Abstract:
A radio frequency transmission module is adapted to generate a first signal to be transmitted and to convert the signal to a radio frequency carrier for its radio transmission, in an operational phase, and is adapted to generate a second signal and to convert the second signal to the radio frequency carrier, in a calibration phase. The module includes a calibration unit having a subsampler adapted to subsample the second converted signal and a calculation unit adapted to calculate Fourier Transform coefficients representative of the signal delivered by the subsampler, for the purposes of processing the first signal to be transmitted, in the operational phase, as a function of at least some of the Fourier coefficients calculated in the calibration phase.

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to integrated electronic circuits, particularly but not exclusively those comprised within radio frequency transmission modules. 
     Such radio frequency transmission modules are encountered in “communications” equipment such as wireless telephones and wireless personal digital assistants. 
     BACKGROUND INFORMATION 
     Certain radio frequency transmission modules are adapted to obtain a signal at a first frequency F 1  carrying the desired information, and to convert this signal to a radio frequency carrier of frequency F LO  for its radio transmission by antenna. The desired information is then in fact carried by a second radio frequency F 1 +F LO . The frequency F 1  can be the zero frequency (baseband) or a frequency called an “intermediate frequency.” 
     It is known that the conversion to the second frequency generates a desired signal at the second frequency F 1 +F LO , a signal corresponding to the carrier at frequency F LO  and an image signal at the frequency F 1 −F LO . 
     There are generally set constraints regarding such a radio frequency transmission module. These constraints include, for example, a maximum limit for the ratio of the image signal level to the desired signal level, called the image rejection ratio (IRR). 
     Let us consider a radio frequency transmission module  110  such as the one represented in  FIG. 1 . It comprises, for example, a digital signal processor (DSP)  100  adapted to deliver a digital signal of frequency F 1  on an I channel (in-phase channel), and a digital signal of frequency F 1  on a Q channel (quadrature channel). 
     Each of these signals is input to a respective digital-to-analog converter (DAC)  101 ,  102  and the analog signals output by the digital-to-analog converters are input to a conversion stage  103  for conversion to a radio frequency F 1 +F LO . 
     The frequency conversion stage  103  comprises two mixers  112 ,  114 , using a Gilbert structure for example. 
     The mixer  112  placed on the I channel is adapted to mix the signal on the I channel provided as input to the conversion stage  103 , for conversion to a signal with a carrier signal LO at radio frequency F LO . 
     The mixer  114  placed on the Q channel is adapted to mix the signal on the Q channel provided as input to the conversion stage  103 , for conversion to a signal with a carrier signal LO′ at radio frequency F LO , and out of phase by 90°relative to the carrier signal LO. 
     In an operational phase, the signals resulting from this mixing and issuing from the I and Q channels are summed, then delivered by the conversion stage  103  before any further processing is applied to them, then sent to a power amplifier  104 . It is then transmitted by a transmitting antenna. 
     In a calibration phase, test digital signals, in the shape of a sine or cosine wave for example, are delivered by the digital signal processor  100  on the I and Q channels. The signal provided by the power amplifier  104  is then input to a calibration loop  105 . 
     The calibration loop  105  comprises a power detector  106 , an analog-to-digital converter (ADC)  107 , and a digital signal processor  108 . 
     The power detector  106  is adapted to determine the envelope of the signal provided as input, to detect the power level of said signal, and to determine the IRR corresponding to said signal. The digital signal processor  108  is adapted, if the calculated IRR exceeds the maximum limit set for the IRR, to determine the calibration coefficients as a function of the signal which is provided as input. These determined calibration coefficients are provided as input parameters to the digital signal processor  100 . 
     In the operational phase, the digital signal processor  100  is adapted to process the digital signals (which are no longer test signals) before they are provided to the I and Q channels. This processing can adapt the amplitude and/or phase of the signal intended for the I channel and/or the signal intended for the Q channel as a function of the calibration coefficients determined during the calibration phase and provided as input to the digital signal processor  100 . 
     Thus, the calibration described here enables the IRR value for the desired signals transmitted during the operational phase to be less than the maximum limit tolerated. Note that other calibrations can be performed, with advantages other than image frequency rejection, for example to compensate for non-linearities of the power amplifier. 
     Such a calibration technique, based on detecting the power level of a signal output by the frequency conversion stage, yields satisfactory results in a certain number of applications. However, it is no longer satisfactory when the maximum limit to be taken into account for the IRR is less than or equal to −40 dB, because it no longer allows sufficient precision. 
