Patent Document

TECHNICAL FIELD 
     This invention relates to a transmitter and, in particular, to a transmitter suitable for transmitting signals in a near-field communication (NFC) frequency band. 
     BACKGROUND OF THE INVENTION 
     Mobile communication devices, such as mobile telephones, smart phones, personal digital assistants (PDA) and laptop computers are often provided with means for communicating wirelessly with other such devices, and with other communication devices. 
     One such means of communicating wirelessly uses near field communication (NFC). Near field communication is the name given to the communication of data over a distance of less than around 5 cm. NFC operates at a frequency of 13.56 MHz, and allows data to be transferred at rates from 106 kbit/s to 848 kbit/s. Data is transmitted between an NFC initiator and an NFC target. The initiator (often referred to as a reader) is a powered device that emits a radio frequency (RF) field. The target need not be powered, and typically takes the form of a key fob, a card or a mobile telephone. When an NFC target is moved into the RF field emitted by the initiator, the target is powered by the RF field, and emits a signal which is detected by the initiator. 
     An example of how NFC technology is used is in a security system for securing access to a restricted area or building. An NFC initiator is installed in a unit positioned near to, say, a restricted entrance or door. The initiator generates a radio frequency (RF) field. When a target, which may take the form of a key card or a key fob, is moved into the RF field generated by the initiator, the target, which is powered by the RF field, emits a signal which is detected by the initiator unit. If the security system recognises the returned signal as one from a card authorised to access the entrance or door, then it sends a signal to another part of the security system to grant access to the restricted area, for example by unlocking the door or deactivating an alarm system. 
     In a known device, an NFC target is installed in a mobile telephone. Therefore, when the mobile telephone is moved near to an NFC initiator, such that the target is moved within the RF field emitted by the initiator, the target is detected and emits a return signal to the initiator. In another known device, an NFC initiator is installed in a mobile telephone, and is capable of detecting NFC targets that are within an RF field emitted therefrom. Of course, a mobile telephone may be provided with both an NFC target and an NFC initiator. 
     An NFC initiator includes an antenna which must be driven in order for it to emit signals. An existing NFC antenna driver uses switching amplifiers to generate signals at the required frequency of 13.56 MHz. However, such known drivers generate harmonics at integer multiples of the desired frequency, in addition to the signal at the desired frequency. The higher frequency harmonics have frequencies equal to or similar to those of signals used by mobile telephone receivers. Therefore, if such a known NFC transmitter is installed in a mobile telephone, the higher frequency harmonics generated in addition to the signal at the desired frequency are likely to interfere with signals received by the mobile telephone receiver. If the mobile telephone is also installed with an NFC receiver, then signals transmitted by the mobile telephone, which are of the same or similar frequency as those received by the NFC receiver, are likely to interfere with received NFC signals. Additionally or alternatively, cross modulation may occur between signals transmitted by the mobile telephone and signals received or transmitted by the NFC receiver and/or transmitter. 
     Interference between the NFC signals and the mobile telephone signals can cause undesirable noise, for example during a telephone conversation. If the interference is too great, then a mobile telephone call may be dropped. Similarly, if the mobile telephone signal interferes with the NFC receiver (that is the NFC target), then the NFC initiator may not be unable to detect a response from an NFC target. 
     Furthermore, particular harmonics of signals transmitted by an NFC transmitter, in particular the sixth, seventh and eighth harmonics, have frequencies falling within the frequency modulation (FM) frequency band. Therefore, mobile telephone devices that have NFC transmitters and FM receivers built in might suffer interference problems when transmitted NFC signals interfere with received FM signals. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention a transmitter for transmitting signals in a near-field communication (NFC) frequency band comprises an antenna; a first current source for generating a first current to excite the antenna; a first detector for detecting a first voltage at an output of the first current source; and means for reducing the current output by the first current source if the detected first voltage exceeds a first predefined threshold. An advantage of the transmitter including means for reducing the current is that distortion in the transmitted signal can be limited, and the risk of components in the transmitter being damaged can be reduced. 
