Calibration in a radio frequency transmission module

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.

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 F1carrying the desired information, and to convert this signal to a radio frequency carrier of frequency FLOfor its radio transmission by antenna. The desired information is then in fact carried by a second radio frequency F1+FLO. The frequency F1can 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 F1+FLO, a signal corresponding to the carrier at frequency FLOand an image signal at the frequency F1−FLO.

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 module110such as the one represented inFIG. 1. It comprises, for example, a digital signal processor (DSP)100adapted to deliver a digital signal of frequency F1on an I channel (in-phase channel), and a digital signal of frequency F1on a Q channel (quadrature channel).

Each of these signals is input to a respective digital-to-analog converter (DAC)101,102and the analog signals output by the digital-to-analog converters are input to a conversion stage103for conversion to a radio frequency F1+FLO.

The frequency conversion stage103comprises two mixers112,114, using a Gilbert structure for example.

The mixer112placed on the I channel is adapted to mix the signal on the I channel provided as input to the conversion stage103, for conversion to a signal with a carrier signal LO at radio frequency FLO.

The mixer114placed on the Q channel is adapted to mix the signal on the Q channel provided as input to the conversion stage103, for conversion to a signal with a carrier signal LO′ at radio frequency FLO, 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 stage103before any further processing is applied to them, then sent to a power amplifier104. 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 processor100on the I and Q channels. The signal provided by the power amplifier104is then input to a calibration loop105.

The calibration loop105comprises a power detector106, an analog-to-digital converter (ADC)107, and a digital signal processor108.

The power detector106is 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 processor108is 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 processor100.

In the operational phase, the digital signal processor100is 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 processor100.

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.

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.

FIG. 2shows a radio frequency transmission module1in one embodiment.

The radio frequency transmission module1is adapted to operate in either a calibration phase or an operational phase.

The radio frequency transmission module1comprises a digital signal processor2, two digital-to-analog converters5,6adapted to convert an input digital signal into an analog signal, a frequency conversion stage9, a switch18, and a calibration loop14.

The digital signal processor2is adapted to deliver two digital signals sIand sQ.

The signal sIis intended for the in-phase channel, called the I channel, of the radio frequency transmission module1, while the signal sQis intended for the quadrature channel, called the Q channel, of the radio frequency transmission module1.

For example, signals sIand sQare identical signals except that they are out of phase with each other by 90°.

On the I channel, the digital signal sIis delivered to the digital-to-analog converter5. On the Q channel, the digital signal sQis delivered to the digital-to-analog converter6. The analog signals respectively issuing from the digital-to-analog converters5and6are then delivered to band-pass filters7and8. The resulting filtered signals are then provided to the frequency conversion stage9.

The frequency conversion stage9comprises two mixers10,11, in a Gilbert structure for example. The mixer10placed on the I channel is adapted to mix the signal provided on the I channel as input to the conversion stage9for impression onto a carrier signal LO at radio frequency FLO. The mixer11placed on the Q channel is adapted to mix the signal provided on the Q channel as input to the conversion stage9for conversion to a signal with a carrier signal LO′ at radio frequency FLO′, 90° out of phase with the carrier signal LO. The signal SIresulting from this frequency conversion is then delivered on the I channel by the frequency conversion stage9, while the signal SQresulting from this frequency conversion is delivered on the Q channel by the frequency conversion stage9.

The spectrum for the signal SI, similarly to the one for the signal SQ, comprises a non-zero carrier component at the radio frequency FLO, a non-zero desired signal component at the radio frequency FLO+F1, and an image signal component at the radio frequency FLO−F1(in other embodiments, the non-zero desired signal component corresponds to the radio frequency FLO−F1, and the image signal component to the radio frequency FLO+F1).

The signals SIand SQresulting from the mixing performed by the frequency conversion stage9and issuing from the I and Q channels are summed into a summed signal S, which is input to a switch18. The switch18is adapted to couple the output from the frequency conversion stage9to a terminal O1, in an operational phase, and to couple the output from the frequency conversion stage9to a terminal O2, in a calibration phase.

In a calibration phase, the signals sIand sQare, for example, digital sine or digital cosine signals of frequency F1(where F1is not a radio frequency).

In an operational phase, these signals sIand sQare digital signals carrying the information for transmission. They comprise a non-zero component at frequency F1.

In an operational phase, the summed signal provided to the terminal O1by the switch18possibly undergoes additional processing within the radio frequency transmission device1, for example amplification, before being transmitted by a transmitting antenna A.

In a calibration phase, the summed signal provided to the terminal O2by the switch18is delivered as input to the calibration loop14.

The operations performed in the calibration phase by the calibration loop14allow, in one embodiment, determining from test signals the processing to be applied by the digital signal processor2to 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 loop14of one embodiment comprises a separating unit16, a subsampler20, a low-pass filter21and a digital processing unit12.

The digital processing unit12of one embodiment comprises a band-pass filter13, an analog-to-digital converter17, followed by a digital signal processor15.

In the calibration phase, the summed signal S equal to the SI+SQissuing from the test signals is provided as input to the subsampler20.

