs may as well be used, such as the Hartley oscillator, the Armstrong oscillator or the Vackar oscillator.

During normal operation, with no input signal at the antenna terminal 73, resistor 70 and the voltage source 72 maintains the gate of the FET transistor 61 below a knee voltage of the transistor and practically no current passes through transistor 61. The circuit is in an idle position or meta-stable position draining practically no current from the power supply. The transistor 61 is in a position 79 on an amplification diagram shown in Fig. 7, in which the amplification is below "one". This is called the first mode of operation, which takes place in a sub-threshold area of the operational diagram of the transistor. In this area, the power or current consumption of the transistor is very low and the amplification of the transistor circuit is also low or below "one". However, if a signal appears at the antenna terminal 73 having an amplitude or power sufficient to pass the transistor over the knee voltage, indicated by line 79, and into the area in which the amplification is above "one", the transistor 61 starts to draw current. Then, the circuit starts to oscillate at the frequency determined by the inductor 65 and the capacitors 66 and 67. The oscillations will cause the circuit to enter the saturation area indicated by line 80 of Fig. 7. This is called the second mode of operation, in which oscillations take place at a specific resonance frequency.

The circuit will resonate for antenna signals selected by the circuit comprising the inductor 65 and the capacitors 66, 67 tuned to a resonance frequency (fl). Such resonating signals will trigger the transistor 61 and start oscillations. An output signal from the transistor 61 is fed to a control circuit 76, which controls a switch 77 via line 78 for resetting the circuit and preventing further oscillations.

Thus, the circuit shown in Fig. 5 in the first mode operates in the sub-threshold area of the transistors 60, 61, wherein practically no power consumption takes place. Only when an antenna signal is received as described above, the circuit starts to consume power in the second mode of operation. Thus, the circuit is always actively listening for radio signals, substantially without draining the power supply.

The circuit of Fig. 5 may be said to have a meta-stable first operation mode, because the amplification in the positive feedback loop is below "one". Thus, the circuit may stay in the meta-stable first operation mode for an indefinite time duration. Metastability in electronics is defined as the ability of a non-equilibrium electronic state to persist for a long and theoretically unlimited period of time.

When a signal above a certain amplitude or power is received, the circuit becomes instable. The amplification increases to above "one", and the circuit starts to oscillate in the second mode. The circuit is pushed into oscillations with rapidly increasing amplitude. However, the "push" may take some time, until the oscillations start, called a latency period. Thus, the reply time of the circuit may be in the area of part of a microsecond or some microseconds, until the circuit has started to oscillate. In an embodiment, this latency period is used for determining the distance of the circuit to a transceiver. The circuit starts to oscillate when the input signal is sufficiently large to push the circuit into the self-oscillating mode. For this purpose, the circuit needs to receive a sufficient signal amplitude and a sufficient amount of power. Thus, if the circuit is close to the transmitter, the signal amplitude is high and the circuit rapidly receives a sufficient amount of power to start self-oscillations. The transmitter may measure the latency and in this way determine the distance.

Another method of determining the distance would be for the transmitter to transmit a signal with short duration. The circuits being closest to the transmitter will receive a sufficient amount of power to respond. Then, the transmitter increases the duration of the transmit signal, which means that circuits at a longer distance will successively receive a sufficient amount of power to respond. The duration may be increased in small steps.

A furter method of determining the distance would be for the transmitter to increase the transmit power, which means that circuits at still larger distances will respond. The power may be increased in small steps. In this way the distance of the circuit to the transceiver or the reader be determined.

It is noted that an antenna signal with a frequency outside the predetermined centre frequency will still trigg the circuit if it is close enough and has sufficiently high signal strength.

Fig. 8 shows a diagram of the Q-value of the circuit of Fig. 5. At the centre frequency, the Q-value is for example "two" when the circuit is in the area with maximum amplification of the circuit, i.e. on the operation area between lines 79 and 80 in Fig. 7. Thus, with an excitation frequency of fc, the circuit will start to oscillate relatively rapidly. However, if the excitation frequency is in the frequency band between fl and f2, the circuit will still start to oscillate (possibly with longer latency period and with a higher required input power), because the amplification will be above "one". However, as soon as the oscillations are initiated, the oscillation frequency will shift towards the centre frequency fc. This feature can be used and is called dissonance below. Suppose that the reader transmits a signal having a frequency fl , which is on the 3 dB line of the response curve 81, shown in Fig. 8. Then, any tag having a frequency band including the reader frequency will start to oscillate and broadcast a signal. The signal broadcasted will be the centre frequency of the tag. Thus, when the reader may transmit a signal having a frequency fl, as indicated in Fig. 8, a tag having the frequency response 81 of Fig. 8 will start to oscillate around a frequency f81 and a tag having a frequency response 82 of Fig. 8 will start to oscillate around a frequency f82.