     BRIEF SUMMARY 
     One embodiment enables the calibration of signals in a radio frequency transmission module, taking into account a maximum authorized IRR limit which is less than or equal to −40 dB. 
     For this purposes, a first aspect provides a radio frequency transmission module adapted to generate a first signal for transmission and to convert said signal to a radio frequency carrier for its radio transmission, in an operational phase, and adapted to generate a second signal and convert said second signal to the radio frequency carrier, in a calibration phase. 
     This radio frequency transmission module comprises a calibration unit adapted to calculate, in the calibration phase, Fourier Transform coefficients from the second converted signal, and comprising a subsampler and a calculation unit, with the subsampler adapted to subsample the second converted signal, and with the calculation unit adapted to calculate the Fourier Transform coefficients representative of the signal delivered by the subsampler, for the purposes of processing the first signal to be transmitted, in the operational phase, as a function of at least some of said Fourier coefficients calculated in the calibration phase. 
     Such a radio frequency transmission module is thus adapted to implement a signal calibration to meet strict IRR constraints. In particular, such a radio frequency transmission module allows calculating with precision, from at least some of the calculated Fourier Transform coefficients, the IRR corresponding to the second signal transposed on the radio frequency, to compare it with a maximum limit less than or equal to −40 dB, and to deduce the phase or amplitude adjustments to make to a signal to be transmitted in an operational phase, so that the signal actually transmitted meets the IRR constraint. 
     One role of the subsampling is to provide a spectrum comprising components corresponding to the components of the second transposed signal, of the signal image at frequencies below that of the radio frequency carrier, from which the analog-to-digital conversion and the Fourier Transform calculations are performed. 
     In one embodiment, the calibration unit additionally comprises a filter placed between the subsampler and the calculation unit, and adapted to extract a portion of the frequency spectrum of the second signal subsampled by the subsampler, said portion comprising a component corresponding to the second converted signal and a component corresponding to an image of the second converted signal, with the calculation unit adapted to calculate Fourier Transform coefficients representative of the signal delivered by the filter. This arrangement thus allows extracting only a portion of the signal of interest, comprising all the information used for the calibration. This simplifies the calculations to be performed. 
     In one embodiment, the filter is a low-pass filter adapted to extract the portion of the spectrum of the second subsampled signal comprising the component corresponding to the second converted signal and the component corresponding to an image of the second converted signal, these being the closest to the zero frequency. This arrangement allows performing the calculation operations at the zero frequency, which simplifies these operations. 
     In one embodiment, the second signal is a sine or cosine test signal. This arrangement allows a simple and direct determination of the Fourier Transform coefficients. 
     In one embodiment, the radio frequency transmission module is adapted to modify the phase and/or amplitude of the first signal to be transmitted in the operational phase, as a function of at least some of the Fourier coefficients calculated in the calibration phase using the second signal. 
     In a second aspect, a radio frequency transmission/reception system comprises a radio frequency transmission module according to the first aspect and a radio frequency reception module, said transmission and reception modules being adapted to operate in phase and in quadrature and to share the use of the calculation unit, with the radio frequency transmission module being adapted to process said first signal to be transmitted in the operational phase as a function of the Fourier coefficients calculated, in the calibration phase by the calculation unit, for one from either an in-phase component or a quadrature component of the second signal provided, excluding the Fourier coefficients calculated, in the calibration phase, for the other from either said in-phase component or said quadrature component of the second signal. Such a system allows reusing the means comprised in the receiving module, for the calculation performed by the calibration loop, while avoiding the inclusion in the calibration of mismatches between the in-phase channel and the quadrature channel of the module. 
     In a third aspect, a processing method in a radio frequency transmission module is adapted, in an operational phase of the module, to generate a first signal to be transmitted and to convert said first signal to a radio frequency carrier for its radio transmission, with said process comprising the following in a calibration phase of said module:
         generate a second signal and convert said second signal to the radio frequency carrier,   subsample said second signal converted to radio frequency,   calculate Fourier Transform coefficients representative of the subsampled signal in order to process the first signal to be transmitted in the operational phase as a function of at least some of the Fourier coefficients calculated in the calibration phase.       

