Double up-conversion modulator

The present invention concerns an in quadrature double frequency up-conversion modulator, used in particular in transmitter circuits for mobile telephones. The double up-conversion modulator includes a first frequency up-conversion stage formed by a first modulation unit. This first modulation unit receives at one input baseband quadriphased signals, formed of a first in-phase signal, a second in phase opposition signal, a third in quadrature signal and a fourth in quadrature-opposition signal. This first unit is controlled by carrier signals quadriphased at a first intermediate frequency formed by a first in-phase carrier signal, a second in phase opposition carrier signal, a third in quadrature carrier signal and a fourth in quadrature-opposition carrier signal. The modulation unit delivers a signal up-converted to the first intermediate frequency at one of its ouptut.

This type of simple up-conversion in quadrature modulator, formed of a single modulation unit, is widely known in the prior art. The modulator, such as that shown in FIG. 1, includes two differential mixers 10 and 12 . The first mixer 10 receives at one input an in-phase baseband signal bbI and the second mixer 12 receives at one input an in quadrature baseband signal bbQ. The two mixers 10 and 12 are controlled by high frequency carrier signals HF_I and HF_Q obtained by a high frequency in quadrature signal generator, which is not shown. The first mixer 10 is controlled by an in-phase high frequency carrier signal HF_I. The second mixer 12 is controlled by an in quadrature high frequency carrier signal HF_Q, namely a signal phase shifted by 90° with respect to carrier signal HF_I. The signals delivered at the outputs of mixers 10 and 12 are in quadrature signals modulated at the frequency of the carrier signals. An adder 14 placed at the output of the modulator adds the two modulated signals to provide the high frequency modulated signal HF_M at its output.

The drawback of such a simple up-conversion modulator lies in the necessity of generating high frequency carrier signals HF_I and HF_Q that are perfectly in quadrature. Such a signal generator quickly becomes complex without however being able to assure a phase shift of 90° with sufficient precision. Moreover, the power consumption of such a generator is quite significant.

Another type of simple up-conversion in quadrature modulator known in the prior art is shown in FIG. 2 . This modulator operates on exactly the same principle as that presented hereinbefore. The difference from the previous modulator lies in the generation of high frequency in quadrature carrier signals HF_I and HF_Q. Here, a high frequency generator 26 is used to generate carrier signals at a frequency 2HF double the frequency desired for the modulated output signal. An in quadrature frequency divider-by-two 28 is placed at one output of generator 26 , which enables carrier signals HF_I and HF_Q to be obtained perfectly in quadrature with the modulation frequency HF desired for controlling differential mixers 20 and 22 of the modulator.

The drawback of this type of modulator lies in the excessive power consumption of high frequency generator 26 , used to generate signals at a frequency that is double the desired frequency. Indeed, for frequencies of the order of one GHz, such as for example 900 MHz (mobile telephone range), the generator has to generate signals at a frequency of 1.8 GHz.

One of the permanent concerns of those skilled in the art is making low power consuming circuits, which are generally integrated in portable communication tools, while still obtaining optimum in quadrature signal modulation quality.

The invention is characterised in that it allows the aforementioned drawbacks to be avoided.

The present invention thus concerns a double up-conversion modulator, as defined in the preamble, characterised in that the first stage is also formed of a second modulation unit, similar to the first unit,

receiving at one input said first, second, third and fourth baseband signals,

controlled by said first, second, third and fourth carrier signals at said first intermediate frequency, and

delivering at one output a second converted signal, in phase opposition to said first up-converted signal, at said first intermediate frequency, and in that the modulator further includes a second up-conversion stage,

receiving at one input said first and second up-converted signals at said first intermediate frequency,

controlled by two other carrier signals at a second intermediate frequency, formed by a fifth in-phase carrier signal and a sixth in phase opposition carrier signal, and

delivering at one output a high frequency signal, said high frequency corresponding to the sum of said first and second intermediate frequencies.

