TRANSMISSION CIRCUIT OF A RADIO FREQUENCY SIGNAL

Abstract: The present invention relates to a transmission circuit of a radio frequency signal, intended to receive an input signal (IN), comprising: a first frequency converter (FC1), having reference inputs intended to receive at least one differential signal, and intended to deliver a signal having a predetermined frequency called intermediate frequency, and a phase representative of that of the input signal (IN), and a second frequency converter (FC2) intended to deliver the radio frequency signal resulting from a conversion of the frequency of the output signal of the first frequency converter (FC1) into a tunable frequency. According to the invention, the first frequency converter (FC1) comprises digital means for selecting one of the reference inputs as a function of the value of the input signal (IN), for connecting it to the output of the first frequency converter (FC1). The invention enables to simplify the structure and the embodiment of the transmission circuit considerably and to improve the performance thereof. Application: Transmission of signals in radiotelephones.

Description: Transmission circuit of a radio frequency signal. FIELD OF THE INVENTION The present invention relates to a transmission circuit of a radio frequency signal, having at least one input intended to receive an input signal that has a phase that is representative of data carried by said signal, comprising: . a first frequency converter, having at least two inputs called reference inputs intended to receive a differential signal coming from a first oscillator, and one output intended to deliver an output signal that has a predetermined frequency called intermediate frequency and a phase that is representative of that of the input signal, and a second frequency converter having an output that forms the output of the transmission circuit and is intended to deliver the radio frequency signal resulting from a conversion of the frequency of the output signal of the first frequency converter into an adjustable frequency. BACKGROUND OF THE INVENTION Such transmission circuits are currently used, among other possible applications, in radiotelephones in which a signal processing circuit delivers to the transmission circuit a digital input signal in view of its transmission via an antenna. In known transmission circuits, the digital input signal is split up into a first and a second digital input signal in phase quadrature with each other. These first and second input signals are subjected, within the first frequency converter, to a digital/analog conversion and to a modulation called IQ modulation during which they are mixed with two phase quadrature signals coming from the first oscillator and having the intermediate frequency, and are then combined again for forming a resulting signal that constitutes the output signal of the first frequency converter. SUMMARY OF THE INVENTION The invention is linked with the following considerations: The IQ modulation makes it necessary to use analog mixers which are subjected to considerable constraints with respect to linearity and are thus complex and costly to manufacture. Furthermore, the digital/analog conversion of the first and second signals calls for two digital/analog converters which also have a complex structure and, moreover, introduce a noise in the converted signals, due to a sampling that is necessary for the conversion. A filtering device must thus be inserted between the digital/analog converters and the analog mixers, which further increases the complexity and manufacturing costs of the first frequency converter. Finally, another filtering device must be inserted between the first and the second frequency converter in order to eliminate undesirable signals which are the product of parasitic modulations introduced in the output signal of the first frequency converter by the analog mixers. It is an object of the present invention to remedy these drawbacks by proposing a transmission circuit in which the structure of the first frequency converter is simplified considerably and generates fewer undesirable signals than in known structures. Indeed, a transmission circuit in accordance with the opening paragraph is characterized according to the invention in that the first frequency converter comprises digital means for selecting one of the reference inputs as a function of the value of the input signal, for connecting said reference input to the output of the first frequency converter. In this transmission circuit, the first frequency converter can use the input signal in its digital form and the digital/analog converters are useless. The omission of these converters enables to reduce the noise generated by the first frequency converter and renders the filtering devices used in the known transmission circuit superfluous, which accordingly reduces the complexity of the transmission circuit according to the invention. Furthermore, it will be demonstrated in the following of the description that a splitting up of the input signal into a first and a second input signal, which are in phase quadrature with each other, indispensable for realizing the IQ modulation in known transmission circuits, may be avoided in certain embodiments of the invention, which will further enable to simplify the structure of the transmission circuit. In one of these embodiments of the invention, a transmission circuit as described above is characterized in that the first frequency converter comprises: . a delta/sigma module having an output intended to deliver an intermediate signal of which a