GB2444179A

GB2444179A - Demodulating radio frequency signals in near-field RF communications enabled devices

Info

Publication number: GB2444179A
Application number: GB0723220A
Authority: GB
Inventors: Joakim Bangs; David Miles
Original assignee: Innovision Research and Technology PLC
Current assignee: Broadcom Innovision Ltd
Priority date: 2006-11-27
Filing date: 2007-11-27
Publication date: 2008-05-28
Also published as: WO2008065375A2; WO2008065375A3; GB0623654D0; GB0723220D0

Abstract

A near-field RF communicator 100, e.g. an RFID device, has an antenna circuit 120 to receive a modulated radio frequency signal by inductive coupling and demodulation circuitry 130 to extract the modulation from a received modulated RF signal. Demodulation circuitry 130 comprises demodulator 114 and differential amplifier 115 which has respective inputs coupled to junctions J1, J2 of antenna circuit 120. Thus, demodulation circuitry 130 receives two modulated RF signals from antenna circuit 120 and extracts modulation using both signals. Amplifier 115 amplifies the modulated RF signal to remove common mode components. Amplifier 115 may have low impedance input circuits 123,124 which may be virtual earth, i.e. current mirror, circuits (fig. 6). Alternatively, the demodulation circuitry may have a differential mixer (400, fig. 3), such as a Gilbert mixer, or be a synchronous IQ demodulator having an I differential mixer (400, fig. 5) and a Q differential mixer (400a). The invention provides near-field RF communicators which are less noise sensitive and which minimise overloading.

Description

NEAR FIELD RF COMMUNICATORS AND NEAR FIELD RF

COMMUNICATIONS-ENABLED DEVICES

This invention relates to near field RF communicators and near field RF communications-enabled devices.

Near field RF (radio frequency) communication requires an antenna of one near field RF communicator to be present within the alternating magnetic field (H field) generated by the antenna of another near field RF communicator by transmission of an RF signal (for example a 13.56 Mega Hertz signal) to enable the magnetic field (H field) of the RF signal to be inductively coupled between the communicators. The RF signal may be modulated to enable communication of control andior other data. Ranges of up to several centimetres (generally a maximum of 1 metre) are common for near field RF communicators.

Near field communication may be referred to as near-field RFID (Radio Frequency Identification) or near-field communication. NFC communicators are a type of near field RF communicator that is capable of both initiating a near field RF communication (through transmission or generation of an alternating magnetic field) with another near field RF communicator and of responding to initiation of a near field RF communication by another near field RF communicator. The term "near field RF communicator" includes not only NFC communicators but also initiating near field RF communicators such as RFID transceivers or readers that are capable of initiating a near field RF communication but not responding to initiation of a near field RF communication by another near field RF communicator and responding near field RF communicators such as RFID transponders or tags that are capable of responding to initiation of a near field RF communication by another near field RF communicator but not of initiating a near field RF communication with another near field RF communicator. Hence NFC communicators can act as both RFID transceivers and RFID transponders and are able to communicate with other NFC communicators, RFID transceivers and RFID transponders. * 2

Examples of near field RF communicators are defined in various standards for example ISO/IEC 18092, ISO/IEC 14443, ISOIIEC 15693 ISO/IEC 21481.

Near field RF communicators may be provided as standalone or discrete devices or may be incorporated within or coupled to larger electrical devices or host devices (referred to below as near field RF communications-enabled devices) to enable those devices to communicate by the near field with other near field RF communicators or devices incorporating or coupled to such near field RF communicators. When incorporated within a larger device or host, a near field RE communicator may be a discrete entity or may be provided by functionality within the larger device or host. Examples of such larger devices or host devices are, for example, mobile telephones, portable computing devices (such as personal digital assistants, notebooks, lap-tops), other computing devices such as personal or desk top computers, computer peripherals such as printers, or other electrical devices such as portable audio and/or video players such as MP3 players, IPODs , CD players, DVD players.

When a first near field RF communicator receives a modulated RE signal from a second near field RF communicator, this modulated RE signal is received by the antenna circuit of the first near field RF communicator and must then be demodulated by a demodulator within the near field RE communicator. However with existing designs the demodulator or near field RF communicator circuit can become overloaded on receipt of large modulated carrier signals and can also be very noise sensitive.

According to an aspect of the present invention, there is provided a near field RF communicator having demodulation circuitry that extracts modulation from a received modulated RF signal coupled to an antenna circuit using first and second modulated radio frequency signals provided by the antenna circuit According to an aspect of the present invention, there is provided a near field RF communicator comprising: an antenna circuit to receive a modulated radio frequency signal by inductive coupling; and demodulation circuitry to extract the modulation from a received modulated radio frequency signal inductively coupled to the antenna circuit, wherein the demodulation circuitry has a first modulated signal input to receive a first modulated radio frequency signal from a first output of the antenna circuit and a second modulated signal input to receive a second modulated radio frequency signal from a second output of the antenna circuit and is operable to extract modulation from a received modulated RF signal using both the first and second modulated radio frequency signals.

In an embodiment, the demodulation circuitry comprises: a differential amplifier having a non-inverting input, an inverting input and an output; and a demodulator coupled to the first output of the antenna circuit and the inverting input is coupled to the second output of the antenna circuit.

In an embodiment, a modulator is provided to modulate a radio frequency signal in accordance with data to enable communication of data by the near field RF communicator.

In an embodiment, the demodulation circuitry comprises a differential mixer.