    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other features of one or more embodiments will become evident upon reading the non-limiting and non-exhaustive description which follows. This is purely illustrative and is to be read while referring to the attached drawings, in which: 
         FIG. 1  shows a radio frequency transmission module as previously described, 
         FIG. 2  represents a transmission module  1  in one embodiment, and 
       the top part of  FIG. 3  schematically represents the spectrum of the signal output from the filter  13  represented in  FIG. 2 , and the bottom part represents the frequencies at which the FFT coefficients are calculated in one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the embodiments. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. 
       FIG. 2  shows a radio frequency transmission module  1  in one embodiment. 
     The radio frequency transmission module  1  is adapted to operate in either a calibration phase or an operational phase. 
     The radio frequency transmission module  1  comprises a digital signal processor  2 , two digital-to-analog converters  5 ,  6  adapted to convert an input digital signal into an analog signal, a frequency conversion stage  9 , a switch  18 , and a calibration loop  14 . 
     The digital signal processor  2  is adapted to deliver two digital signals s I  and s Q . 
     The signal s I  is intended for the in-phase channel, called the I channel, of the radio frequency transmission module  1 , while the signal s Q  is intended for the quadrature channel, called the Q channel, of the radio frequency transmission module  1 . 
     For example, signals s I  and s Q  are identical signals except that they are out of phase with each other by 90°. 
     On the I channel, the digital signal s I  is delivered to the digital-to-analog converter  5 . On the Q channel, the digital signal s Q  is delivered to the digital-to-analog converter  6 . The analog signals respectively issuing from the digital-to-analog converters  5  and  6  are then delivered to band-pass filters  7  and  8 . The resulting filtered signals are then provided to the frequency conversion stage  9 . 
     The frequency conversion stage  9  comprises two mixers  10 ,  11 , in a Gilbert structure for example. The mixer  10  placed on the I channel is adapted to mix the signal provided on the I channel as input to the conversion stage  9  for impression onto a carrier signal LO at radio frequency F LO . The mixer  11  placed on the Q channel is adapted to mix the signal provided on the Q channel as input to the conversion stage  9  for conversion to a signal with a carrier signal LO′ at radio frequency F LO′ , 90° out of phase with the carrier signal LO. The signal S I  resulting from this frequency conversion is then delivered on the I channel by the frequency conversion stage  9 , while the signal S Q  resulting from this frequency conversion is delivered on the Q channel by the frequency conversion stage  9 . 
     The spectrum for the signal S I , similarly to the one for the signal S Q , comprises a non-zero carrier component at the radio frequency F LO , a non-zero desired signal component at the radio frequency F LO +F 1 , and an image signal component at the radio frequency F LO −F 1  (in other embodiments, the non-zero desired signal component corresponds to the radio frequency F LO −F 1 , and the image signal component to the radio frequency F LO +F 1 ). 
     The signals S I  and S Q  resulting from the mixing performed by the frequency conversion stage  9  and issuing from the I and Q channels are summed into a summed signal S, which is input to a switch  18 . The switch  18  is adapted to couple the output from the frequency conversion stage  9  to a terminal O 1 , in an operational phase, and to couple the output from the frequency conversion stage  9  to a terminal O 2 , in a calibration phase. 
     In a calibration phase, the signals s I  and s Q  are, for example, digital sine or digital cosine signals of frequency F 1  (where F 1  is not a radio frequency). 
     In an operational phase, these signals s I  and s Q  are digital signals carrying the information for transmission. They comprise a non-zero component at frequency F 1 . 
     In an operational phase, the summed signal provided to the terminal O 1  by the switch  18  possibly undergoes additional processing within the radio frequency transmission device  1 , for example amplification, before being transmitted by a transmitting antenna A. 
     In a calibration phase, the summed signal provided to the terminal O 2  by the switch  18  is delivered as input to the calibration loop  14 . 
     The operations performed in the calibration phase by the calibration loop  14  allow, in one embodiment, determining from test signals the processing to be applied by the digital signal processor  2  to signals on the I and Q channels in an operational phase. Some of this processing is, for example, for the purposes of constraining the IRR of the radio frequency signals provided based on these signals to be below a set maximum IRR. 