mean value is equal to the value of the input signal, and . a multiplexer having two signal inputs that form the reference inputs of the frequency converter, one control input connected to the output of the delta/sigma module and one signal output forming the output of the frequency converter. This embodiment illustrates well the great simplicity the first frequency converter may feature thanks to the invention. In accordance with the operation principle of a delta/sigma module, well known to a person of ordinary skill in the art and notably described in the publication "Delta/sigma Converters" by Messers, Norsworthy, Schreier and Temes, the delta/sigma module produces a signal that oscillates between two values -1 and +1 at a sampling frequency that is higher than that of the input signal. Depending on the value assumed by the output signal of the delta/sigma module, the multiplexer alternatively switches to the output of the first frequency converter a first or a second signal, which both have the intermediate frequency, but are in phase opposition to each other, since they together form the differential output signal of the first oscillator. The first frequency converter thus easily carries out a phase modulation of the output signal of the first oscillator and delivers an output signal that has for its frequency the intermediate frequency and a phase that is representative of that of the input signal. In another embodiment, a transmission circuit according to the invention is characterized in that it is intended to receive a first and a second intermediate signal whose phases are in quadrature and represent data carried by said signals, and in that the first frequency converter comprises: . a first delta/sigma module having an output intended to deliver a first control signal of which a mean value is equal to the value of the first intermediate signal, . a second delta/sigma module having an output intended to deliver a second control signal of which a mean value is equal to the value of the second intermediate signal, and . a multiplexer having four signal inputs which form four reference inputs of the frequency converter and are intended to receive two differential signals which are in phase quadrature with each other, coming from the first oscillator, the multiplexer having a first and a second control input connected to the outputs of the first and second delta sigma modules, and a signal output forming the output of the frequency converter. This embodiment enables to double the resolution of the phase modulation carried out by the first frequency converter at the cost of the splitting up of the input signal of the transmission circuit into two phase quadrature signals. In a variant of this embodiment, the first frequency converter comprises: . a single delta/sigma module intended to deliver an intermediate signal of which an average value is equal to the value of the input signal, . a quadratic module intended to split up the intermediate signal into a first and a second control signal whose phases are in quadrature with each other and are representative of data carried by the input signal, and . a multiplexer having four signal inputs which form four reference inputs of the first frequency converter and are intended to receive two differential signals which are in phase quadrature with each other, coming from the first oscillator, the multiplexer having a first and a second control input respectively, intended to receive the first and a second control signal, and a signal output which forms the output of the first frequency converter. This variant is advantageous in that the first and second control signals are correlated. Thus, the noise generated by the modulation performed by the first frequency converter is solely linked with the phase quantification and is independent of the amplitude quantization. This implies that it is possible to use a limiter on the output of the first frequency converter, which limiter is then intended to transform the output signal of the first frequency converter into a square-wave signal by subjecting it to a large amplification that causes a systematic peak limiting. The useful information contained in the output signal of such a limiter will thus solely be represented by the switchings of the output signal of the limiter, and thus the useful information will be purely digital. This will enable to considerably simplify the structure of the second frequency converter. In an advantageous embodiment of the invention, the delta/sigma modules have an adjustable gain. Such an embodiment permits to influence the gain of the whole transmission circuit and thus to influence the gain of the second frequency converter, which enables to simplify the design and the mode of operation thereof. In a particular embodiment of the invention, the second frequency converter is formed by a phase-locked loop, comprising: . a phase detector having a first input connected to the output of the first frequency converter, a second input, and an output intended to deliver a signal representative of a phase difference between signals present on the first and second inputs, . a second oscillator having a tuning input connected to the output of the phase detector, and an output forming the output of the second frequency converter, and . a mixer having a first input connected to the output of the second oscillator, a second input connected to an output of a third oscillator, and an output connected to the second input of the phase detector. In another particular embodiment of the invention, the second frequency converter is formed by a