FIELD [0001] The present invention relates to the field of sensors, such as for example wireless sensors.

BACKGROUND

[0002] Sensors may be used to measure physical quantities, such as, for example, temperature, magnetic flux density, humidity or acceleration. Sensors may be embedded in larger products, such as for example vehicles, such as for example cars or aircraft.

[0003] An aircraft may measure its velocity with regard to the surrounding air with a pitot tube sensor, for example, whereas a car may measure its velocity with regard to a road it is traversing by measuring a rotational speed of its tyres. [0004] Sensors may be active sensors, wherein active sensors are powered by an electrical power source, such as a stable mains power or a battery, for example. Alternatively to active sensors, a sensor may be passive in the sense that it does not have a supply of power in its own. Passive sensors may be powered by solar power, in case the sensors are operationally coupled with a photovoltaic cell, for example. In some aspects, a passive sensor may be viewed as semi-active, or energy harvesting, in case it is enabled to obtain energy from its surroundings.

[0005] One category of energy harvesting sensors is sensors that are powered by electromagnetic induction from a transmitter device. Such sensors are passive and unpowered when not being read, but when provided with electromagnetic waves from a transmitter device, these sensors are capable of harvesting energy from the provided electromagnetic waves, measuring a physical quantity and providing a result of the measurement to the transmitter device, or indeed another device. [0006] Active, passive or energy harvesting sensors may be embedded in machinery or buildings to enable better use of such assets. For example, a temperature sensor embedded in a wall may be usable in triggering a fire alarm or warning as a response to a temperature reading that exceeds a threshold.

SUMMARY OF THE INVENTION

[0007] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0008] According to a first aspect of the present invention, there is provided an apparatus comprising a sensor, a resonance of which is affected by a physical or environmental parameter, an antenna, and a mixing element coupled to the antenna and to the sensor, the mixing element being arranged to receive from the antenna at least two carrier tones and to provide to the sensor at least one difference frequency of the at least two carrier tones, to thereby excite the resonance of the sensor.

[0009] Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:

• the sensor comprises a passive sensor configured to measure the physical or environmental parameter

• the passive sensor comprises an inductively coupled electrical resonance circuit sensor

• the sensor comprises at least one of a temperature sensor, a humidity sensor, a strain sensor, a moisture sensor, a magnetic flux density sensor and an acceleration sensor

• the mixing element comprises at least one of a diode and a junction of dissimilar metals

• the diode comprises a varactor diode • the mixing element is configured to enable the at least one difference frequency to mix with at least one of the at least two carrier tones

• the at least two carrier tones comprise tones in a set of frequencies commencing at 471.25 megahertz and extending therefrom toward higher frequencies at 8 megahertz increments

• the set of frequencies terminates at 855.25 megahertz

• the apparatus further comprises a configurable front-end filter and the apparatus is configured to configure the front-end filter based on instructions received in the apparatus

• the apparatus is further configured to cause the difference frequency to mix with at least one of the at least two carrier tones to produce an intermodulation frequency

• the at least two carrier tones comprise at least two digital television carrier tones.

[0010] According to a second aspect of the present invention, there is provided a method comprising receiving at least two carrier tones, and providing to a sensor a resonance of which is affected by a physical or environmental parameter at least one difference frequency of the at least two carrier tones, to thereby excite the resonance of the sensor.

[0011] Various embodiments of the first aspect may comprise at least one feature from the following bulleted list:

• the sensor comprises a passive sensor configured to measure the physical or environmental parameter

• the passive sensor comprises an inductively coupled electrical resonance circuit sensor

• the at least two carrier tones comprise at least two digital television carrier tones

• the sensor comprises at least one of a temperature sensor, a humidity sensor, a strain sensor, a moisture sensor, a magnetic flux density sensor and an acceleration sensor • the method further comprises producing the at least one difference frequency at least in part in a mixing element that comprises at least one of a diode and a junction of dissimilar metals

• the diode comprises a varactor diode

• the method further comprises enabling the at least one difference frequency to mix with at least one of the at least two carrier tones

• the at least two carrier tones comprise tones in a set of frequencies commencing at 471.25 megahertz and extending therefrom toward higher frequencies at 8 megahertz increments

• the set of frequencies terminates at 855.25 megahertz

• the method further comprises receiving instructions and causing a configurable front-end filter to be configured based at least in part on the received instructions

• the method further comprises causing the difference frequency to mix with at least one of the at least two carrier tones to produce an intermodulation frequency.

