Abstract:
An integrated electronic radio-frequency transceiver circuit, including: at least one terminal intended to receive a signal to be transmitted or to transmit a received signal; at least one planar antenna, with a settable resonance frequency; at least one bidirectional coupler having a primary line interposed between the terminal and the antenna and having the respective terminals of a secondary line providing data representative of the transmitted power and of the power reflected on the primary line side; at least one detector of the transmitted power and of the reflected power; and a circuit for selecting the resonance frequency of the antenna according to the ratio between the transmitted power and the reflected power.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 08/51486, filed on Mar. 7, 2008, entitled “CIRCUIT INTEGRATING A TUNABLE ANTENNA WITH A STANDING WAVE RATE CORRECTION,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to electronic circuits and, more specifically, to radio-frequency transceiver circuits, intended for very high frequencies (greater than 100 MHz). 
     2. Discussion of the Related Art 
     A problem which is particularly critical for high frequencies is that the system environment has a direct influence upon the impedance of the antenna. As a result, even for an antenna having good nominal characteristics in terms of ratio of the transmitted power to the reflected power (RL—Return Loss), this ratio may be disturbed by the environment, for example, when a user&#39;s hand comes close to the antenna. Now, high frequency ranges are widely used in mobile applications (cell phone, wireless connection of a portable computer, etc.) so that the effect of the human body (or another disturbing element) on the impedance of the antenna is not negligible. 
     Such modifications of the antenna&#39;s impedance have led, up to now, to interposing impedance matching circuits. 
       FIG. 1  is a block diagram illustrating a usual impedance matching solution. A transmit circuit  1  (SEND) is connected, via an integrated circuit  2 , to a transceiver antenna  3 . Circuit  2  comprises an adjustable impedance matching circuit  21  (MATCH). The impedance adjustment is performed by means of a first coupler  22  of distributed type, interposed between transmit circuit  1  and impedance matching circuit  21 , and a second coupler  23 , interposed between impedance matching element  21  and antenna  3 . Coupler  22  provides, on an output terminal ISO of its secondary line, data relative to the power reflected by the antenna. Coupler  23  provides data relative to the power transmitted to the antenna to a detector  25  (DETECT) to reduce the insertion losses of circuit  21 . The two detectors  24  and  25  provide the measured data to a control circuit  26  (CTRL) which adjusts the parameters of impedance matching circuit  21  according to a reference value (for example, 50 ohms) to reduce the insertion losses of circuit  21  and to improve the impedance matching at the level of transmit circuit  1 . In the shown example, the case of a twin-wire connection between circuits  26  and  21 , transmitting voltage data enabling matching of circuit  21 , is considered. The matching circuit most often is an inductive and capacitive circuit (LC) having its capacitive elements settable by circuit  26 . 
     When the antenna is disturbed by an external element, the modification of its input impedance is detected in the form of a variation of the transmitted and/or reflected power, which enables circuit  26  to modify the impedance of circuit  21  to maintain a matching supposed to be optimal between circuit  1  and antenna  3 . 
     However, matching circuits generally have narrow operation bands, that is, they must be selected according to the frequency range for which the transceiver circuit is intended. 
     Further, the presence of a matching circuit adds losses in the transmission chain by the capacitive and inductive elements in series between the output of transmit circuit  1  and antenna  3 . 
     Moreover, the power capacity is altered for the components forming circuit  21  when the mismatch is significant. 
     SUMMARY OF THE INVENTION 
     It would be desirable to have a transceiver circuit which operates in a wide frequency range. 
     It would also be desirable to have a transceiver circuit with a decreased sensitivity to the outer environment. 
     It would also be desirable to have a transceiver circuit in which line losses are decreased. 
     To achieve all or part of these objects as well as others, at least one embodiment of the present invention provides an integrated electronic radio-frequency transceiver circuit, comprising: 
     at least one terminal intended to receive a signal to be transmitted or to transmit a received signal; 
     at least one planar antenna, with a settable resonance frequency; 
     at least one bidirectional coupler having a primary line interposed between said terminal and the antenna and having the respective terminals of a secondary line providing data representative of the transmitted power and of the power reflected on the primary line side; 
     at least one detector of the transmitted power and of the reflected power; and 
     a circuit for selecting the resonance frequency of the antenna according to the ratio between the transmitted power and the reflected power. 