In an embodiment, the modulated radio frequency signals have a carrier at a first frequency, the demodulation circuitry comprises a differential mixer having the first and second modulated signal inputs and coupled also to receive an oscillator signal having a second frequency, and wherein the differential mixer is operable to mix the first and second modulated radio frequency signals with the oscillator signal to provide sum components at a third frequency which is the sum of the first and second frequencies and difference components at a fourth frequency which is the difference of the first and second frequencies and wherein the demodulation circuitry is operable to extract the modulation on the basis of the difference components.

In an embodiment, the modulated radio frequency signals have a carrier at a first frequency, the demodulation circuitry comprises a differential mixer having the first and second modulated signal inputs and coupled also to receive an oscillator signal having a second frequency the same as the first frequency, and wherein the differential mixer is operable to mix the first and second modulated radio frequency signals with the oscillator signal to provide sum components at a third frequency which is the twice the first frequency and difference components at a fourth frequency which is zero and wherein the * 4 demodulation circuitry is operable to extract the modulation on the basis of the difference components.

In an embodiment, the modulated radio frequency signals have a carrier at a first frequency, the demodulation circuitry comprises a differential mixer having the first and second modulated signal inputs and first and second oscillator inputs coupled to receive, respectively, a first oscillator signal at the first frequency and a second oscillator signal at the first frequency but phase-shifted with respect to the first oscillator signal, and wherein the differential mixer is operable to mix the first and second modulated radio frequency signals and the first and second oscillator signals to provide two output signals each comprising sum components at a further frequency which is the twice the first frequency and difference components at a fourth frequency which is zero and wherein the demodulation circuitry comprises a subtractor to subtract the two output signals to extract the modulation.

In an embodiment, the differential mixer has first and second transistors having corresponding ones of their main electrodes coupled to the first modulated signal input and third and fourth transistors having corresponding ones of their main electrodes coupled to the second modulated signal input, the control gates of the first and fourth transistors being coupled to the first oscillator input and the control gates of the second and third transistors being coupled to the second oscillator input, the other main electrodes of the second and fourth transistors being coupled to one of the two outputs of the differential mixer and the other main electrodes of the first and third transistors being coupled to the other of the two outputs of the differential mixer. The transistors may be field effect transistors, for

example MOSFETs.

In an embodiment, the demodulation circuitry comprises two such differential mixers, one of the first and second oscillator inputs of one of the two such differential mixers being coupled to receive an oscillator signal that is phase quadrature with the oscillator signal coupled to the corresponding oscillator input of the two such differential mixers to provide a IQ demodulator.

In an embodiment, a phase locker is provided to lock together the phase of the carrier of a received modulated radio frequency signal and the oscillator signal. * 5

In an embodiment, the demodulation circuitry has a low pass filter to filter out the third frequency.

In an embodiment, a signal generator is provided to provide the oscillator signal, the signal generator also providing a modulator operable to modulate a radio frequency signal in accordance with data to enable communication of data by the near field RF communicator.

In an embodiment, the inputs of the demodulation circuitry comprise virtual earth inputs.

In an embodiment, the inputs of the demodulation circuitry comprise current mirrors. The current mirrors may each comprise a diode-coupled transistor having a main electrode coupled to the corresponding one of the inputs and a further transistor having its control gate coupled to the control gate of the diode-coupled transistor such that, in operation, a current at the input of the demodulation circuitry is mirrored by the further transistor to provide an input current to the demodulation circuitry. The transistors may be

field effect transistors, for example MOSFETs.

In an embodiment, a power denver is provided to derive power from a radio frequency signal inductively coupled to the coupler.

The near field RF communicator may be an NFC communicator.

Embodiments of the present invention provide significantly less noise sensitive near field RF communicators and minimisation of overloading.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a representational diagram illustrating communication between two devices comprising NFC communicators; * 6 Figure 2 shows one embodiment of an NFC communicator according to the present invention; Figure 3 shows another embodiment of an NFC communicator according to the present invention; Figure 4 shows an example of a differential mixer that may be used in the NFC communicator shown in Figure 3; Figure 5 shows another embodiment of an NFC communicator according to the present invention; Figure 6 shows a virtual earth (virtual ground) circuit that that may be used in the NFC communicator shown in Figure 2 or 3; and Figure 7 shows an example of a virtual earth (virtual ground) circuit that may be used in the NFC communicator shown in Figure 5.

With reference to the drawings in general, it should be understood that any functional block diagrams are intended simply to show the functionality that exists within the device and should not be taken to imply that each block shown in the functional block diagram is necessarily a discrete or separate entity. The functionality provided by a block may be discrete or may be dispersed throughout the device or throughout a part of the device. In addition, the functionality may incorporate, where appropriate, hard-wired elements, software elements or firmware elements or any combination of these. The NFC communicator may be provided wholly or partially as an integrated circuit or collection(s) of integrated circuits.

Referring now specifically to Figure 1, there is shown a representational diagram illustrating communication between two NFC communications-enabled devices. In Figure I the representations of the NFC communications-enabled devices have been shown partly cut-away and the functionality provided by the NFC communications-enabled devices illustrated by way of a functional block diagram within the NFC communications-enabled device. * 7

As shown in Figure 1, one NFC communications-enabled device comprises a mobile telephone (cellular telephone, "cellphone") 1 and the other NFC communications-enabled device comprises a portable computer 2 such as a notebook or laptop computer.

