Abstract:
A mixer-amplifier of an RF signal including at least an amplifier circuit and a mixing circuit controlled at a local oscillator frequency, for amplifying a signal applied on at least one input terminal and converting a first frequency of this signal into a second, lower, frequency, and including a reverse feedback loop switched at the local oscillator frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 08/56298, filed on Sep. 18, 2008, entitled MIXER-AMPLIFIER FOR AN RF RECEIVE CHAIN,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to RF receive chains and, more specifically, to chains provided with a so-called low-noise amplifier (LNA) associated with a mixer to convert an RF signal at a relatively high frequency into a signal at a relatively low intermediary frequency with respect to the frequency of the received signal, to recover the envelope of the RF modulation. 
         [0004]    The present invention more specifically applies to mobile telephony receive chains. 
         [0005]    2. Discussion of the Related Art 
         [0006]      FIG. 1  is a block diagram of a conventional example of an RF chain. AN RF signal received by an antenna  1  is processed by a low-noise amplifier  2  (LNA), then by a mixer  3  receiving an intermediary frequency and switching at a so-called local oscillator frequency OL. The output of mixer  3  is intended to be processed by receive circuits  4  (Rx). Most often, filters  5  and  6  respectively at the frequency of the received signal and at the intermediary frequency are provided upstream and downstream of the series-association of amplifier  2  and of mixer  3 . Further, other circuits may be interposed between antenna  1  and filter  5 . These circuits have been symbolized by an interruption  8  in the path of the signal and may comprise path separators, couplers, impedance matching circuits, etc. 
         [0007]    To improve the stability and the frequency matching, as well as the linearity of the reception chain, a reverse feedback  21  is generally performed on low-noise amplifier  2  by feeding back on its input a portion of its output signal. This reverse feedback loop  21  may comprise a filter  22  at the frequency of the RF signal. Such a reverse feedback enables doing away with the use of inductive elements used in impedance-matching circuits, but adversely affects the gain as well as the signal-to-noise ratio of the chain. 
         [0008]    Document WO-A-2007/057324 describes an example of an RF reception system in which a signal sampled from the output of a mixer is applied to a low-noise amplifier. This amounts to permanently applying a reverse feedback signal. 
         [0009]    Document US-A-2006/0208791 describes another example of an RF receiver using a switched-capacitor filter. 
       SUMMARY OF THE INVENTION 
       [0010]    It would be desirable to have an RF receive chain which takes advantage of a reverse feedback on the low-noise amplifier without adversely affecting the signal-to-noise ratio. 
         [0011]    It would also be desirable to decrease the gain losses introduced by the reverse feedback. 
         [0012]    To achieve all or part of these objects as well as others, at least one embodiment of the present invention provides a mixer-amplifier of an RF signal comprising at least an amplifier circuit and a mixing circuit controlled at a local oscillator frequency, for amplifying a signal applied on at least one input terminal and converting a first frequency of this signal into a second, lower, frequency, and comprising a reverse feedback loop switched at the local oscillator frequency. 
         [0013]    According to an embodiment of the present invention, the reverse feedback loop loop comprises, in addition to a switch, a filter at the local oscillator frequency. 
         [0014]    According to an embodiment of the present invention, the amplifier amplifies the signal at the first frequency and is followed by the mixer. 
         [0015]    According to an embodiment of the present invention, the mixer is arranged upstream of the amplifier that amplifies the signal at the second frequency. 
         [0016]    According to an embodiment of the present invention, the mixer-amplifier is capable of processing differential signals. 
         [0017]    According to an embodiment of the present invention, the mixer-amplifier is capable of processing non-differential signals. 
         [0018]    At least one embodiment of the present invention also provides an RF receive chain for circuits for interpreting this signal, comprising at least one mixer-amplifier. 
         [0019]    At least one embodiment of the present invention also provides an RF reception system. 
         [0020]    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 
         [0021]      FIG. 1 , previously described, shows a common RF receive chain; 
           [0022]      FIG. 2  is a block diagram of a partial embodiment of an RF receive chain; 
           [0023]      FIG. 3  is a block diagram of another embodiment of a partial RF receive chain; 
           [0024]      FIG. 4  is a block diagram of another embodiment applied to a differential structure; and 
           [0025]      FIG. 5  illustrates an embodiment of the mixer of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The same elements have been designated with the same reference numerals in the different drawings. 
         [0027]    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 structure of a low-noise amplifier has not been detailed, the present invention being compatible with common designs of such amplifiers. Further, the circuits upstream and downstream of the described mixer-amplifier have not been detailed either, the present invention being here again compatible with usual circuits. 