     An embodiment provides such a circuit having no impedance matching circuit. 
     According to an embodiment, the antenna with a settable frequency comprises one or several miniature electromechanical switches interposed between conductive elements. 
     According to an embodiment, the antenna with a variable frequency comprises one or several elements of settable capacitance. 
     According to an embodiment, a radio-frequency transceiver circuit is provided. 
     An embodiment provides such a device having no impedance matching circuit. 
     The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 , previously described, is intended to illustrate the state of the art and the problem to solve; 
         FIG. 2  is a block diagram of an example of a radio-frequency transceiver chain; 
         FIG. 3  is a block diagram of another radio-frequency transceiver chain example; 
         FIG. 4  is a block diagram of still another radio-frequency transceiver chain example; 
         FIG. 5  is a block diagram of an antenna circuit according to an embodiment of the present invention; 
         FIG. 6  schematically shows an embodiment of an adjustable antenna; 
         FIG. 7  shows another embodiment of an adjustable antenna; and 
         FIG. 8  shows still another embodiment of an adjustable antenna. 
     
    
    
     DETAILED DESCRIPTION 
     The same elements have been designated with the same reference numerals in the different drawings. 
     For clarity, only those elements which are useful to the understanding of the present invention have been shown and will be described. In particular, the circuits for generating the signals to be transmitted and processing the received signals have not been detailed, the present invention being compatible with usual circuits. 
       FIG. 2  is a block diagram of an example of a radio-frequency transceiver chain of the type to which the present invention applies. 
     On the transmit side, a signal Tx to be transmitted proceeds through an amplifier  31  (PA) before being processed by a band-pass filter  32  (BPF) for transmission by an antenna  41  or  42 . A so-called diversified switch  40  is in charge of routing the signal to be transmitted from filter  32  to antenna  41  or  42 . On the receive side, switch  40  routes a received signal from antenna  41  or  42  to a band-pass filter  33 . Filter  33  is, in receive mode, followed by a balun transformer  34  (BALUN) and of a low-noise amplifier  35  (LNA) providing a signal Rx to processing circuits. The diagram of  FIG. 2  for example corresponds to a Bluetooth-type transceiver architecture. 
       FIG. 3  illustrates another example of application of a radio-frequency transceiver chain. In this case, signal Tx to be transmitted crosses a transmit amplifier  31 , then a switch  45  (Rx/Tx) in charge of routing the received signal with respect to the transmitted signal. Switch  45  is followed by a band-pass filter  36 , common to the transmission and to the reception, connected to a common antenna  43 . In receive mode, a signal originating from antenna  43  and having passed through filter  36  passes through switch  45 , then a mode-switching transformer  34  and an amplifier  35 , to provide signal Rx. The embodiment of  FIG. 3 , for example, corresponds to a transceiver circuit of ultra wide band type (UWB). 
       FIG. 4  illustrates another example of application in which an antenna  44  is shared by several transceiver circuits by means of an antenna switch  46 . For example, paths of a first group  37 , each comprising a band-pass filter  33  and a low-noise amplifier  35 , are intended for the reception of mobile telephony signals in different frequency bands. Paths of a second group  38 , each comprising a low-pass filter  39  and a transmit amplifier  31 , are intended for the transmission of mobile telephony signals in different frequency bands. A third path comprises a duplexer  47  (typically of band-pass filter type) between an amplifier  31  of transmission and an amplifier  35  of reception of signals to be transmitted and of received signals. This path, for example, corresponds to data transmissions. 
     In all the above applications, a disturbance in the environment of the antenna risks generating significant losses in the transmission or the reception under the effect of a mismatch. 
       FIG. 5  is a block diagram of an antenna circuit  5  according to an embodiment. This circuit integrates a planar antenna  51  having its access  511  connected to a first end  522  of a main line of a coupler  52  with distributed lines, the other end  521  of this main line of the coupler being intended to be connected to radio-frequency transceiver circuits  1  (E/R). The two ends  523  and  524  of a secondary (or coupled) line of coupler  52  are respectively connected to detection circuits  53  and  54  (DETECT) having respective outputs connected to an integrated circuit  56  (CTRL) for controlling an adjustment of the tuning frequency of antenna  51 . Antenna circuit  5  is for example intended to form antenna  41 ,  42 ,  43 , or  44  of the circuits of  FIGS. 2 to 4 , head  1  being then supposed to contain the different filters, baluns, antenna switches, etc. 