The mobile telephone I has the usual features of a mobile telephone including mobile telephone functionality 10 (in the form of, usually, a programmed controller, generally a processor or microprocessor with associated memory or data storage, for controlling operation of the mobile telephone in combination with a SIM card), an antenna 8 for enabling connection to a mobile telecommunications network, and a user interface 3 with a display 4, a keypad 5, a microphone 6 for receiving user voice input and a loudspeaker 7 for outputting received audio to the user. The mobile telephone also has a chargeable battery 11 coupled to a charging socket 12 via which a mains adapter (not shown) may be connected to enable charging of the battery 11. The mobile telephone I may have an alternative or additional power supply (not shown), for example a reserve battery or emergency battery.

Similarly the portable computer 2 has the usual features of a portable computer including portable computer functionality 20 in the form of, usually, a processor with associated memory in the form of ROM, RAM and/or hard disk drive, one or more removable media drives such as a floppy disk drive and/or a CDROM or DVD drive, and possibly a communications device for enabling the portable computer to connect to a network such as the Internet. The portable computer 2 also includes a user interface 21 including a display 22, a keyboard 23 and a pointing device, as shown a touchpad 24. The portable computer 2 also has a chargeable battery 25 coupled to a charging socket 26 via which a mains adapter (not shown) may be connected to enable charging of the battery 25.

In addition, as shown in Figure 1, the NFC communications-enabled devices 1 and 2 each have an NFC communicator 15 and 30. As shown, the NFC communicators 15 and are incorporated within the larger devices and, as with the other functional blocks, may be discrete entities within the host devices or may be provided by features dispersed throughout or integrated within the host device or a part of the host device. * 8

Each NFC communicator 15 and 30 comprises NFC operational components 16 and 31 for, as will be described below, enabling control of the NFC functionality and generation, modulation and demodulation of an RF signal. Each NFC communicator 15 and 30 also comprises a coupler or antenna circuit 17 and 32 comprising an inductor or coil in the form of an antenna 18 and 33. The couplers 17 and 32 enable an alternating magnetic field (H field) generated by the antenna of one near field RF communicator 15 (or 30) by transmission of an RF signal (for example a 13.56 Mega Hertz signal) to be inductively coupled to the antenna of the other near field RF communicator 30 (or 15) when that antenna is within the near field of the RF signal generated by the one near field RF communicator 15 (or 30).

The NFC communicators 15 and 30 are coupled to the mobile telephone and portable computer functionality 10 and 20, respectively, to enable data and/or control commands to be sent between the NFC communicator and the host device and to enable user input to the NFC communicator. Communication between the user interface 3 or 21 and the NFC communicator 15 or 30 is via the host device fI.inctionality 11 or 20, respectively.

Each NFC communicator 15 and 30 also comprises a power provider 19 and 34.

The power providers 19 and 34 may be power supplies within the host device or specific to the NFC communicators 15 and 30, for example a button cell battery, or other small battery. As another possibility or additionally as shown by dashed lines in Figure 1, one or both of the power providers 19 and 34 may simply comprise a coupling to derive power from the corresponding device battery 11 or 25.

It will be appreciated that Figure 1 shows only examples of types of host devices.

A host device may be another type of electrical device such as a personal digital assistant (PDA), other portable electrical device such as a portable audio and/or video player such as an MP3 player, an IPOD , CD player, DVD player or other electrical device.

Rather than being incorporated within the host device, the NFC communicator 15 or 30 may be associated with the host device, for example by a wired or wireless coupling.

In such a case, a housing of the NFC communicator may be physically separate from or * 9 may be attached to the housing of the host device; in the later case, the attachment may be permanent once made or the NFC communicator may be removable. For example, the NFC communicator may be housed within: a housing attachable to another device; a housing portion, such as a fascia of the NFC communications-enabled device or another device; an access card; or may have a housing shaped or configured to look like a smart card. For example an NFC communicator may be coupled to a larger device by way of a communications link such as, for example, a USI3 link, or may be provided as a card (for example a PCMCIA card or a card that looks like a smart card) which can be received in an appropriate slot of the larger or host device.

Figure 2 shows a functional block diagram of an NFC communications-enabled device 100 in accordance with the invention to illustrate one way in which the NFC operational components I OOa of an NFC communications-enabled device embodying the invention may be implemented.

As shown in Figure 2, the NFC operational components include a controller 107 to control overall operation of the NFC communicator, demodulation circuitry 130 (described in greater detail below) to enable demodulation of a modulated RF signal coupled to an antenna circuit 120 to enable data to be communicated to the NFC communicator, and modulation circuitry to enable modulation of an RF signal to enable data to be communicated by the NFC communicator. Data (information and/or control data) to be communicated may be read from, and received demodulated data may stored, in any appropriate data store, for example one or more of an internal memory of the controller 107, a data store 108 associated with the controller 107, a host device data store where the NFC communicator has a host, or an associated standalone data store.

The NFC communicator may operate using any appropriate modulation scheme that is in accordance with the standards and/or protocols under which the NFC communicator operates.