     The calibration loop  14  of one embodiment comprises a separating unit  16 , a subsampler  20 , a low-pass filter  21  and a digital processing unit  12 . 
     The digital processing unit  12  of one embodiment comprises a band-pass filter  13 , an analog-to-digital converter  17 , followed by a digital signal processor  15 . 
     In the calibration phase, the summed signal S equal to the S I +S Q  issuing from the test signals is provided as input to the subsampler  20 . 
     The subsampler  20  is adapted to subsample the signal S, at a given frequency f e  (to avoid spectrum aliasing problems, f e  is chosen to be greater than 2F 1 ). 
     This subsampling has the effect of reproducing the spectrum of signal S every f e  frequency. 
     Thus components similar to the one situated at frequency F LO  are reproduced at frequencies F LO +if e , where i is any whole number. Similarly, components similar to the ones respectively located at frequency F LO +F 1  and F LO −F 1 , are reproduced at the respective frequencies F LO +F 1 +if e  and F LO −F 1 +if e , where i is any whole number. 
     This subsampled signal is then provided to the low-pass filter  21 , which is adapted to extract, from the other spectrum reproductions, the reproduction of the spectrum for the signal S comprising the components at frequencies F LO −k 0 f e , F LO +F 1 −k 0 f e , and F LO −F 1 −k 0 f e , which are the closest to 0 (meaning k 0  is such that no matter what the integer value of k, the absolute value of F LO −k 0 f e  is less than or equal to the absolute value of F LO −kf e ). 
     Then the obtained filtered signal is provided to the digital processing unit  12 . The signal will then be filtered using a band-pass filter  13 , which will isolate the part of the spectrum corresponding to the three frequency components of interest F LO −k 0 f e , F LO +F 1 −k 0 f e , and F LO −F 1 −k 0 f e , representing the carrier components, of the desired signal and the image signal. 
     Then the signal output from the band-pass filter  13  is provided to the analog-to-digital converter  17 , which converts the analog signal received as input into a digital signal, then delivers the obtained digital signal to the digital signal processor  15 . 
     This processor  15  calculates the Fourier coefficients from the signal which is input to it, using a Fast Fourier Transform or FFT for example. 
     The Fourier coefficients calculated for the frequencies closest to frequencies F LO −k 0 f e , F LO +F 1 −k 0 f e , and F LO −F 1 −k 0 f e  are then processed by the digital signal processor  15 . 
     The digital signal processor  15  is especially adapted to deduce, as a function of at least the amplitude of the Fourier coefficient calculated for the frequency closest to F LO +F 1 −k 0 f e  (corresponding to the desired signal) and the amplitude of the Fourier coefficient calculated for the frequency closest to F LO −F 1 −k 0 f e  (corresponding to the image signal), the IRR for the signal S, and then to compare the calculated IRR with the set maximum IRR. 
     The digital signal processor  15  is adapted to determine one or more calibration coefficients as a function of this comparison. 
     These calibration coefficients are communicated to the digital signal processor  2 . The processor is adapted to process, in the next operational phase, the digital signals s I  and/or s Q  before they are provided to the digital-to-analog converters  101 ,  102 , as a function of at least one calibration coefficient determined by the calibration loop  14 . 
     The processing applied to the digital signals s I  and/or s Q  as a function of at least some of the calibration coefficients can be modifications to the phase and/or amplitude of said signals. 
     In one embodiment, a calibration coefficient is additionally determined by the digital signal processor  15  as a function of at least the amplitude of the Fourier coefficient calculated for the frequency closest to F LO −k 0 f e , corresponding to the component of the carrier F LO . 
     In one embodiment, F LO  and f e  are chosen such that the frequency F LO −k 0 f e  is non-zero and is a multiple of a reference frequency F REF  (in other words, the frequency F LO  is equal to the sum of a multiple of the sampling frequency and a multiple of the reference frequency) and the frequency F 1  is chosen such that it is also a multiple of the reference frequency F REF . The FFT is then defined, using its size N and its sampling frequency F FFT , such that the Fourier coefficients are calculated in each of the frequencies F LO −k 0 f e , F LO +F 1 −k 0 f e , and F LO −F 1 −k 0 f e  (the frequency F FFT  is then a multiple of the reference frequency), as represented in  FIG. 3 . The frequency F FFT  can be chosen to be equal to the reference frequency, for example. 
     The top part of  FIG. 3  represents the part of the signal filtered by the band-pass filter  13 . 
     The lower part of  FIG. 3  represents the N frequencies 
                 ⅈ   ×     F   FFT       N     ,   ⅈ         
being a positive integer from 0 to N−1 for which the FFT provides a Fourier coefficient
 