[0012] According to a third aspect of the present invention, there is provided an apparatus comprising means for receiving at least two carrier tones, and means for providing to a sensor a resonance of which is affected by a physical or environmental parameter at least one difference frequency of the at least two carrier tones, to thereby excite the resonance of the sensor.

[0013] According to a fourth aspect of the present invention, there is provided a method comprising selecting, for each sensor comprised in a plurality of sensors, an intermodulation frequency, configuring each of the plurality of sensors with a set of carrier tone frequencies, each set of carrier tone frequencies producing the respective intermodulation frequency, and reading out sensor information from at least one of the sensors comprised in the plurality of sensors, the sensor information being read out at the respective intermodulation frequency selected for each of the at least one sensor.

[0014] According to a fifth aspect of the present invention, there is provided a system comprising an apparatus according to the first aspect, and a reader device, the reader device not being configured to output the at least two carrier tones, the reader comprising a receiver configured to read out sensor information from the from the apparatus.

[0015] According to a sixth aspect of the present invention, there is provided a use of an apparatus in accordance with the first aspect to harvest energy from carrier tones, such as, for example, DVB carrier tones.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIGURE 1 illustrates a system in accordance with at least some embodiments of the present invention; [0017] FIGURE 2 illustrates a system in accordance with at least some embodiments of the present invention;

[0018] FIGURE 3 is a flow graph illustrating a first method in accordance with at least some embodiments of the present invention;

[0019] FIGURE 4 illustrates signalling in accordance with at least some embodiments of the present invention, and

[0020] FIGURE 5 is a flow graph illustrating a second method in accordance with at least some embodiments of the present invention.

EMBODIMENTS

[0021] An energy harvesting sensor may be configured to rely in a difference frequency derived from at least carrier tones, such that the difference frequency corresponds to a resonance frequency of the sensor. The sensor may be designed so that the physical quantity the sensor measures affects the resonance characteristics of the sensor, such that a result of measurement may be read out from the sensor by determining a voltage characteristic associated with the resonance. For example, a ratio between an amplitude of the carrier tones and a resonance of the sensor may be used to determine the physical quantity the sensor is arranged to measure. The read out may take place on an intermodulation frequency of the at least two television carrier tones. In various embodiments, in addition to, or alternatively to, amplitude, a frequency and/or phase may be determined in read out from the intermodulation frequency to deduce the physical quantity. [0022] FIGURE 1 illustrates a system in accordance with at least some embodiments of the present invention. In the example of FIGURE 1, a television broadcaster 130 emits a plurality of carrier tones 130a, such as, for example, digital television carrier tones or wireless local area network, WLAN, carrier tones. Carrier tones 130a may be comprised, for example, in the frequency interval {471.25, 855.25} MHz, at an interval of 8MHz. Carrier tones 130a may be encoded with television programmes, which may be present in the signals transmitted from television broadcaster 130 in the form of a suitable modulation. Examples of suitable modulation schemes may comprise, for example, variants of quadrature amplitude modulation, such as QAM- 16, QAM-32 or QAM-64, for example. Modulation of a carrier tone with a representation of information may cause the carrier tone to change shape in a minor way, such that the carrier tone is no longer a perfect sinusoid. However, for the purposes of the present invention this minor deviation is not foreseen to be problematic.

[0023] While a single broadcast tower is illustrated in FIGURE 1, it is to be understood that in general, television broadcaster 130 may operate a plurality of broadcast units, such that carrier tones broadcast from the plurality of broadcast units may be considered to be ubiquitously present in a coverage area of a network of television broadcaster 130. While an absolute level of transmitted carrier tones 130a may vary in dependence of a local transmit power and distance from a transmission unit, carrier tones 130a are likely to be present in the coverage area of the network of television broadcaster 130 at an amplitude that generally exceeds a certain minimum amplitude level, to ensure that television programmes can be successfully received within the coverage area. For example, in semi-urban areas, power levels in the range of several tens of microwatts may be harvestable from television transmissions.