As shown in Figure 2, the modulation circuitry comprises a signal generator 109 coupled via a driver 11 1 to the antenna circuit 120. The driver may be a Class D output driver, although a Class A, a Class B or a Class AB output driver may be used, or an emitter-follower or source-follower may be used, for example. In this example, the signal generator 110 is controlled by the controller 107 to cause modulation by gating or switching on and off an RF signal in accordance with the data to be communicated. As another possibility, a separate or further signal controller or modulator 110 (shown in phantom lines in Figure 2) may be incorporated within the NFC operational components and coupled to the signal generator 109 to control modulation of the signal generated by the signal generator 109 in accordance with data or instructions received from the controller 107. As another possibility, the signal generator 109 may provide an unmodulated signal and modulation may be achieved by using, rather than the single-input driver shown in Figure 2, a differential driver having its other input coupled to a data output of the controller 107 to modulate the output of the driver, for example by gating or switching on and off the driver 111, in accordance with the data to be communicated.

Generally the signal generator 109 will include an oscillator to generate the RF signal, although it may be possible for the signal generator to generate the RF signal from a clock of the controller 107 or a clock or oscillator of a host device. The controller 107 may, as shown in Figure 2, be coupled to the driver 111 to control the gain of the driver and thus the modulation depth of a modulated RF signal.

In the example shown in Figure 2, the antenna circuit 120 comprises an antenna coil 112 coupled in parallel with a capacitor 113. As is known in the art, the antenna circuit 120 may also include filtering capacitors to reduce harmonics and comply with electromagnetic energy emission regulations. As shown in Figure 2, the driver 111 has its non-inverting output coupled via a capacitor 104 to one junction J2 between the capacitor 113 and the antenna coil 112 and its inverting output coupled via a capacitor 117 to the other junction ii between the capacitor 113 and the antenna coil 112. As another possibility a single-ended driver may be used if the other side of the antenna coil is grounded.

The controller 107 controls overall operation of the NFC communicator. The controller 107 may be, for example, a microprocessor, for example a RISC processor or other microprocessor, or a state machine. Program instructions for programming the * 11 controller and/or control data for communication to another near field RF communicator may be stored in an internal memory of the controller 107 and/or the data store 108.

The NFC communicator I OOa also has or is associated with a power provider 104 coupled between power supply lines Vdd and Vss. The power provider 104 may be any one or more of the types of power providers discussed above. In the interests of simplicity, power supply couplings from the power provider 104 to other components are not shown in Figure 2.

The NFC communicator I OOa may or may not have or be capable of being connected or coupled with at least one of other functionality 105 (for example functionality of a host device such as described above) and a user interface 106. One or both of the NFC communications-enabled devices may be a standalone NFC communicator, that is it may have no functionality beyond its NFC communications functionality, although it may possibly have its own user interface and perhaps further functionality specific to the standalone NFC communicator. The other functionality 105 may provide at least some of the control functions of the controller 107. For example the NFC communicator controller 107 may control modulation and modulation protocols whereas the data to be transmitted may be provided by other functionality 105.

In Figure 2, the controller 107, the power provider 104, data store 108, other functionality 105 and user interface 106 are shown as part of the NFC communicator. One or more of these may, however, at least in part be provided by a host or another device couplable by a wired or wireless coupling to the NFC communicator.

The NFC communicator lOOa may operate in an initiator mode (that is as an initiating near field RF communicator) or a target mode (that is as a responding near field RF communicator), dependent on the mode to which the NFC communicator is set. The mode may be determined by the controller 107 or may be determined in dependence on the nature of a received near field RF signal. When in initiator mode, an NFC communicator initiates communications with any compatible responding near field RF communicator capable of responding to the initiating NFC communicator (for example an NFC * 12 communicator in target mode or an RFID tag or transponder) that is in its near field range, while when in target mode an NFC communicator waits for a communication from a compatible initiating near field RF communicator (for example an NFC communicator in initiator mode or an RFID initiator or transceiver). As thus used, compatible means operable at the same frequency and in accordance with the same protocols, for example in accordance with the protocols set out in various standards such as ISO/IEC 18092, ISOIIEC 21481, ISO/IEC 14443 and ISO/JEC 15693.

When in initiator or target mode, the NFC communicator may communicate in accordance with an active or passive protocol. When using an active protocol the initiating NFC communicator will transmit an RF field and following completion of its data communication turn off its RF field. The responding near field RF communicator (target) will then transmit its own RF field and data before again turning off the RF field and so on.

When using a passive protocol the NFC communicator (initiator) will transmit and maintain its RF field throughout the entire communication sequence. The protocol used will depend on instructions received form the controller 107 and the response received

from a responding near field RF communicator.

In the example shown in Figure 2, the demodulation circuitry comprises a differential amplifier 115 having one input (as shown its non-inverting input) coupled via capacitor 121 to the junction J 1 of the antenna circuit 120 and its other input (as shown its inverting input) coupled via capacitor 122 to the junction J2 of the antenna circuit 120 and a demodulator 114 coupled to the output of the differential amplifier 119. When a modulated RF signal is inductively coupled to the antenna circuit 120, a differential signal is developed across the antenna coil junctions J I and J2 and as these are coupled to respective inputs of the differential amplifier 115, the differential amplifier 115 acts to amplify the modulated RF signal and to remove common mode components. The demodulator 114 acts to extract the modulation from the amplified modulated RF signal output by the differential amplifier. In the example shown in Figure 2, the demodulator 114 is coupled to supply the extracted data directly to the controller 107 for processing.

There may, however, be some prior signal processing, for example an A/D converter may be provided to convert the output of the demodulator 114 to a digital input signal for the controller 107. O 13

The capacitors 121 and 122 are shown as being outside the NFC operational components lOOa to indicate that, where the NFC operational components lOOa are provided by an integrated circuit, the capacitors 121 and 122 are, like the antenna circuit, external to the integrated circuit.