     According to a second aspect of the present invention, an integrated circuit comprises a transmitter according to the first aspect. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  is a circuit diagram showing part of a circuit having an antenna and an NFC driver; and 
         FIG. 2  is a circuit diagram showing a circuit having an antenna and two NFC drivers. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings,  FIG. 1  shows a circuit  10  which forms a near field communication (NFC) driver. The circuit  10  forms part of a larger circuit, shown by dashed line  100 , and which will be discussed with reference to  FIG. 2 . The circuit  10  includes an antenna  12  which is formed of a plurality of coil turns. In the drawings of this specification, the antenna  12  is shown to have four turns. However, one skilled in the field of antenna design would appreciate that the antenna  12  may be formed of a coil having any number of turns. 
     The antenna  12  has a first end  14  and a second end  16 . The ends  14 ,  16  of the antenna  12  are connected to circuitry which will be discussed in greater detail below. 
     A device in which the circuit  10  is installed is capable of operating in a transmit mode and in a receive mode. In the receive mode, the antenna  12  receives a signal in an NFC frequency band, and the signal is demodulated by demodulation means (not shown) via circuitry (not shown). This invention relates to the circuit  10  in a transmit mode, in which a signal is transmitted from the antenna  12 . 
     A carrier signal  18 , having a frequency of 13.56 MHz, is generated by a signal generator  20 . In transmit mode, the carrier signal  18  is modulated with data to be transmitted by the antenna. 
     The carrier signal  18  is input into a variable amplifier  22 . An amplified version of the carrier signal  18  is output from the variable amplifier  22  via a first output  24  and a second output  26 . The first output  24  is an amplified version of the original drive (i.e. input) signal, and the second output  26  is an output driver bias, which is proportional to the drive signal level. Those skilled in the art will appreciate that the output driver bias  26  is not important in the context of the present invention, and is shown in  FIG. 1  for completeness. The signal output by the first and second outputs  24 ,  26  of the variable amplifier  22  drives a current source  28 . In this embodiment, the current source  28  is an N-type metal-oxide-semiconductor field-effect transistor (NMOS) device. However, one skilled in the field of circuit design will appreciate that a bipolar (junction) transistor current source device could alternatively be used. 
     A first output  30  from the current source  28  is connected to a negative terminal  32  of a power supply (not shown), via a resistor  34 . A second output  36  of the current source  28  is connected to a positive terminal  38  of the power supply (not shown), via a load inductor  40 . The second output  36  of the current source  28  is also connected, via a node  42 , to the antenna  12 , via circuitry which will now be discussed. 
     A capacitor  44  is connected in series with a resistor  46  between the node  42  and the first end  14  of the antenna  12 . A capacitor  48  is connected in parallel with the antenna  12 , between the capacitor  44  and the resistor  46 . A resistor  50  is connected between the capacitor  48  and the second end  16  of the antenna  12 . A node  52  is provided between the capacitor  48  and the resistor  50 . Two further capacitors  54  and  56  are connected between the node  52  and a node  58  which, itself, is located between the node  42  and the capacitor  44 . The capacitor  56  is a shunt capacitor which can be used to tune the antenna  12 . 
     A node  58 ′ located between the capacitor  54  and the capacitor  56  forms a point of connection to a second part (not shown) of the circuit  100 , which will be discussed in connection with  FIG. 2 . 
     The second output  36  of the current source  28  is also connected to the variable amplifier  22  via a feedback loop which will now be described in greater detail. The feedback loop is connected between the output  36  of the current source  28  and the load inductor  40  at a node  60 . An attenuator  62  and a diode  64  are connected in series between the node  60  and an inverting input  66  of an operational amplifier  70 . An RF level indicator  71  provides a DC indication of the voltage of the signal at the node  42 , and forms the inverting input  66  of the operational amplifier  70 . The RF level indicator  71  is also capable of tracking the AM component of the signal  18 , and can function as an AM output in a manner known to those skilled in the art. A node  65  positioned between the diode  64  and the inverting input  66  of the operational amplifier  70  is connected to ground  67  via a reservoir capacitor  69 . In an alternative embodiment, the system can be connected to a polar receiving system, which allows the system to function on amplitude and phase independently. In that alternative embodiment, for optimum performance, those skilled in the art will appreciate that there is a need to recover the phase of the received signal as well as the amplitude. 
     A threshold reference  72  (a DC signal) is input into the operational amplifier  70  via a non-inverting input  68 . The purpose of the threshold reference  72  will be discussed with reference to  FIG. 2 . An output  74  of the operational amplifier  70  is input into the variable amplifier  22 . The output  74  of the operational amplifier  70  determines the amount by which the signal  18  is amplified. 