The centre frequency fc of a tag may be set from the start or may be adjustable, for example as indicated in Fig. 5. A variable capacitor or a variable inductor may be used for tuning the circuit. A variable capacitor or varactor is well known and comprises a diode in which the width of the barrier layer between the p-doped and n-doped areas is controlled by a gate voltage.

A variable inductor or capacitor may also be constructed by using one or several switches for including one or several inductors or capacitors in the LC circuit. The switches may be controlled during or before operation. For example, the LC circuit may be tuned to the reader frequency fl from the start and when a signal indicating oscillation is present on the output, another parallel capacitor may be switched into the circuit shifting the oscillation frequency to a lower value (f82), or another series capacitor may be switched into the circuit shifting the oscillation to a higher value (f81).

Alternatively, the control circuit may adjust the LC circuit before operation to an expected reader transmission frequency. After the receipt of the reader signal and oscillation, the control circuit resets the oscillator. Then, the control circuit may shift the LC circuit to a new expected frequency from the reader.

The filter has been shown as a LC circuit comprising an inductor and two capacitors. However, any filter circuit can be used having a resonance frequency, i.e. a band-pass filter. Such filters can include MEMS circuits, a crystal filter, ceramic filters, LC-filters with more than three components, active filters.

In another embodiment, the receiver circuit shown may be used only for "wake-up" of the tag. Such an embodiment is shown in Fig. 9. The device comprises an antenna 131. A band-pass filter 132 and a non-linear amplifier 133 are connected to form an oscillator, which starts to oscillate when the signal from the antenna is sufficiently large for starting the oscillations. A comparator senses the oscillations and controls a switch 135, which may switch on the power supply to control circuit 136. The control circuit emits a signal to the oscillator to deactivate the oscillations by opening a switch 137 thereby including a resistor 138 in the oscillating circuit, whereby the oscillations cease. At the same time, the control circuit closes another switch 139 to the power supply of the control circuit. The control circuit 136 comprises another transceiver 140 according to any known principle, such as a heterodyne receiver, which takes over the continued operation of the tag. The transceiver is connected to the antenna 131. When the operation is finalized according to any algorithm, the control circuit deactivates itself by opening the switch 139 (switch 135 is open), and the tag returns to the first mode of operation awaiting a new wake-up signal at the predetermined frequency.

In the above-mentioned embodiment, if the transceiver comprises a heterodyne receiver, such heterodyne receiver comprises a VCO. Such a VCO may be the oscillator circuit of the receiver circuit. Such an embodiment is shown in Fig. 4. An antenna 181 receives the signal from the transceiver of for example a reader. The antenna 181 is connected to the input circuit of a mixer 182, which is connected to the input terminal 183 of a receiver device 184, for example of the type shown in Figs. 3 or 5. The antenna signal will pass through the mixer 182 to the input terminal 183 without any substantial attenuation. If the signal is a sufficiently strong, the reveiver device will be triggered and start self-oscillating. Such self-oscillations are assessed by a control circuit 185, which closes a switch 186 passing battery power to the mixer 182 and an IF amplifier 187. The oscillations from the receiver circuit 184 are used by the mixer 182 to form a heterodyne receiver according to conventional technique. The heterodyne receiver operates at an operation frequency which is the oscillation frequency plus or minus the IF frequency. Thus, the reader may transmit further information at said operation frequency, which information is used by the receiver device in a specific manner, for example for identification purpose.

When the receiver circuit is used as a local oscillator in the heterodyne receiver, the receiver circuit will never be quenched. Once started, it will continue to oscillate until the battery power has been consumed. The heterodyne receiver or the control circuit 185 may include means for quenching the oscillator, for example if the activation was unintentional.

The feature of using the receiver circuit as a local oscillator in a heterodyne receiver may be used in combination with any of the other embodiments described in this specification.

In addition, the receiver circuit may be used as a transmitter of the transceiver system included in the control circuit, for example as described in connection with Fig. 9.