The use of the differential amplifier 115, in place of a traditional RFID single-ended demodulator front-end, enables the signal input to the demodulator 114 to have a bigger receive amplitude and a better signal-to-noise ratio because of common-mode rejection.

The differential amplifier 115 may have low impedance input circuits 123 and 124 (shown by phantom blocks in Figure 2) to provide current inputs A and B for the differential amplifier. These low impedance input circuits 123 and 124 may be virtual earth, that is current mirror, circuits responsive to the current flowing into the corresponding input. The two virtual earth circuits 123 and 124 will be the same to enable common mode rejection.

Figure 6 shows an example of such a virtual earth circuit 123 or 124. In the example shown in Figure 6, node 301 represents the input of the differential amplifier 115.

The virtual earth circuit 123 or 124 comprises a current mirror having a first transistor 302 having its first main electrode coupled via a bias current source 307 to power supply line Vdd (see Figure 2) and its second main electrode coupled to the power supply line Vss.

The first main electrode of the first transistor 302 is coupled to the input node 301 and to its control electrode so that the transistor is diode-coupled. The control electrode of the first transistor 302 is also coupled to the control electrode of a second transistor 303 having itssecond main electrode coupled to the power supply line Vss and its first main electrode coupled to an output 308 of the current mirror to provide output current A or B. In this example, the first and second transistors 302 and 303 are n channel field effect transistors (FET5), for example NMOS transistors and so the first main electrode is the drain electrode and the second main electrode is the source electrode. The bias current * 14 source 307 may be provided by a PMOS current mirror coupled to Vdd. It may be possible to use other forms of transistors such as bipolar transistors.

In operation of the virtual earth circuit 123 or 124 shown in Figure 6, the capacitors 121 and 122 which are of very low capacitance, for example 0.5 or I picofarad, act as voltage-to-current converters for the virtual earth circuits 123 and 124. These capacitors may, where better tolerances are required than can be achieved with discrete capacitor components, be formed by copper tracks on a printed circuit board of the NFC communicator. The current 305 input at node 301 is mirrored as current 304 flowing through transistor 303 and the mirrored input current 304 provides the actual input to the differential amplifier 115 and thus "represents" the input current within the differential amplifier 115. The current mirror may provide any appropriate current mirror ratio. The current mirror ratio may be adjusted by, for example, replacing transistor 303 by plural transistors coupled in parallel and/or by varying the channel lengthIwidth ratio. The current mirror ratio of the virtual earth circuits 123 and 124 should of course be the same.

The modulated carrier signals supplied to the differential amplifier 115 inputs comprise an AC voltage. Providing the virtual earth circuits 123 and 124 causes, because of the resulting very low input impedances, current to flow through capacitor 121 into the amplifier non-inverting input and current to flow through capacitor 122 into the amplifier inverting input. The differential amplifier 115 outputs a voltage signal Vout representing the modulated carrier signal where Vout = G (i31 -i12); where G is the transimpedance gain of the differential amplifier 119, and i11 and m2 are the respective input currents.

The virtual earth or current mirror circuits provide low input impedances for the differential amplifier 115 to the extent that the AC-component of the voltage 306 at the amplifier inputs (represented by node 301 in Figure 6) is virtually equal to zero. The DC-component of voltage 306 is close to the FET threshold voltage, which is usually about 0.7 volts.

One advantage of such low impedance inputs to the demodulator circuitry 130 is that no significant reduction occurs in the Q of the antenna circuit, and hence no significant loss of coupling strength between near field RF communicators. In addition, in * 15 contrast to the case where high input impedance voltage couplings are made to the demodulation circuitry, it should not be necessary to divide-down the voltage of a received modulated carrier signal to avoid over-voltage damage which may otherwise occur when very high magnetic fields couple to the antenna coil and so the modulated carrier signal input to the demodulator 114 may be increased or maximised. The use of such low impedance inputs may also make it easier to adjust the dynamic range in current mode by for example using switchable parallel coupled transistors in the current mirror to control the current mirror ratio.

Figure 3 shows a functional block diagram of an NFC communications-enabled device 100 in accordance with the invention to illustrate another way in which the NFC operational components I OOa of an NFC communications-enabled device embodying the invention may be implemented. The same numbering is used as in Figure 2 where parts of the NFC communicator have equivalent functions.

In the example shown in Figure 3, the demodulation circuitry 131 comprises a differential mixer 400 having a first set of signal inputs comprising first and second modulated RF signal inputs 118 and 119 coupled via respective capacitors 121 and 122 to junctions Ji and J2, respectively, of the antenna circuit 120 to receive RF modulated signals RF1 and RF2 and a second set of signal inputs comprising a local oscillator signal LO input 404 and a phase shifted local oscillator signal LO input 405.

In the example shown in Figure 3, the signal generator 109 has an oscillator I 09b coupled to the driver Ill to provide the RF carrier, a phase locker 1 09a to achieve phase-locking between the oscillator 109b output and an RF signal inductively coupled to the antenna circuit 120 and a phase shifter 1 09c. As mentioned above, the signal generator 109 may not necessarily have its own oscillator but may generate the RF signal from a clock of the controller 107 or a clock or oscillator of a host device.