The modulator according to the invention thus enables low power consuming modulation to be achieved. Indeed, the carrier signals used to transpose the input baseband signals into a high frequency output signal, are signals at intermediate frequencies lower than the high output modulation frequency. Thus, the lower the frequency, the lower the amount of power consumed by the generator, which is used to provide these carrier signals.

Another constant concern of those skilled in the art is the necessity for generating the cleanest possible modulated signals. In order to do this, it is indispensable to remove all the stray signals or feedthrough signals which appear in unbalanced structures.

This is why, according to an advantageous embodiment of the invention, the double up-conversion modulator is characterised in that the two modulation units of the first up-conversion stage include respectively, first and second differential mixers, and third and fourth differential mixers,

in that the first and second mixers provide at their output, respectively, first and second modulated signals at the first intermediate frequency (IF 1 ), the first and second modulated signals being added at the output of the first unit to obtain the first up-converted signal,

in that the third and fourth differential mixers provide at their output, respectively, third and fourth signals modulated to the first intermediate frequency, the third and fourth modulated signals being added at the output of the second unit to obtain the second up-converted signal, and

in that the second up-conversion stage is formed of a fifth differential mixer.

Finally, in this type of in quadrature modulator, it is very important that the carrier signals are in quadrature and with great precision. Indeed, particularly in a field such as mobile telephony where the modulation frequencies are of the order of one GHz, any phase shift between the in-phase signals and the in quadrature signals leads to the appearance of additional stray signals.

In a preferred embodiment of the invention, the double up-conversion modulator is characterised in that the carrier signals at the second intermediate frequency controlling the second up-conversion stage are directly generated by a signal generator at the second intermediate frequency and in that the carrier signals at the first intermediate frequency controlling the first up-conversion stage are obtained after dividing the frequency by two in an in quadrature frequency divider placed at one output of the signal generator, the second intermediate frequency being twice as high as the first intermediate frequency.

The invention will be explained hereinafter in detail via an embodiment given solely by way of example, this embodiment being illustrated by the annexed drawings, in which:

FIG. 1 shows an in quadrature modulator according to the prior art;

FIG. 2 shows another in quadrature modulator according to the prior art;

FIG. 3 is a schematic diagram of the modulator according to the invention;

FIG. 4 is a detailed diagram of the modulator according to the invention; and

FIG. 5 is a diagram of the carrier signal generator circuit.

FIGS. 1 and 2 have already been described within the scope of the prior art.

FIG. 3 is a schematic diagram of the double up-conversion modulator according to the invention. The modulator divided into two stages called frequency up-conversion stages. Indeed, the role of each of the two stages is to modulate and transpose the signals received at one input to a higher frequency. More specifically, first stage 1 enables the modulator input signals to be modulated at a first intermediate frequency IF 1 . The signals obtained are then “up-converted” to a greater, or high frequency HF, through second stage 2 .

Stage 1 is formed of two so-called “modulation” units 101 and 102 . These two units are similar as regards their internal structure. The difference between the two lies in the input and control signals which they receive.

The first modulation unit 101 is formed of two differential mixers 103 and 104 and adder 107 . The second modulation unit 102 is formed of two differential mixers 105 and 106 and adder 108 . One will preferably use differential mixers formed by a pair of MOS technology transistors. The advantage of using such mixers lies in the cancelling of stray signals or feedthrough signals at the output of the mixer. These feedthrough signals are actually cancelled if the two input signals and the two control signals received by the mixer are in phase opposition.

Unit 101 receives four low frequency or baseband input signals at four inputs. These signals are quadriphased. Mixer 103 receives at a first input the in-phase signal bbI and at a second input the in phase opposition signal bbI_b, and mixer 104 receives at a third input the in-phase quadrature signal bbQ and at a fourth input the in-phase quadrature opposition signal bbQ_b.

Unit 101 also receives four control signals or carrier signals which are also quadriphased. These carrier signals enable the baseband signals (bbI, bbI_b, bbQ and bbQ_b) to be modulated at the carrier signal frequency. Mixer 103 receives the in-phase carrier signal IF 1 I and the in-phase opposition carrier signal IF 1 I_b. Mixer 104 receives the in-phase-quadrature carrier signal IF 1 Q and the in-phase-quadrature opposition carrier signal IF 1 Q_b. All the carrier signals are at intermediate frequency IF 1 .