     Turning to  FIG. 2 , it will be apparent that the circuit  10  shown in  FIG. 1  forms the left-hand side  10  of the larger circuit  100  of  FIG. 2 . The right-hand side  10 ′ of the circuit  100  is effectively the circuit  10  of  FIG. 1  mirrored about the antenna  12 . Features of the left-hand side  10  of the circuit  100  in  FIG. 2  are provided with reference numerals matching those used in  FIG. 1 . Features of the right-hand side  10 ′ of the circuit  100  are provided with apostrophised reference numerals corresponding to those used in the left-hand side  10  of the circuit  100  and in  FIG. 1 . In other words, the general reference numeral given to the right-hand side of the circuit  100  is  10 ′, the variable amplifier in the right-hand side of the circuit  100  is given the reference numeral  22 ′, and so on. 
     The signal  18  generated by the signal generator  20 , is input into the variable amplifier  22 ′ as well as the variable amplifier  22 . 
     Referring particularly to the right-hand side  10 ′ of the circuit  100  of  FIG. 2 , a second output  36 ′ from a current source  28 ′ is connected to a positive terminal  38 ′ of a power supply (not shown) via a load inductor  40 ′. The second output  36 ′ from the current source  28 ′ is also connected to the second end  16  of the antenna  12  via a node  42 ′ between the current source  28 ′ and the load inductor  40 ′. The connection between the node  42 ′ and the second end  16  of the antenna  12  is connected via a node  58 ′, positioned between the capacitor  54  and the capacitor  56 . 
     A first RF output  76  and a second RF output  76 ′ are connected to the nodes  58  and  58 ′ respectively, either side of the antenna  12 . The RF outputs  76 ,  76 ′ are used by the device  100  in receive mode. Briefly, in receive mode, the signal  18  is not modulated with data. Instead, the signal  18  a continuous wave signal and, using the signal  18 , the antenna  12  emits a generally continuous RF field having a frequency of 13.56 MHz. When a target (such as an NFC tag in a billboard or poster) interrupts the field, the field emitted by the antenna  12  powers the target, the target&#39;s presence is detected by the RF outputs  76 ,  76 ′, and an exchange of data between the target and the receiver  100  takes place. This data transfer mechanism is known as load modulation. The target switches the impedance it presents to the initiator (the mobile phone&#39;s NFC receiver in this example) at a frequency of 847 KHz (that is 13.56 MHz/16). The 847 KHz switching signal is then modulated with the data that the target is sending to the mobile phone&#39;s NFC receiver. The data is received by the RF outputs  76 ,  76 ′ and input, via connections  82 ,  82 ′, into receiver front end circuitry  84  where it is processed. The receiver front-end circuitry  84  will not be discussed in detail, but will be familiar to a person skilled in the relevant field. The signal  18  is also input into the receiver front-end circuitry  84  via input  86 . 
     The first outputs  30 ,  30 ′ from the respective current sources  28 ,  28 ′ are summed in a summation unit  78 . An output  80  of the summation unit  78  is used in a negative feedback loop (not shown) to linearise the output current sources, in a manner that will be familiar to those skilled in the art. 
     The operation of the NFC driver  100  will now be described. 
     As indicated above, the NFC driver  100  includes the two parts  10 ,  10 ′ of the circuit, which are substantially identical, and mirrored about the antenna  12 . 
     A carrier signal  18 , having a frequency of 13.56 MHz is generated by the signal generator  20 , and input into the variable amplifiers  22  and  22 ′. The driver  100  is configured such that the variable amplifiers  22 ,  22 ′ operate 180 degrees out of phase with one another with respect to the carrier signal. In other words, the variable amplifier  22  outputs a drive signal  24  which is 180 degrees out of phase with the drive signal  24 ′ output by the variable amplifier  22 ′. Thus, the variable amplifiers  22 ,  22 ′ operate in a “push-pull” configuration. The signals input into each of the variable amplifiers need to be out of phase with one another to ensure that a balanced system is maintained. Considering, first, the circuit  10 , the drive signal  24  drives the current source  28 . The current source  28  powers the antenna  12  with its output  36 . The output  36  from the current source  28  is also input into the inverting input  66  of the operational amplifier  70 . The non-inverting input  68  of the operational amplifier  70  receives a threshold reference input  72 . The operational amplifier  70  compares the inverting input  66  with the threshold reference input  72  and, if the inverting input is greater than the threshold reference input, then the signal  74  output by the operational amplifier acts to adjust the variable amplifier  22  so as to attenuate the drive signal  24 . Thus, the output  36  from the current source  28  is kept below the threshold reference  72 . 