The reader may transmit a single frequency signal or a broadband wake-up signal comprising several frequencies or a band of frequencies, such as a "Dirac" pulse. The tags that receive a signal at the predetermined frequency of sufficient signal strength will wake up. In this way, the tag will be in a low-power mode during a long time, until a wake-up signal is received. If a spurious wake-up signal is received, the tag will only wake up for a very short time, until the control circuit has verified that the wake-up signal was not a valid wake-up signal issued by a reader. The sensitivity of the receiver circuit can be adjusted so that a proper compromise is obtained between safe operation and too may unintentional wake-up signals. This is obtained by adjusting the bias of the transistor to be closer or longer from the trigger point in which self-oscillations start. The circuit of Fig. 5 comprises a bias circuit comprising a voltage source 72 which is connected to the gate of the transistor 61. By adjusting the voltage source 72 and the resistor 70, a proper sensitivity can be obtained, i.e the operating point may be positioned to the left of line 79 in Fig. 7 and at a suitable distance there from.

In an embodiment, the receiver circuit comprises means for adjusting the sensitivity of the receiver circuit. The voltage source 72 which is connected to the gate of the transistor 61 may be adjustable.

A bias circuit for obtaining such a variable bias voltage is shown in Fig. 6, which shows only the encircled portion marked with VI in the circuit of Fig. 5. In the series circuit of resistor 70 and voltage source 72, a normally open switch 188 is arranged. A resistor 189 is connected in parallel with the volage source and the switch 188, as shown in Fig. 6. If the switch 188 is open, the gate of the transistor 61 is connected to ground via resistors 70 and 189. Thus, Vin is substantially zero and the receiver circuit is operated close to the origo of the diagram of Fig. 7. The transistor 61 is substantially blocked and the receiver bias circuit is in a first low-sensitivity mode. When the switch 188 is closed, the operating point is moved to the right in Fig. 7 closer to the line 79, whereby the receiver bias circuit is in a second high- sensitivity mode.

In the first low-sensitive mode of the bias circuit, a large signal is required at the input terminal 73 for triggering the device. When such a large signal is received, the circuit is triggered and the control circuit 76 may be arranged to close the switch 188, whereby the bias circuit is transferred to the second high-sensitivity mode.

Such an activation of the receiver device may take place when the goods carrying the receiver circuit or tag is moved from an intermediate store to a shop, in which the goods is exposed for selling. The intermediate store, or a transport device, can include a strong transmitter, which is used for activating the bias circuit to a more sensitive mode. The activation can take place in another manner, such as by ultrasound, infrared light, light, laser, radioactive radiation.

The switch 188 may be a spring loaded switch, which is kept open by a fusable material. When the fuseable material is influenced upon, for example by ultrasound or IR light, it may melt and the switch is moved to the closed position. Radiactive radiation, such as gamma, beta or alfa radiation, may be used for influencing upon the switch 188 to close the switch.

The receiver bias circuit may have several sensitivity modes, for example three modes with low, intermediate and high sensitivities. The sensitivity may be continuously adjustable. Such an adjustable device may be arranged based on the cirucit of Fig. 12, by replacing the resistors 174 and 177 and the switch 178 by a single adjustable resistor. The adjustable resistor may be embodied as a MOSFET transistor, the resistance of which is dependent on the source voltage. The source voltage may be adjusted continuously or in steps.

For example, the receiver circuit may have a very low sensitivity from the start, and be exposed to activation with high energy, for example after leaving an intermediate store as described above. At the same time, a clock circuit is started which increases the sensitivity further after a predetermined time, for example three days, corresponding to the expected time from the intermediate store to the shop.

If the goods leaving the intermediate store is to pass a second intermediate store of less size, the clock may increase the sensitivity from the first low-sensitive mode, to a second intermediate mode, after for example thee days, and then to a third high-sensitivity mode after fourteen days, when the goods is expected to be in the shop. The feature with adjustable sensitivity may be used with any of the other embodiments of this specification.

Fig. 10 shows another embodiment, wherein several circuits 142, 143 are connected to the same antenna 141. Each circuit comprises a deactivation switch 144 and 145, respectively. Two circuits 142, 143 are shown in Fig. 10, but any number of circuits can be used. The first circuit 142 is tuned to the frequency f81 corresponding to a logical "one" and the second regenerative circuit 143 is tuned to the frequency f82 corresponding to a logical "zero". A control circuit 146 controls deactivation switches 144 and 145 of each circuit in dependence on a signal received by a sensor 147.