In the example shown in Figure 3, the output of the oscillator 109b is coupled to the local oscillator input 404 of the differential mixer 400 as well as to the driver 111 so that the oscillator 109b provides both the RF carrier and the local oscillator signal. The * 16 output of the oscillator 109b is also coupled to the phase shifter 109c which phase-shifts the output of the oscillator I 09b and provides a phase-shifted output to the phase shifted local oscillator input 405. The phase shifter 1 09c phase shifts the oscillator output signal by, in this example, 180 degrees or an integer multiple thereof. There are other ways in which the required signals may be provided. For example, the oscillator (or clock divider) could provide a differential signal to start with The differential mixer 400 acts to mix the local oscillator signals LO and LO and the RF modulated signals RF I and RF2 and provides two outputs each of which contains components at a first frequency which is the sum of the frequencies of RF carrier and the local oscillator and at a second frequency which is the difference between the frequencies of RF carrier and the local oscillator. As the two sets of input signals are at the same frequency, the second frequency will be a zero so that difference component of the output of the differential mixer 400 will comprise the modulation signal without the carrier, whereas the sum component will be a signal of twice the frequency of the carrier. A subtractor or differential amplifier 402 is provided to determine the difference between the two outputs of the differential mixer 400, thereby removing noise and DC common mode components.

A filter 402 is again provided to remove the unwanted signal at twice the carrier frequency, leaving the desired modulation signal, that is the extracted data. This filtered signal is, possibly after analogue-to-digital conversion by an A/D converter 403, input to the controller 107. The differential mixer circuit 400 thus outputs the extracted modulation to the controller 107.

The RF signal inputs to the differential mixer 400 may have low impedance virtual earth or current mirror input circuits 141 and 142 (shown in phantom lines in Figure 3) as discussed above with reference to Figures 2 and 6.

Figure 4 shows an example of a differential mixer 400, in this case a Gilbert mixer circuit * 17 The differential mixer has four matched transistors providing a first transistor pair Ti and T2 having corresponding ones (the sources where the transistors are NMOS) of their main electrodes coupled to the input 118 and a second transistor pair T3 and T4 having corresponding ones of their main electrodes (the sources where the transistors are NMOS) coupled to the input 119. The control gates of transistors TI and T4 are coupled to the local oscillator LO signal input 404 while the control gates of transistors T2 and T3 are coupled to a phase-shifted local oscillator signal LO input 405. The other main electrodes of transistors T2 and T4 are both coupled to a first output 407 and, via a low pass filter circuit, to the power supply line Vdd while the other main electrodes of transistors TI and T3 are both coupled to a second output 408 and, via a low pass filter circuit, to the power supply line Vdd. In this example, the transistors TI to T4 are field effect transistors (PETs), although it may be possible to use other forms of transistors such as bipolar transistors.

In this example, the transistors Ti to T4 are n channel field effect transistors (FETs), for example NMOS transistors, and so the one main electrodes are their drain electrodes and the other main electrodes are their source electrodes.

Figure 4 shows on example of a low pass filter that may be used. In this example, the low pass filter circuitry is provided by a resistor Ri coupled in parallel with a capacitor Cl between the other main electrodes of transistors T2 and T4 and the power supply line Vdd, a resistor R2 coupled in parallel with a capacitor C2 between the other main electrodes of transistors T3 and Ti and the power supply line Vdd, and a capacitor C3 coupled between the two resistor-capacitor circuits RI, CI and R2, C2. This low pass filter circuitry may obviate the need for the separate low pass filter 402 shown in Figure 3.

As shown in Figure 4, the coupling of the low pass filter to the power supply line Vdd is via a single conmion mode resistor R3 to enable the correct common mode output voltage at 407 and 408. Of course, other forms of low pass filter may be used.

As discussed in, for example, the paper "A precise four-quadrant multiplier with subnanosecond response" by B. Gilbert published in the IEEE Journal of Solid-State Circuits, Volume 3, Issue 4, Dec 1968 Page(s):365 -373, the transistors each effectively * 18 act to multiply the RF signal and the control gate local oscillator signal which from the trigonometric relationship: cos(a)cos(b)= 1 /2cos(a-b) + 1/2 cos(a+b) results in the output signals each containing components at a first frequency which is the sum of the frequencies of RF carrier and the local oscillator and at a second frequency which is the difference between the frequencies of RF carrier and the local oscillator. As explained above, because the two sets of input signals are at the same frequency, the second frequency will be a zero so that difference component of the output of the differential mixer will comprise the modulation signal without the carrier, whereas the sum component will be a signal of twice the frequency of the carrier. Each output from the Gilbert mixer circuit 400 thus provides a signal containing the sum and the difference of the two sets of input signals and subtracting one output from the other as discussed above achieves common mode rejection and provides an output signal that is proportional to the multiplied input signals. In this case, because the two sets of input signals are at the same frequency, the difference signal component is a zero frequency signal component comprising the modulation signal, whereas the sum signal component is a signal of twice the frequency of the carrier which can be removed by filtering as discussed above.

The use of a single differential mixer does not necessarily provide for reliable demodulation because the modulation is generally a combination of phase and amplitude modulation. Accordingly, it may be preferable to use a synchronous IQ demodulator.

Figure 5 shows an example of an NFC communicator that differs from the NFC communicator described above with reference to Figures 3 and 4 in that the demodulation circuitry 132 has an I differential mixer 400 that has inputs 404 and 405 coupled to receive the local oscillator signal from oscillator I 09b and the phase-shifted local oscillator signal from phase shifter 1 09c (which is in anti-phase to its input) and a Q differential mixer 400a that has inputs 404a and 405a coupled to receive a quadrature local oscillator signal from a quadrature phase shifter I 09d of the signal generator 109 and a phase-shifted quadrature local oscillator signal from a phase shifter 1 09e (which is in anti-phase to its input).