At the output of unit 101 , there is placed an adder 107 enabling the two modulated signals IF 1 Im and IF 1 Qm, delivered at the outputs of mixers 103 and 104 , to be added. These two modulated signals IF 1 Im and IF 1 Qm are at intermediate frequency IF 1 , thus the up-converted signal IF 1 C at the output of adder 107 , and thus at the output of unit 101 , is at intermediate frequency IF 1 . This signal IF 1 C has the advantage of not having feedthrough signals relating in particular to the stray capacitances and quantities linked to the transistors used in differential mixers.

Unit 102 is formed, in an entirely similar way to unit 101 , of two differential mixers 105 and 106 and an adder 108 . The signals received at the four inputs are the same four quadriphased baseband signals, and the control carrier signals are the same four signals that are also quadriphased at intermediate frequency IF 1 .

The difference between the two units 101 and 102 lies in the combination of signals received at their input with the control signals. Indeed, the purpose of this unit 102 is to provide at its one output another up-converted signal IF 1 C_b in phase opposition with respect to the up-converted signal provided at the output of unit 101 .

In order to do this, one need only invert the baseband signals or invert the carrier signals received by each of the mixers, which is the same thing, since the mixers are symmetrical. This inversion will cause a modification in the modulated signals obtained at the output of the mixers. Modulated signal IF 1 Im-b at the output of mixer 105 is in phase opposition with respect to signal IF 1 Im. Likewise, the modulated signal IF 1 Qm_b at the mixer 106 output is in phase opposition with respect to signal IF 1 Qm. Thus, the up-converted signal IF 1 C_b obtained at the output of adder 108 is in phase opposition with respect to up-converted signal IF 1 C.

At the output of up-conversion stage 1 , there are thus two up-converted in opposition signals IF 1 C and IF 1 C_b at intermediate frequency IF 1 . These two up-converted signals are provided to up-conversion stage 2 .

It should be noted in this embodiment example that the signals up-converted in phase opposition to each are obtained by similar units 101 and 102 , but it is however, possible to envisage using other means than mirror unit 102 to obtain the up-converted in phase-opposition signal IF 1 C_b, like for example a 180° phase shifter.

Stage 2 is formed of a single differential mixer 110 receiving at one input the up-converted signal IF 1 C and at another input the up-converted signal IF 1 C_b at intermediate frequency IF 1 . Mixer 110 is controlled by two carrier signals in phase opposition IF 2 I and IF 2 I_b, at intermediate frequency IF 2 . The modulated signal HF_M obtained at the output of mixer 110 corresponds to the signals received at its inputs but frequency transposed or up-converted. Indeed, modulated signal HF_M is a high frequency signal, whose frequency HF is equal to the sum of intermediate frequencies IF 1 +IF 2 .

The use of a differential mixer for the frequency transposition or up-conversion not only eliminates the feedthrough signals, due to stray capacitances of the transistors forming mixer 110 , but also eliminates the signals at intermediate stray frequencies IF 1 and IF 2 .

FIG. 4 shows a double up-conversion modulator according to the invention in detail. The global structure of the modulator has already been described with reference to FIG. 3 . Thus, the two up-conversion stages 1 and 2 and the two modulation units 201 and 202 of stage 1 can be seen again.

The differential mixers used are each formed of two transistors. The transistors used are preferably MOS technology transistors, which enables a high level of linearity to be obtained for the mixer and thus for the global structure.

Transistors 203 A to 206 A and 203 B to 206 B of FIG. 4 correspond to mixers 103 to 106 of FIG. 3, and transistors 210 A and 210 B of FIG. 4 correspond to mixer 110 of FIG. 3 .