The operation for polling the tag is as follows. The reader has identified the tag in any manner. The tag is polled at a frequency determined in advance, such as frequency fl indicated in Fig. 8. The control circuit operates switches 144 and 145 in dependence of the most significant bit of the sensor value of sensor 147, which may be a temperature sensor having an eight bit value to be transmitted to the reader. If the most significant bit is "one", switch 144 is closed and switch 145 is opened and if the most significant bit is "zero", switch 145 is closed and switch 144 is opened. Then, the reader transmits a polling signal at frequency fl. If the most significant bit is "one", oscillation circuit 142 starts to oscillate at frequency f81. If the most significant bit is "zero", oscillation circuit 143 starts to oscillate at frequency f82. The reader detects the response. Then, the control circuit disables the oscillations, and controls switches 144 and 145 according to the next bit in the signal and the reader sends a second polling signal at frequency f 1 , and so on. After eight polling signals, eight bits of the temperature signal has been transmitted to the reader.

The same principle can be used with a single circuit, wherein the control circuit controls a varactor or similar device in dependence on the sensor signal. If it is determined that certain frequency areas are occupied by other transmitters, the electronic device according to any of the above embodiments may include means for avoiding such frequency domains, for example under control of a reader.

The device according to any of the embodiments described above can be manufactured in CMOS technology. However, other technologies may also be used, such as discrete components, for example MOSFET transistors, having a very low power consumption in the low-power mode.

Fig. 11 is a circuit diagram of another embodiment of the oscillation circuit. The circuit is similar to the circuit shown in Fig. 5, but is arranged in a balanced manner. The circuit requires only one inductor, which makes it more suitable for being embodied on a silicon chip with all components on the chip.

The oscillation circuit comprises a first NFET transistor 151 and a second PFET transistor 152. Between the sources of the transistors, an inductor 153 is connected. Parallel with the inductor, three capacitors 154, 155, 156 are connected, mutually in series. The gates of the transistors are fed with a gate voltage via two voltage sources 157, 158 and two resistors 159, 160 so that the transistors normally are in an area below the normal operation area of the transistor, in a sub-threshold area. The gates of the transistors are connected to an antenna terminal 161 via a balun 162 which converts the unbalanced antenna signal to a balanced signal and via each two isolation capacitors 163, 164, 165, 166. A diode 167 provides a control circuit 170 with an output signal via output terminal 168 and output capacitor 169. A control line 171 controls two reset switches 172 and 173. The operation is similar to the circuit in Fig. 5.

The circuit operates as a combination of a bistable and astable multivibrator. In the first mode of operation, the circuit is meta-stable, meaning that a trigger signal having sufficient power in the relevant frequency band will pass the circuit into the second mode of operation. In the second mode of operation, the circuit operates like an astable multivibrator, oscillating at a specific frequency. The oscillations will continue until some outer event stops the oscillations and return the circuit to the first meta-stable mode.

The balanced receiver circuit according to Fig. 11 can include the option to amend the sensitivity as disclosed in Fig. 12. Other manners of adjusting the sensitivity or adjusting the operating point of the transistors 151, 152 may occur to the skilled person.

Fig. 12 discloses the circuit of Fig. 11 with a different bias circuit, comprising three resistors 174, 175, 176 connected in series from the positive rail to the gate of the first transistor, from the gate of the first transistor to the gate of the second transistor and from the gate of the second transistor to the negative rail. By means of these resistors 174, 175, 176, the circuit is biased to a first sensitivity mode with low sensitivity, because the resistor 175 is smaller than the resistors 174, 176. By means of two switches 178, 180, another resistor 177, 179 may be connected in parallel with resistors 174, 176 so that the parallel connected resistors changes the bias point of the circuit to a second, more sensitive mode. A single switch 190 connects the gates of the two transistors to stop oscillations and to form another, still less sensitive mode, compared to the first sensitivity mode.

Fig. 13 shows a conventional oscillation circuit, the oscillation of which is controlled by a current source idc=ibias. As in the circuit of Fig. 11, the antenna signal is fed to the gates of the transistors via a balun and two capacitors.