There are other ways in which the required signals may be provided. For example, the oscillator (or clock divider) I 09b could provide a differential signal which would be O 19 coupled directly to the I mixer and via a 90 degree Q phase shifter to provide a 90 degree phase shifted differential signal to the Q mixer.

The outputs from junctions ii and J2 of the antenna circuit are each coupled to each of the differential mixers 400 and 400a. Again, the inputs to the differential mixers may be provided with virtual earth input circuits. In this case, each virtual earth input circuit provides two duplicate current outputs. This may be achieved, as shown in phantom lines in Figure 6, simply by coupling the control gate of a further transistor 303' matched to transistor 303 to the control gate of transistor 302 so that the further transistor 303' has its second main electrode coupled to the power supply line Vss and its first main electrode coupled to an output 308a of the current mirror to provide an output current QA or QB. As another possibility, the virtual earth circuit of Figure 6 may be modified as shown Figure 7 so that each transistor 303 and 303' is replaced by a bank of transistors (as shown three transistors 303a, 303b and 303c and 303'a, 303'b and 303'c) that are switchable as pairs (pairs 303a and 303'a, 303b and 303'b, and 303c and 303'c in the example shown) by switches (as shown switches 310a, 310b and 3lOc and 310'a, 310'b and 3 10'c) under the control of, for example, the controller 107 to control the current mirror ratio 310 of the virtual earth circuit. Of course in this example, when a pair is switched in or out in one virtual earth circuit the corresponding pair is switched in or out in the other virtual earth circuit. Such a switched transistor approach may also be applied in this case of the examples shown in Figures 2 and 3 where the virtual earth provides only one output current.

The I differential mixer 400 and the Q differential mixer 400a may each be as shown in Figure 4.

In other respects the NFC communicator shown in Figure 5 is the same as that shown in Figure 3.

The transistors described above will be of the appropriate conductivity type, generally n-conductivity type although it may be possible to use p-conductivity type transistors with appropriate circuit modification. Where FETs are used they will generally be enhancement mode FETs although it may be possible to use depletion mode transistors * 20 with appropriate circuit modification. Also bipolar transistors may be used where appropriate or desired. Other forms of current mirror than that described above may be used to provide virtual earth low impedance inputs where desired.

In embodiments described above, a near field RF communicator has an antenna circuit 120 to receive a modulated radio frequency signal by inductive coupling; and demodulation circuitry 130, 131 or 132 to extract the modulation from a received modulated radio frequency signal inductively coupled to the antenna circuit. The demodulation circuitry has a first modulated signal input 118 to receive a first modulated radio frequency signal from a first output J2 of the antenna circuit and a second modulated signal input 119 to receive a second modulated radio frequency signal from a second output J I of the antenna circuit. The demodulation circuitry extracts modulation from a received modulated RF signal using both the first and second modulated radio frequency signals. The demodulation circuitry may have a differential amplifier and a demodulator or may have one or more differential mixers.

As described above the inputs to the differential amplifier or mixer are taken from opposite ends of the antenna coil. As another possibility, the antenna coil may have a grounded centre tap so that the signals at J 1 and J2 are each equal to half the differential and in anti-phase.

Although particular types of current mirrors have been described above to provide the virtual earth, other known forms of current mirror may be used.

As described above, the high Z (impedance) inputs from the antenna circuitry are provided by capacitors 121 and 122. However, other forms of matched high Z couplings may be used. As an example, the high Z coupling could be a much higher value capacitor (for DC isolation) in series with a resistor (for V to I conversion).

As described above the capacitors are external to the integrated circuit. It may however be possible for the capacitors to be integrated, for example thick oxide capacitors may be integrated onto the circuit. * 21

The examples described above are NFC communicators or NFC communications-enabled devices. The present invention may however also be applied in any near field RF communicator where there is a requirement to receive and demodulate an RF signal, for example in an RFID transceiver or RFID transponder.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims. * 22

Claims

1. A near field RF communicator comprising: an antenna circuit to receive a modulated radio frequency signal by inductive coupling; and demodulation circuitry to extract the modulation from a received modulated radio frequency signal inductively coupled to the antenna circuit, wherein the demodulation circuitry has a first modulated signal input to receive a first modulated radio frequency signal from a first output of the antenna circuit and a second modulated signal input to receive a second modulated radio frequency signal from a second output of the antenna circuit and is operable to extract modulation from a received modulated RF signal using both the first and second modulated radio frequency signals.

2. A near field RF communicator according to claim 1, wherein the demodulation circuitry comprises: a differential amplifier having a non-inverting input, an inverting input and an output; and a demodulator coupled to the output of the differential amplifier, the non-inverting input being coupled to the first output of the antenna circuit and the inverting input being coupled to the second output of the antenna circuit.

3. A near field RF communicator according to claim 2, wherein the non-inverting and inverting inputs of the differential amplifier comprise virtual earth inputs.

4. A near field RF communicator according to claim 2, wherein the non-inverting and inverting inputs of the differential amplifier comprise current mirrors.