In order to be able to obtain up-converted signals IF 1 C and IF 1 C_b at the output of units 201 and 202 , the baseband signals and the carrier signals provided to these two units are combined as follows:

transistors 203 A and 203 B receive at their drain, respectively, the in-phase baseband signal bbI and the in phase opposition baseband signal bbI_b, and at their gate, respectively, the in-phase carrier signal IF 1 I and the in phase opposition carrier signal IF 1 I_b, their source being connected 213 to the modulated signal IF 1 Im;

transistors 204 A and 204 B receive at their drain, respectively the in quadrature baseband signal bbQ and the in quadrature opposition baseband signal bbQ_b, and at their gate, respectively, the in quadrature carrier signal IF 1 Q and the in quadrature opposition carrier signal IF 1 Q_b, their source being connected 214 to modulated signal IF 1 Qm;

transistors 205 A and 205 B receive at their drain, in reverse with respect to transistors 203 A and 203 B, respectively, the in phase opposition baseband signal bbI_b and the in-phase baseband signal bbI, and at their gate, respectively, the in-phase carrier signal IF 1 I and the in phase opposition carrier signal IF 1 I_b, their source being connected 215 to modulated signal IF 1 Im_b;

transistors 206 A and 206 B receive at their drain, in reverse with respect to transistors 204 A and 204 B, respectively, the in quadrature opposition baseband signal bbQ_b and the in quadrature baseband signal bbQ, and at their gate, respectively, the in quadrature carrier signal IF 1 Q and the in quadrature opposition carrier signal IF 1 Q_b, their source being connected 216 to modulated signal IF 1 Qm_b;

adder 207 adds the modulated signals IF 1 Im and IF 1 Qm to give the up-converted signal IF 1 C and adder 208 adds the modulated signals IF 1 Im_b and IF 1 Qm_b to give the up-converted signal IF 1 C_b.

The two up-converted signals are supplied to the input of stage 2 . Mixer 110 of FIG. 3 corresponds to transistors 210 A and 210 B of FIG. 4 . In order to keep only the signal modulated at the desired frequency at the output of stage 2 , the transistors are controlled by in-phase and in phase opposition carrier signals at intermediate frequency IF 2 .

Transistor 210 A receives at its drain the in-phase up-converted signal IF 1 C, and at its gate the in-phase carrier signal IF 2 I. Transistor 210 B receives at its drain the in phase opposition up-converted signal IF 1 C_b and at its gate the in-opposition carrier signal IF 2 I_b. The sources of transistors 210 A and 210 B are connected 220 to the high frequency modulated signal HF_M, the frequency HF of this signal being equal to the sum of intermediate frequency IF 1 of the up-converted signals and intermediate frequency IF 2 of the carrier signals of stage 2 .

FIG. 5 is an example of the means used to generate the carrier signals at intermediate frequencies IF 1 and IF 2 . A generator 301 generating signals at intermediate frequency IF 2 supplies at output an in-phase signal IF 2 I, which is directly supplied to stage 2 of the modulator, means being provided to phase shift the in-phase signal by 180° and thus obtain in-phase signal IF 2 I and in phase opposition signal IF 2 I_b. Signal IF 2 I delivered by generator 301 is also supplied to an in quadrature frequency divider 302 . This divider supplies at one output in-phase signal IF 1 I and at another output in quadrature signal IF 1 Q at an intermediate frequency IF 1 that is two times lower than intermediate frequency IF 2 . These signals IF 1 I and IF 1 Q are supplied to stage 1 of the modulator as carrier signals, where means are also provided for generating the in phase opposition signals.

In the example of mobile telephony, the modulation frequency is 900 MHz. It will thus be advantageous to take a generator generating signals at 600 MHz and a frequency divider-by-two. Intermediate frequency IF 2 will thus be 600 MHz, intermediate frequency IF 1 will be two times smaller i.e. 300 MHz. This gives a high modulated signal HF at output of 900 MHz.

It is clear that the description is given only by way of example and that other embodiments, in particular in the means for generating opposite signals such as the mirror modulation unit ( 102 , 202 ) are comprised within the scope of the present invention.