[0024] In the example of FIGURE 1 , vehicle 1 10 comprises two sensors, sensor 112 and sensor 114. Sensor 112 may be read from outside vehicle 110 by a reader 120, via wireless link 120a. Sensor 114 is to be read by controller 116 that is comprised in vehicle 110, like sensor 114 itself. Controller 116 may comprise an on board computer, for example, that is configured to provide a display to a driver of vehicle 110 that displays a current velocity of vehicle 110. Sensor 114 may be read by controller 116 via wireless link 116a. Alternatively to a wireless link, the link between sensor 114 and controller 116 may be a wire-line link. Alternatively to a vehicle, sensors 112 and 114 may be embedded in a building, for example. In some embodiments, there is only one sensor, and in yet further embodiments more than two sensors may be provided.

[0025] Sensor 112 and sensor 114 may each be configured to harvest energy from television carrier tones 130a. In detail, each of these sensors may comprise an antenna element that is capable of receiving at least two of carrier tones 130a. Each of the sensors may further comprise a mixing element, such as, for example, a diode, that is arranged to produce an intermodulation product of at least two of the received carrier tones.

[0026] Inductively coupled electrical resonance-circuit sensors may be utilized to measure a physical quantity, such as strain, temperature, acceleration and/or moisture. These sensors may consist of an electrical resonance circuit, whose resonance frequency is sensitive to the measured physical quantity. For example surface acoustic wave, SAW, resonance circuit sensors may be used, since propagation properties of SAWs may be inherently sensitive to a physical quantity of interest, such as strain, moisture or temperature, for example. The resonance circuit may comprise a MEMS resonance circuit. The sensor may comprise at least one of a capacitive sensing element, an inductive sensing element and a resistive sensing element.

[0027] Examples of capacitive sensors include MEMS microphones, MEMS inertial sensors, MEMS pressure sensors and ceramic humidity sensors. Piezo -resistive strain gauges may be employed as deformation sensors. Inductive humidity sensors are examples of sensors based on inductive principles. [0028] Moisture sensors embedded in a structure of a building may be employed to provide early indications of moisture damage. For example, where a moisture protection structure has been damaged, moisture may begin to seep into concrete that forms the structure of the building. The earlier the seeping moisture is detected, the faster and less invasive it may be to repair. Moisture protection structures may be damaged by vibration caused by nearby traffic or excavation work, for example. [0029] A frequency of the intermodulation product may be predicted based on the incoming carrier tone frequencies. In detail, where a first one of the incoming carrier tones has a frequency ft and a second one of the incoming carrier tones has a frequency ft, a difference frequency Af = ft - ft may be provided from the mixing element to a resonant sensor part of a sensor, such that the sensor part has a resonance at this difference frequency Af. The sensor part will, responsive to frequency Aft resonate in a way that is dependent on a prevailing value of the physical quantity the sensor is arranged to measure. This has the effect that the sensor information of the sensor becomes encoded in the resonance. For example, where the sensor is a moisture sensor, an amplitude of the resonance, or its frequency, may be dependent on the moisture of the sensor part. Likewise, where the sensor is a temperature sensor, an amplitude or a frequency of the resonance may be dependent on the temperature of the sensor part.

[0030] The resonance may be further caused to mix with the incoming frequencies ft and ft, which yields an intermodulation frequency ¾M = 2 · ft - ft. By reading the sensor's response at the intermodulation frequency, the output of the sensor, that is, the sensor information can be read out without needing to provide input power separately from a reader into the sensor. Rather, the incoming carrier tones 130a are used to cause the sensor to emit information that allows the sensor to be read.