The oscillator circuit may as well be embodied using a bipolar transistor. The sensitivity may be adjusted by adjusting the base current, for example by connecting several resistors in series or parallel with one or several switches. Herein above, several embodiments have been described with reference to a RFID- system comprising a reader and several tags. However, the inventions is applicable in many other areas, as those mentioned in the introduction of the specification. In such cases, the reader is replaced by a transmitter and a receiver which are operated in a specific manner. Thus, a reader is synonymous with any device comprising at least one transmitter and at least one receiver. Both may be operating by radiowaves, such as in the microwave area around 1 to 10 Ghz. However, other combinations, such as a radiowave transmitter and another type of receiver may be used.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

It should also be emphasized that the singular forms "a", "an" and "the" as used herein are intended to comprise the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms "includes", "comprises", "including" and/or "comprising" when used in this specification, is taken to specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The present invention has been described above with reference to specific embodiments. However, other embodiments than the above described are equally possible within the scope of the invention. Different method steps than those described above, performing the method by hardware or software or a combination of hardware and software, may be provided within the scope of the invention. The different features and steps of the invention may be combined in other combinations than those described. The different embodiments described above do not limit the scope of the invention, but the scope of the invention is only limited by the appended patent claims.

Claims

PATENT CLAIMS

1. A method of operating a battery powered electronic device, comprising: initially operating the electronic device in a meta-stable, first mode of operation; operating the electronic device in an oscillating, second mode of operation, wherein the electronic device oscillates at a predetermined oscillation frequency; transferring the electronic device from said first mode of operation to said second mode of operation by means of a bias circuit, which is triggered by a signal having a predetermined signal frequency and power level and being wirelessly received by an antenna of the electronic device; transferring the bias circuit from a first sensitivity mode, in which the bias circuit has a first sensitivity, to a second sensitivity mode, in which the bias circuit has a second sensitivity, which is larger than said first sensitivity, by means of a wireless signal.

2. The method according to claim 1, further comprising: transferring said bias circuit from said first sensitivity mode to said second sensitivity mode upon receipt of a wireless transfer signal by said antenna.

3. The method according to claim 2, further comprising: transferring said bias circuit from said first sensitivity mode to said second sensitivity mode by radiofrequency, microwave, infrared, laser, radiactive radiation or ultrasound means.

4. The method according to claim 1, 2 or 3, further comprising: using said electronic device in a heterodyne receiver as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier.

5. The method according to any one of the previous claims, further comprising: terminating the second mode of operation and transferring said device to said meta- stable, first mode of operation by means of a switch circuit.

6. A battery powered electronic device, comprising: an amplification member (61) and a filter member (65, 66, 67) connected to form an oscillation circuit; a bias circuit (72, 70, 62) for maintaining the oscillation circuit in an inactive, meta- stabile, first mode of operation, having low power dissipation; an input circuit (73, 68, 69) comprising an antenna for receiving a signal for said amplification member for transferring said device into a second mode of operation, wherein the device oscillates at a predetermined oscillation frequency; wherein said bias circuit comprises a first sensitivity mode, in which the bias circuit has a first sensitivity, and a second sensitivity mode, in which the bias circuit has a second sensitivity, which is larger than said first sensitivity.

7. The device according to claim 6, further comprising: a device for transferring said bias circuit from said first sensitivity mode to said second sensitivity mode upon receipt of a wireless transfer signal.

8. The device according to claim 7, wherein said transfer signal is a radiofrequency, microwave, infrared, laser, radiactive radiation or ultrasound signal.

9. The device according to claim 6, 7 or 8, further comprising: a heterodyne receiver, wherein said electronic device is used as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier.

10. The device according to claim 6, 7, 8 or 9, further comprising: a device for terminating the second mode of operation and transferring said device to said meta-stable, first mode of operation by means of a switch circuit, for example after a predetermined time period.

11. A method of operating a battery powered electronic device, comprising: initially operating the device in a meta-stable, first mode of operation; operating the electronic device in an oscillating, second mode of operation, wherein the electronic device oscillates at a predetermined oscillation frequency; transferring the device from said first mode of operation to said second mode of operation by means of a signal having a predetermined signal frequency and power level and being wirelessly received by an antenna of the electronic device; using said electronic device in a heterodyne receiver as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier.

12. A battery powered electronic device, comprising: an amplification member (61) and a filter member (65, 66, 67) connected to form an oscillation circuit; a bias circuit (72, 70, 62) for maintaining the oscillation circuit in an inactive, meta- stabile, first mode of operation, having low power dissipation; an input circuit (73, 68, 69) comprising an antenna for receiving a signal for said amplification member for transferring said device into a second mode of operation, wherein the device oscillates at a predetermined oscillation frequency; a heterodyne receiver wherein said device is used as a local oscillator for mixing with an antenna signal to produce an intermediate frequency to be amplified by an intermediate frequency amplifier.