         [0028]      FIG. 2  is a block diagram of an embodiment of a mixer-amplifier. Such a mixer-amplifier is capable of being interposed on an RF receive line, for example, between an antenna and an electronic processing circuit of the type illustrated in  FIG. 1 , with or without interposed filtering and matching elements. 
         [0029]    A low-noise amplifier  2  (LNA) is series-connected with a mixing circuit  3 , the output of amplifier  2  being directly connected to the input of mixer  3 . Input IN of amplifier  2  receives an RF signal originating, for example, from an antenna of the type of antenna  1  of  FIG. 1  (possibly with one or several interposed filters). Output OUT of mixer  3  is intended to be connected (possibly with one or several interposed filters) to a circuit (for example  4 ,  FIG. 1 ) for processing the baseband signals. A reverse feedback circuit  7  samples the output of mixer  3  to feed back a portion of the signal at input IN of amplifier  2 . Reverse feedback path  7  comprises a switch  71  controlled by the signal at the intermediary frequency (local oscillator OL). Optionally, a filtering element  72  is interposed in reverse feedback loop  7  in series with switch  71 . 
         [0030]    The fact of switching reverse feedback loop  7  at the frequency of the local oscillator to return to the intermediary frequency limits gain losses in this loop. 
         [0031]    Further, optional filter  72  is a filter at the intermediary frequency and no longer at the high frequency of the RF signal. It may, for example, often have a simpler construction. 
         [0032]    Further, one filter ( 5 ,  FIG. 1 ) upstream of amplifier  2  may be removed provided that the application allows it. 
         [0033]      FIG. 3  shows another embodiment of a mixer-amplifier in which a first variation with respect to the embodiment of  FIG. 2  is an inversion of the respective positions of the amplifier and of mixer  3  in the series association. As a result, amplifier  2 ′ is designed to operate at the intermediary frequency and not at that of the RF signal. Another modification is an inversion of the respective positions of switch  71  and of filter  72  in reverse feedback loop  7 ′. 
         [0034]      FIG. 4  shows a further embodiment of a mixer-amplifier applied to an embodiment with a differential structure of low-noise amplifier  2 ″ and of mixer  3 ″. In this example, respective inputs IN+and IN− of the mixer-amplifier are directly connected to input terminals  21  and  22  of differential amplifier  2 ″, which respectively receive the outputs of two reverse feedback paths  7 +and  7 −. Differential outputs  23  and  24  of amplifier  2 ″ are connected to inputs  31  and  32  of mixer  3 ″ having respective differential outputs  33  and  34  defining respective outputs OUT+and OUT− of the mixer-amplifier towards the processing circuits of the reception device. Each reverse feedback loop  7 +,  7 − comprises two paths bringing respective outputs  33  and  34  of mixer  3 ″ onto inputs IN+and IN− of amplifier  2 ″ via switches. For example, output  33  is connected via a MOS transistor  711  to terminal IN+and via a MOS transistor  712  to terminal IN−. Further, output terminal  34  is connected via a MOS transistor  713  to terminal IN+and via a MOS transistor  714  to terminal IN−. Mixer  3 ″ further comprises two differential inputs LO+and LO− of the signal at the intermediary frequency, which receive complementary signals. Transistors  712  and  713  are controlled by positive local oscillator signal LO+while transistors  711  and  714  are controlled by negative local oscillation signal LO−. 
         [0035]      FIG. 5  is a simplified example of a mixer  3 ″ formed of four transistors  36 ,  37 ,  38 , and  39 . Transistors  36  and  37  connect terminal  31  respectively to terminals  33  and  34  while transistors  38  and  39  connect terminal  32  respectively to terminals  34  and  33 . Transistors  36  and  38  are controlled by positive signal LO+while transistors  37  and  39  are controlled by signal LO−. The operation of the circuit of  FIG. 5  is that of a usual mixer. 
         [0036]    The described mixer-amplifier enables performing the impedance-matching of the amplifier with no external inductance. The filtering in the reverse feedback enables to further improve the linearity. The reverse feedback further enables a wide-band matching for a multiple-standard use. 
         [0037]    Various embodiments have been described. Different variations and modifications are within the abilities of those skilled in the art. In particular, the respective constructions of the amplifier and of the mixer are within the abilities of those skilled in the art using common structures, be they differential or non-differential, provided to provide a reverse feedback on the series-association of the mixer and of the amplifier, switched at the frequency of the local oscillator of the mixer. Further, several amplifiers may be provided in the series association, for example, with the mixer being interposed between two amplifiers respectively operating at the frequency of the input signal and at the intermediary frequency. Further, the different variations may be combined. Finally, any switch type (MOS, bipolar transistors, etc.) may be used according to the technology used for the rest of the elements of the mixer-amplifier. 
         [0038]    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.