[0031] An advantage of intermodulation frequencies have over harmonic frequencies, for example, in this regard is that a frequency offset between the carrier tones and the intermodulation frequency is smaller, which facilitates circuit design. In particular, television carrier tones are evenly and closely distributed in frequency, which makes circuit design utilizing their intermodulation frequencies advantageous. The even spacing of television carrier tones enables building sensor apparatuses that use the carrier spacing as the difference frequency that excites the resonance of the sensor. While the difference frequency may be the same, intermodulation frequencies will differ based on the carrier tones used. For example, frequencies 100 and 108 MHz have a 8-Mhz difference frequency and yield intermodulation products at 92 MHz and 116 MHz. Frequencies 108 and 116 MHz have the same difference frequency, 8MHz, but yield intermodulation products at 100 MHz and 124 MHz. Thus the physical construction of a plurality of sensor apparatuses may be the same, based on the difference frequency, but they may be configured to emit their sensor information at different intermodulation frequencies by configuring them to use different carrier tones. [0032] Also a plurality of sensors may be utilized using separate difference frequencies. For example, two sensors having difference frequencies 2 MHz and 6 MHz will generate mtermodulation responses at different frequencies, when excited by a single OFDM DVB-T channel, comprising more than thousand subcarriers in a 8 MHz band. The sensor may use all or only some of these subcarriers.

[0033] As a specific example, let ft = 471.25 MHz and f ₂ = 479.25 MHz. The sensor can be designed in such a way that its resonance frequency corresponds to the channel spacing, which is the difference frequency, here 8 MHz or ft - f ₂ . The response of the sensor can subsequently be read out at the mtermodulation frequency 463.25 MHz. Another mtermodulation frequency, which can alternatively or additionally be used, produced by these two frequencies, is 2 · f ₂ - ft = 487.25 MHz.

[0034] The sensor information can be read on the mtermodulation frequency, for example, by comparing at least one of an amplitude, frequency and phase of the mtermodulation response to amplitudes of the all or some of the carrier tones ft and f ₂ . The physical quantity measured by the sensor information can then be determined by combining the at least one of the amplitudes, frequencies and phases of the mtermodulation response and carrier tones. A possible implementation is the use of a look-up table, or advanced signal analysis, to obtain the sensor information from ratios between amplitudes between f _1M , ft and f ₂ . In advanced signal analysis, the behaviour of signals in sensors can be modelled to enable determination of the physical quantity based on the mtermodulation response. When using frequency and/or phase in addition to, or alternatively to, amplitude, a look-up table may be employed to determine the value of the physical quantity, alternatively to or in addition to advanced signal analysis.

[0035] Where a plurality of carrier tones 130a are emitted, a plurality of sensors may be read out. In detail, the sensor devices may comprise front-end filters to select two incoming carrier tones in such a way that no two sensors have the same resulting mtermodulation frequency. For example, in terms of FIGURE 1, sensor 112 and sensor 114 may use different incoming carrier tones to enable their simultaneous non- interfering read out. Thus controller 116 and reader 120 can obtain the sensor values they are interested in without interference from the other sensor. In general, a system in accordance with the present invention may comprise a plurality of sensors configured to select incoming carrier tones in such a way that each of the plurality of sensors emits its sensor information on a different intermodulation frequency.

[0036] In some embodiments, a plurality of sensors may be employed as described above, where the sensors have configurable front-end filters. In these embodiments, a central node, which may correspond to a reader, may transmit instructions to the sensors, for example wirelessly. Based on the instructions, individual sensors may then configure their respective front-end filters so as to selectively admit digital television carrier tones. Acting thus, the central node may configure the plurality of sensors in such a way that each of the plurality of sensors becomes remotely readable on an intermodulation frequency, each of the plurality of sensors being readable at a different intermodulation frequency. Being remotely readable may comprise that the sensor emits its sensor information on a specific intermodulation frequency as a response to incoming carrier tones.

[0037] Adding a new sensor to the plurality of sensors may be accomplished by the central node by transmitting instructions to the new sensor, the instructions being configured to cause the new sensor to configure its front-end filters in such a way that the new sensor will begin to emit its sensor information on an intermodulation frequency that is different from each of the intermodulation frequencies at which sensors already present in the plurality of sensors emit their sensor information.