               f   ⁡     (       ⅈ   ×     F   FFT       N     )       ,         
where N is the size of the FFT and F FFT  its sampling frequency.
 
     In the case in question, there exist integers k 1 , k 2  and k 3  between 0 and N−1, such that 
                     k   1     ×     F   FFT       N     =       F   LO     -     F   1     -       k   0     ⁢     f   e           ;                     k   2     ×     F   FFT       N     =       F   LO     -       k   0     ⁢     f   e               
and
 
     
       
         
           
             
               
                 
                   k 
                   3 
                 
                 × 
                 
                   F 
                   FFT 
                 
               
               N 
             
             = 
             
               
                 F 
                 LO 
               
               + 
               
                 F 
                 1 
               
               - 
               
                 
                   k 
                   0 
                 
                 ⁢ 
                 
                   f 
                   
                     e 
                     . 
                   
                 
               
             
           
         
       
     
     In one embodiment, F 1  is equal to 1.25 MHz, and F LO  is within the frequency band [2400 MHz-2484 MHz] (as specified in the 802.22 b/g standards) or within the frequency band [4900 MHz-5850 MHz] (as specified in the 802.11a standard). The frequency f e  is chosen from within the range [15 MHz-50 MHz]. 
     f e  is chosen such that, for example, F LO −k 0 f e  is equal to 5 MHz. Then F LO −F 1 −k 0 f e  is equal to 3.75 MHz and F LO +F 1 −k 0 f e  is equal to 6.25 MHz. The frequencies F LO −k 0 f e  and F 1  are multiples of 312.5 kHz. The frequency F FFT  is equal to the reference frequency 312.5 kHz. 
     In one embodiment, the digital processing unit  12  is part of a radio frequency reception module comprised for example in the terminal (a telephone for example) comprising the radio frequency transmission module  1 . The resources are therefore shared between the transmitting part and the receiving part of the terminal. 
     In such an embodiment, in the calibration phase, the summed signal S used for the calibration is provided to only one of the I or Q channels of the digital processing unit  12  of the radio frequency reception module  1 , for determining the calibration coefficients as a function of the processing performed on only one of the two channels of the processing unit  12 . Alternatively, the total signal S is provided to both the I and Q channels of the digital processing unit  12  of the radio frequency reception module  1 , meaning upstream from the mixers of the radio frequency reception module  1 , for determining the calibration coefficients as a function of the processing performed on the two channels by the processing unit  12 . However, this latter arrangement has one disadvantage, in that the calibration then takes into account the characteristics introduced by the processing performed on the I and Q channels of the radio frequency reception module  1  and not those introduced by the processing performed on the Q channel (or respectively the I channel) of the radio frequency reception module  1  (these characteristics not reproduced between the I and Q channels are called a mismatch). 
     In one embodiment, the low-pass filter  21  is replaced with a filter adapted to extract, from the subsampled spectrum, a reproduction of the spectrum of the signal S comprising the components at frequencies F LO −k p f e , F LO +F 1 −k p f e , and F LO −F 1 −k p f e  (k p  is a non-zero integer) which are not the closest to 0. For example, k p  can be equal to k 0 +2. Other values for k p  are possible. In such a case, the digital processing unit  12  is adapted to, once the signal is received as a digital input, convert this digital signal to the zero frequency before applying the Fourier transformation. 
     One embodiment has been described above in a transmission module comprising an in-phase channel and a quadrature channel. In another embodiment, a radio frequency transmission module only comprises one signal processing channel. 
     A calibration of an embodiment allows, for example, calculating during a calibration phase the value of the IRR for a radio frequency signal delivered by the radio frequency emission module  1 , comparing it with a limit value less than −40 dB, and processing the digital input signals at least once during an operational phase, as a function of at least the value of the IRR. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.