     In addition to delivering the carrier signal  18  to the antenna  12 , the output  36  from the current source is also input into the load inductor  40 . The load inductor  40  is charged by the output  36  from the current source  28 . 
     Although a signal  18  is provided to both of the current sources  28 ,  28 ′, only one of the current sources operates at any one time. In other words, for a first predetermined duration, the carrier signal  18  drives the variable amplifier  22  in the left hand side  10  of the circuit. After the first predetermined duration, the signal  18  drives the variable amplifier  22 ′ in the right hand side  10 ′ of the circuit for a second predetermined duration. The first and second predetermined durations are preferably substantially the same. 
     Thus, during the second predetermined duration, an output  24 ′ from the variable amplifier  22 ′ drives the current source  28 ′ which, in turn outputs current via an output  36 ′ to the antenna  12 , to an inverting input  66 ′ of an operational amplifier  70 ′, and to a load inductor  40 ′. 
     During the second predetermined duration, the output  36  stops providing power to the antenna  12 , and stops charging the load inductor  40 . Instead, the load inductor  40  begins to discharge its stored current, which is input into the antenna  12 , and through the current source  28 ′, to a terminal of the power supply  32 ′. After the second predetermined duration, the current source  28  on the left hand circuit  10  again generates an output  36 , and so on. Each predetermined duration referred to above is a single half-cycle of the 13.56 MHz signal. 
     To understand the effect of the circuit  100 , it is useful to consider an example. The current supplied to the antenna  12  from each of the load inductors  40 ,  40 ′ will, at its maximum, be approximately equal to the current supplied by the current sources  28 ,  28 ′. For example, if the power supplied to each current source by respective power supplies  32 ,  38  and  32 ′,  38 ′ gives rise to a current output I from outputs  36 ,  36 ′, then the current I will be supplied to the antenna  12  and to each of the load inductors  40 ,  40 ′. This gives rise to a voltage V across the nodes  42 ,  42 ′ and  58 ,  58 ′. When the load inductors  40 ,  40 ′ are fully charged, they will each store a current I and, when each load inductor discharges, it will supply a maximum current I to the antenna. It is important to ensure that the discharging load inductors  40 ,  40 ′ do not cause the voltages across nodes  58  and  58 ′ to increase beyond 2V. If the voltage does increase beyond 2V, there is a risk that components within the circuit will be damaged. Furthermore, a large increase in voltage can cause interference or distortion over a range of frequencies emitted by the antenna  12 . The voltage V is approximately equal to a supply voltage V dd  (that is the voltage supplied by the power supply  32 ,  32 ′,  38 ,  38 ′) minus the voltage used by the current source  28 ,  28 ′. The supply voltage V dd  is the voltage of the battery (3.6 v), and the voltage used by the current source is approximately 0.5 v, so V=3.1 v. The invention is, therefore, arranged to adjust the variable amplifier  22  so that the voltage across the nodes  58  and  58 ′ does not exceed a maximum voltage, V max =V dd +V(=3.6 v+3.1 v=6.7 v). If the voltage across the nodes  58  and  58 ′ increases beyond V max  (6.7 v in this example) by a small amount then, initially, the transmitted signals from the antenna  12  will experience increased distortion. This could lead to regulatory failure of the transmission mask. If the distortion is significant, then the target may fail to receive the data transmitted in the signal. 
     If the voltage across the nodes  58  and  58 ′ increases significantly beyond V max , the driver is likely to fail, and there becomes a significant risk of irreparable damage occurring to the receiver. 
     So far, the invention has been described in terms of individual embodiments. However, one skilled in the art will appreciate that various embodiments of the invention, or features from one or more embodiments, may be combined as required. It will be appreciated that various modifications may be made to these embodiments without departing from the scope of the invention, which is defined by the appended claims. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Technology Category: 5