[0038] By using carrier tones 130a to cause a sensor to emit its sensor information on an intermodulation frequency, reader 120 need not emit any signals to provide power to the sensor. Thus when compared to a prior solution where a reader powers sensors or sensor tags, an advantage may be obtained in the construction of reader 120, wherein reader 120 needn't comprise a transmitter enabled to transmit signals to power the sensor. Rather reader 120 may comprise a receiver, such as for example a tuneable receiver, usable in receiving the sensor information on an intermodulation frequency from the sensor. In some embodiments, reader 120 is enabled to transmit instructions to sensors to configure their front-end filters, but a transmitter enabled to do this can use a lower output power than a transmitter capable of remotely powering a sensor.

[0039] In various jurisdictions, there may exist regulations concerning radio emissions that take place within a frequency band allocated to digital television. For example, it may be required that any emissions must have a power of at least 50 dB below the carrier tone. This can be achieved in embodiments of the present invention, since intermodulation frequencies are due to their innate nature emitted at a lower amplitude than the incoming frequencies that cause them to occur. In case the invention is to be practiced in a jurisdiction with such limits, measurements may be conducted to verify the limit is respected. [0040] In practical terms, for example, if it is assumed that a power level of a carrier tone received in a sensor is one microwatt and spurious noise exists at a level of -lOOdBm at the receiver, a maximum reading distance of the sensor may be 4.7 meters. Assuming that a carrier tone input power is -20 dBm, an antenna gain is 10 dBi, sensitivity is -105 dBm and the sensor is equipped with an antenna with a 3 dBi gain, a read-out range of the sensor may be 9 meters at 1.2 GHz, for example.

[0041] FIGURE 2 illustrates a system in accordance with at least some embodiments of the present invention. Sensor apparatus 200 may correspond to sensor 112 or 114 of FIGURE 1, for example. Sensor apparatus 200 comprises an antenna 210, which may be internal to a shell of sensor apparatus 200 or at least in part attached to sensor apparatus 210 and extending outside the shell of sensor apparatus 200. Antenna 210 may be capable of receiving carrier tones, such as for example television carrier tones. In FIGURE 2, first carrier tone 202 and second carrier tone 203 are received, at least in part, by antenna 210.

[0042] Antenna 210 may be coupled with a matching circuitry 220. Matching circuitry 220 may comprise, for example, a shunt admittance in parallel with antenna 210 and a series inductance, although other matching topologies may be used as well.

[0043] Coupled to matching circuitry 220, the sensor apparatus 200 of FIGURE 2 comprises a mixing element 230, which in this example comprises a diode, such as for example a varactor diode. Further, coupled to mixing element 230 is sensor 240, which, as described above, may comprise a resonant sensor element, such as for example a SAW sensor element.

[0044] In use, first carrier tone 202 and second carrier tone 203 are received via antenna 210, matched in matching circuitry 210 and allowed to mix via mixing element 230. As described above, a difference frequency between first carrier tone 202 and second carrier tone 203 causes sensor 240 to resonate in a way that is affected by the physical quantity that is to be measured by sensor apparatus 200 of FIGURE 2. Finally, an intermodulation frequency 204 of first carrier tone 202 and second carrier tone 203 enables remote read-out of the sensor information produced in the resonance of sensor 240.

[0045] FIGURE 3 is a flow graph illustrating a first method in accordance with the present invention. The phases of the illustrated method may take place in sensor apparatus 200 of FIGURE 2, for example, or in sensor 112 or sensor 114 of FIGURE 1.

[0046] Phase 310 comprises receiving at least two digital television carrier tones.

Phase 320 comprises providing to a sensor, a resonance of which is affected by a physical or environmental parameter, at least one difference frequency of the at least two digital television carrier tones, to thereby excite the resonance of the sensor. The physical or environmental parameter may comprise the physical quantity that the sensor is arranged to measure, for example.

[0047] FIGURE 4 illustrates signalling in accordance with at least some embodiments of the present invention. On the vertical axes are disposed, from left to right, television broadcaster 130, sensor 112, sensor 114 and, finally, reader 120. Like numbering denotes like structure as in FIGURE 1.