     Coupler  52  is a bidirectional coupler and is thus capable, for example in transmit mode, of providing on access  523  (CPLD) data relative to the transmitted power P F  between accesses  521  (IN) and  522  (OUT) of the coupler and, on the other access  524  (ISO) of the coupled line, data relative to the power P R  reflected by the antenna. The exploitation of both data, measured by circuits  53  and  54  and provided to circuit  56 , enables determining the ratio between the reflected and transmitted powers, and accordingly modifying the resonance frequency of antenna  51 . 
     Coupler  52  may also be used, via detector  53 , to provide data (connection  531 ) to circuit  1  to adjust the transmit power of the amplifier comprised in the circuit, by providing it with data relative to the transmitted power. 
     Coupler  52  preferably is a wide-band bidirectional coupler able to operate over the entire frequency band for which circuit  5  is intended. It further exhibits a good directivity, to make out the transmitted power from the reflected power. For a bidirectional coupler, it is considered that a good directivity corresponds to a power difference between ports CPLD and ISO of at least 25 dB while all ports are loaded with 50-ohm impedances. 
     As compared with the insertion of impedance matching circuits, circuits  5  decreases insertion losses since there now only is one coupler between circuit  1  and antenna  51 . Low insertion losses correspond to losses smaller than 1 dB and, preferably, smaller than 0.5 dB. 
     The frequency adjustment of antenna  51  by means of circuit  56  is performed under control of signals  56   1  to  56   n  (n≧1) provided by circuit  56 . Number n of signals and their type depends on the provided type of adjustable antenna. 
       FIG. 6  shows a first example of a planar antenna with an adjustable resonance frequency. It is a wire antenna formed of a conductive serpentine  60 , deposited on an insulating substrate (not shown). Serpentine  60  may be interrupted, for example, in two places (switches  61  and  62 ). The opening of one of the switches causes a shortening of the antenna length, and thus a change in its tuning frequency from its access  511 . For example, the switches are of micro-electromechanical type (MEMS) and receive, for example, D.C. control signals  56   1  to  56   2  from circuit  56 . 
       FIG. 7  schematically shows a second example of a planar antenna formed of a so-called slot antenna. A planar conductive section  71  is formed in a slot or window  72  made in a ground plane  73  on an insulating substrate (not shown). The slot has an approximate T shape and section  71  extends in the entire vertical branch of the T. In this example, two switches  75  and  76 , respectively  77  and  78 , are provided on either side of the vertical branch of the T to connect the two edges of conductive plane  73  to two locations on the horizontal portions of the T. Here again, a closing of one of switches  75  to  78  modifies the resonance frequency of the antenna. The switches, for example miniature electromechanical switches, are individually controlled by signals  56   1  to  56   4 . 
       FIG. 8  is a simplified perspective view of a third embodiment of a PIFA-type adjustable antenna. A planar conductive section  81  is formed on an insulating layer  82  above a ground plane  83 . One end of strip  81 , intended to form access  511  of the antenna, is for example brought under ground plane  83  by a conductive via  84  crossing a window  834  of the ground plane. A connection  85  to a capacitive element of variable capacitance  86  (schematically illustrated in dotted lines) is provided at the other end of section  81 . Variable-capacitance element  86  may be a Varicap diode, a switched capacitor network, a PIN diode, etc. 
     The discussed embodiments enable avoiding the use of an impedance matching network. 
     Further, a same antenna may be used for several frequencies and for several transmission types (for example, for several mobile telephony transmission-reception bands). 
     For a matching of the antenna according to reference values provided by the transceiver head (for example for a frequency band switching), control circuit  56  receives one or several reference signals (connection  57  in dotted lines,  FIG. 5 ) enabling it to adjust the exploited reference value according to the results provided by detectors  53  and  54 . Thus, it is possible not only to control the antenna frequency to maintain a ratio between the transmitted power and the reflected power for a given frequency band, but also to modify the tuning frequency according to the application. 
     An adaptable voltage standing wave ratio (VSWR) correction antenna has thus been obtained. 
     Various embodiments have been described. Different alterations, modifications and improvements are within the abilities of those skilled in the art, especially as to the selection of the type of adjustable antenna according, for example, to the control circuit available or that can easily be formed in the circuit. Further, the practical implementation of the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.