The subsampler20is adapted to subsample the signal S, at a given frequency fe(to avoid spectrum aliasing problems, feis chosen to be greater than 2F1).

This subsampling has the effect of reproducing the spectrum of signal S every fefrequency.

Thus components similar to the one situated at frequency FLOare reproduced at frequencies FLO+ife, where i is any whole number. Similarly, components similar to the ones respectively located at frequency FLO+F1and FLO−F1, are reproduced at the respective frequencies FLO+F1+ifeand FLO−F1+ife, where i is any whole number.

This subsampled signal is then provided to the low-pass filter21, which is adapted to extract, from the other spectrum reproductions, the reproduction of the spectrum for the signal S comprising the components at frequencies FLO−k0fe, FLO+F1−k0fe, and FLO−F1−k0fe, which are the closest to 0 (meaning k0is such that no matter what the integer value of k, the absolute value of FLO−k0feis less than or equal to the absolute value of FLO−kfe).

Then the obtained filtered signal is provided to the digital processing unit12. The signal will then be filtered using a band-pass filter13, which will isolate the part of the spectrum corresponding to the three frequency components of interest FLO−k0fe, FLO+F1−k0fe, and FLO−F1−k0fe, representing the carrier components, of the desired signal and the image signal.

Then the signal output from the band-pass filter13is provided to the analog-to-digital converter17, which converts the analog signal received as input into a digital signal, then delivers the obtained digital signal to the digital signal processor15.

This processor15calculates 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 FLO−k0fe, FLO+F1−k0fe, and FLO−F1−k0feare then processed by the digital signal processor15.

The digital signal processor15is especially adapted to deduce, as a function of at least the amplitude of the Fourier coefficient calculated for the frequency closest to FLO+F1−k0fe(corresponding to the desired signal) and the amplitude of the Fourier coefficient calculated for the frequency closest to FLO−F1−k0fe(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 processor15is adapted to determine one or more calibration coefficients as a function of this comparison.

These calibration coefficients are communicated to the digital signal processor2. The processor is adapted to process, in the next operational phase, the digital signals sIand/or sQbefore they are provided to the digital-to-analog converters101,102, as a function of at least one calibration coefficient determined by the calibration loop14.

The processing applied to the digital signals sIand/or sQas 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 processor15as a function of at least the amplitude of the Fourier coefficient calculated for the frequency closest to FLO−k0fe, corresponding to the component of the carrier FLO.

In one embodiment, FLOand feare chosen such that the frequency FLO−k0feis non-zero and is a multiple of a reference frequency FREF(in other words, the frequency FLOis equal to the sum of a multiple of the sampling frequency and a multiple of the reference frequency) and the frequency F1is chosen such that it is also a multiple of the reference frequency FREF. The FFT is then defined, using its size N and its sampling frequency FFFT, such that the Fourier coefficients are calculated in each of the frequencies FLO−k0fe, FLO+F1−k0fe, and FLO−F1−k0fe(the frequency FFFTis then a multiple of the reference frequency), as represented inFIG. 3. The frequency FFFTcan be chosen to be equal to the reference frequency, for example.

The top part ofFIG. 3represents the part of the signal filtered by the band-pass filter13.

The lower part ofFIG. 3represents the N frequencies

ⅈ×FFFTN,ⅈ
being a positive integer from 0 to N−1 for which the FFT provides a Fourier coefficient

f⁡(ⅈ×FFFTN),
where N is the size of the FFT and FFFTits sampling frequency.

In the case in question, there exist integers k1, k2and k3between 0 and N−1, such that

k1×FFFTN=FLO-F1-k0⁢fe;k2×FFFTN=FLO-k0⁢fe
and

In one embodiment, F1is equal to 1.25 MHz, and FLOis 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 feis chosen from within the range [15 MHz-50 MHz].

feis chosen such that, for example, FLO−k0feis equal to 5 MHz. Then FLO−F1−k0feis equal to 3.75 MHz and FLO+F1−k0feis equal to 6.25 MHz. The frequencies FLO−k0feand F1are multiples of 312.5 kHz. The frequency FFFTis equal to the reference frequency 312.5 kHz.

In one embodiment, the digital processing unit12is part of a radio frequency reception module comprised for example in the terminal (a telephone for example) comprising the radio frequency transmission module1. 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 unit12of the radio frequency reception module1, for determining the calibration coefficients as a function of the processing performed on only one of the two channels of the processing unit12. Alternatively, the total signal S is provided to both the I and Q channels of the digital processing unit12of the radio frequency reception module1, meaning upstream from the mixers of the radio frequency reception module1, for determining the calibration coefficients as a function of the processing performed on the two channels by the processing unit12. 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 module1and not those introduced by the processing performed on the Q channel (or respectively the I channel) of the radio frequency reception module1(these characteristics not reproduced between the I and Q channels are called a mismatch).

In one embodiment, the low-pass filter21is 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 FLO−kpfe, FLO+F1−kpfe, and FLO−F1−kpfe(kpis a non-zero integer) which are not the closest to 0. For example, kpcan be equal to k0+2. Other values for kpare possible. In such a case, the digital processing unit12is 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 module1, 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.