[0048] In phase 410, television broadcaster emits carrier tones, such as for example digital television carrier tones. The carrier tones may be broadcast, meaning the transmission is not directed to any node in particular, rather the carrier tones, which may be encoded with information, may be emitted from a transmitter in an at least in part non- directional manner, to receivers within a coverage area of the transmitter. Phase 410 may be continuous, in other words the transmission of carrier tones may continue during the other phases of FIGURE 4 as well.

[0049] In optional phase 420, reader 120 provides an instruction to sensor 1 12, so as to configure sensor 112 to use a first set of carrier tones transmitted by television broadcaster 130. The first set of carrier tones may be selected by reader 120 in a way that an intermodulation frequency produced by the first set of carrier tones is a desired frequency. In other words, reader 120 may select an intermodulation frequency it wants to use when reading sensor information from sensor 112, and then instruct sensor 112 to use a carrier tones that result in the desired intermodulation frequency. [0050] In optional phase 430, sensor 112 configures at least one filter so as to cause the first set of carrier tones to be admitted toward a mixing element of sensor 112, and to cause other carrier tones to be suppressed in a front end of sensor 112. Where phase 420 is absent, also phase 430 is absent.

[0051] In optional phase 440, reader 120 configured sensor 114, so as to configure sensor 112 to use a second set of carrier tones transmitted by television broadcaster 130. The second set of carrier tones produces an mtermodulation frequency that is different from the mtermodulation frequency emitted by sensor 112, enabling simultaneous non- interfering wireless readout of sensor 112 and sensor 114. In optional phase 450, sensor 114 acts on the instruction received in phase 440 and configured at least one filter accordingly. Where phase 440 is absent, also phase 450 is absent. [0052] Where phases 420, 430, 440 and 450 are absent, sensor 112 and sensor 114 may be pre-configured, for example during manufacture, to use the first set of carrier tones and the second set of carrier tones, respectively.

[0053] In phase 460, sensor 112 emits electromagnetic radiation at the mtermodulation frequency produced by the first set of carrier tones. The emitted radiation comprises embedded therein sensor information of sensor 112, for example, information on a strain experienced by sensor 112. Phase 460 may be seen as a continuous, in that sensor 112 may emit the mtermodulation frequency continuously once front-end filters of sensor 112 are configured and transmission 410 is present.

[0054] In phase 470, sensor 114 emits electromagnetic radiation at the mtermodulation frequency produced by the second set of carrier tones. The emitted radiation comprised embedded therein sensor information of sensor 114, for example, information on a moisture level experienced by sensor 114. Phase 470 may be seen as a continuous phase after phase 450, in that sensor 114 may emit the mtermodulation frequency continuously once front-end filters of sensor 114 are configured and transmission 410 is present.

[0055] FIGURE 5 is a flow graph illustrating a second method in accordance with at least some embodiments of the present invention. The phases of the illustrated method may be performed in reader 120 of FIGURE 1, or in a control device of reader 120, the control device being configured to control the functioning of reader 120 when implanted therein. [0056] Phase 510 comprises selecting, for each sensor comprised in a plurality of sensors, an mtermodulation frequency. Phase 520 comprises configuring each of the plurality of sensors with a set of digital television carrier tone frequencies, each set of digital television carrier tone frequencies producing the respective intermodulation frequency. Finally, phase 530 comprises reading out sensor information from at least one of the sensors comprised in the plurality of sensors, the sensor information being read out at the respective intermodulation frequency selected for each of the at least one sensor. A set of digital television carrier tone frequencies may comprise at least two digital television carrier tone frequencies.

[0057] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0058] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed. [0059] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0060] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0061] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0062] The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0063] At least some embodiments of the present invention find industrial application in enabling passive sensors to be used to gather sensor information. ACRONYMS LIST

MEMS microelectromechanical

MHz megahertz

SAW surface acoustic wave

WLAN wireless local area network

REFERENCE SIGNS LIST

Controller

a Wireless link

a Wireless link

Television broadcaster

a Carrier tones

Reader

Sensor ap aratus

First carrier tone

Second carrier tone

Intermodulation frequency

Antenna

Matching circuitry

Mixing element

Sensor

-320 Phases of the method illustrated in FIGURE 3-470 Phases of signaling illustrated in FIGURE 4-530 Phases of the method illustrated in FIGURE 5