5. A near field RF communicator according to claim 4, wherein each current mirror comprises a diode-coupled transistor having a main electrode coupled to the corresponding one of the non-inverting and inverting inputs and a further transistor having its control gate coupled to the control gate of the diode-coupled transistor such that, in operation, a current at the input of the differential amplifier is mirrored by the further transistor to provide an input current to the differential amplifier.

6. A near field RF communicator according to claim 5, wherein the diode-coupled transistor and the further transistor are field effect transistors.

7. A near field RF communicator according to claim I, wherein the demodulation circuitry comprises a differential mixer.

8. A near field RF communicator according to claim 1, wherein the modulated radio frequency signals have a carrier at a first frequency, the demodulation circuitry comprises a differential mixer having the first and second modulated signal inputs and coupled also to receive an oscillator signal having a second frequency, and wherein the differential mixer is operable to mix the first and second modulated radio frequency signals with the oscillator signal to provide sum components at a third frequency which is the sum of the first and second frequencies and difference components at a fourth frequency which is the difference of the first and second frequencies and wherein the demodulation circuitry is operable to extract the modulation on the basis of the difference components.

9. A near field RF communicator according to claim 1, wherein the modulated radio frequency signals have a carrier at a first frequency, the demodulation circuitry comprises a differential mixer having the first and second modulated signal inputs and coupled also to receive an oscillator signal having a second frequency the same as the first frequency, and wherein the differential mixer is operable to mix the first and second modulated radio frequency signals with the oscillator signal to provide sum components at a third frequency which is the twice the first frequency and difference components at a fourth frequency which is zero and wherein the demodulation circuitry is operable to extract the modulation on the basis of the difference components.

10. A near field RF communicator according to claim 1, wherein the modulated radio frequency signals have a carrier at a first frequency, the demodulation circuitry comprises a differential mixer having the first and second modulated signal inputs and first and second oscillator inputs coupled to receive, respectively, a first oscillator signal at the first frequency and a second oscillator signal at the first frequency but phase-shifted with respect to the first oscillator signal, and wherein the differential mixer is * 24 operable to mix the first and second modulated radio frequency signals and the first and second oscillator signals to provide two output signals each comprising sum components at a further frequency which is the twice the first frequency and difference components at a fourth frequency which is zero and wherein the demodulation circuitry comprises a subtractor to subtract the two output signals to extract the modulation.

11. A near field RF communicator according to claim 10, wherein the differential mixer has first and second transistors having corresponding ones of their main electrodes coupled to the first modulated signal input and third and fourth transistors having corresponding ones of their main electrodes coupled to the second modulated signal input, the control gates of the first and fourth transistors being coupled to the first oscillator input and the control gates of the second and third transistors being coupled to the second oscillator input, the other main electrodes of the second and fourth transistors being coupled to one of the two outputs of the differential mixer and the other main electrodes of the first and third transistors being coupled to the other of the two outputs of the differential mixer.

12. A near field RF communicator according to claim 11, wherein the

transistors are field effect transistors.

13. A near field RF communicator according to any of claims 7 to 12, wherein the demodulation circuitry comprises two such differential mixers, one of the first and second oscillator inputs of one of the two such differential mixers being coupled to receive an oscillator signal that is phase quadrature with the oscillator signal coupled to the corresponding oscillator input of the two such differential mixers to provide a IQ demodulator.

14. A near field RF communicator according to any of claims 7 to 13, wherein the modulated signal inputs of each differential mixer comprise virtual earth inputs.

15. A near field RF communicator according to any of claims 7 to 13, wherein modulated signal inputs of each differential mixer comprise current mirrors.

16. A near field RF communicator according to claim 15, wherein the current mirrors each comprises a diode-coupled transistor having a main electrode coupled to the corresponding modulated signal input and a further transistor having its control gate coupled to the control gate of the diode-coupled transistor such that, in operation, a current at the modulated signal input is mirrored by the further transistor to provide an input current to the differential mixer.

17. A near field RF communicator according to claim 16, wherein the diode-coupled transistor and the further transistor are field effect transistors.

18. A near field RF communicator according to any of claims 7 to 17, wherein a phase locker is provided to lock together the phase of the carrier of a received modulated radio frequency signal and the oscillator signal.

19. A near field RF communicator according to any to any of claims 7 to 18, wherein the demodulation circuitry has a low pass filter to filter out the third frequency.

20. A near field RF communicator according to any of the preceding claims, further comprising a modulator operable to modulate a radio frequency signal in accordance with data to enable communication of data by the near field RF communicator.

21. A near field RF communicator according to any of claims 7 to 19, wherein a signal generator is provided to provide the oscillator signal, wherein the signal generator also provides a modulator operable to modulate a radio frequency signal in accordance with data to enable communication of data by the near field RF communicator.

22. A near field RF communicator according to any of claims 1 to 19, wherein further comprises a data store operable to store data, a modulator operable to modulate a radio frequency signal in accordance with data, and a controller operable both to initiate near field radio frequency communication with another near field RF communicator and to respond to near field radio frequency communication initiated by another near field RF communicator. * 26

23. A near field RF communicator according to any of the preceding claims, further comprising a power denver operable to derive power from a radio frequency signal inductively coupled to the coupler.

24. A near field RF communicator according to any of the preceding claims, wherein a coil of the antenna circuit is coupled across the first and second outputs of the antenna circuit.

25. A near field RF communicator according to any of the preceding claims, wherein the near field RF communicator is an NFC communicator.

26. A device comprising a near field RF communicator in accordance with any of the preceding claims.