phase-locked loop, comprising: . a phase detector having a first input connected to the output of the first frequency converter, a second input, and an output intended to deliver a signal representative of a phase difference between signals present on the first and the second input, . a second oscillator having a tuning input connected to the output of the phase detector, and an output forming the output of the second frequency converter, . a third oscillator of which an output is connected to the second input of the phase detector, and . a mixer having a first input connected to the output of the first oscillator, a second input connected to the output of the second oscillator, and at least one symmetrical output connected to the reference inputs of the first frequency converter. In a variant of this embodiment, a frequency divider is inserted between the first and the second frequency converter, which enables to mitigate the constraints that exist with respect to the oscillation frequencies of the various oscillators included in the transmission circuit. If the present invention can be used in all types of devices intended for the transmission of radio signals, its use is particularly advantageous in radiotelephones. The invention thus also relates to a radiotelephony device comprising a signal processing unit and an antenna intended to transmit a radio frequency signal, characterized in that it further includes a transmission circuit as described above, intended to receive an input signal from the signal processing unit and to deliver the radio frequency signal to the antenna. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment(s) described hereinafter. In the drawings: - Fig. 1 is a function diagram describing a transmission circuit in accordance with a first particular embodiment of the invention, - Fig. 2 is a Fresnel diagram describing the splitting up of an input signal performed in this particular embodiment of the invention, - Fig. 3 is a function diagram describing a delta/sigma module used in this particular embodiment of the invention, - Fig. 4 is a timing diagram describing the evolution of the input and output signals of such a delta/sigma module, - Fig. 5 is a function diagram describing a transmission circuit in accordance with a second embodiment of the invention and - Fig. 6 is a function diagram describing a transmission circuit in accordance with a third embodiment of the invention. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 diagrammatically shows a signal transmitter, for example a radiotelephone, including a transmission circuit according to the invention. This circuit is intended to receive from a signal processing unit PU a digital input signal IN and to deliver a radio frequency signal to an antenna ANT. In this particular embodiment, the transmission circuit comprises: . a first frequency converter FCl having four inputs called reference inputs intended to receive from a first oscillator OSC1 two differential signals (0; 180) and (90; 270) which have an intermediate frequency, and one output intended to deliver an output signal which has for its frequency the intermediate frequency and a phase representative of that of the input signal, and . a second frequency converter FC2 having one output, which forms the output of the transmission circuit and is intended to deliver the radio frequency signal resulting from a conversion of the frequency of the output signal of the first frequency converter FCl into a tunable frequency. In the embodiment described here, the digital input signal LN is split up into a first and a second input signal INI and LNQ, having phases in quadrature and representative of data carried by the digital input signal LN, by means of a quadratic module IQM. The first frequency converter FCl comprises a first delta/sigma module SDI which has an output intended to deliver a first control signal MI of which a mean value is equal to the value of the first input signal LNI. The first frequency converter FCl further includes a second delta/sigma module SDQ which has an output intended to deliver a second control signal MQ of which a mean value is equal to the value of the second input signal INQ. The first frequency converter FCl finally includes a multiplexer MUX which has four signal inputs forming the four reference inputs of the first frequency converter FCl and intended to receive the two difference signals (0; 180) and (90; 270) coming from a first oscillator OSC1, via another quadratic module QM. The multiplexer MUX further has a first and a second control input connected to the outputs of the first and second delta/sigma modules, and a signal output that forms the output of the first frequency converter FCl. Each delta sigma module SDI and SDQ produces a signal that oscillates between two values -1 and +1 at a sampling frequency that is higher than that of its input signal LNI or LNQ. Depending on the pair of values assumed by the pair of control signals (MI; MQ), the multiplexer MUX alternatively switches to the output of the first frequency converter FCl one of the signals (0, 90, 180, 270) that it receives on its reference inputs which have all the intermediate frequency, two successive signals being in phase quadrature with each other since they constitute in pairs differential signals in phase quadrature coming from the first oscillator OSC1. The first frequency converter FCl then easily carries out a phase modulation of the output signal of the first oscillator OSC1 and delivers an output signal that has the intermediate frequency and a phase representative of that of the input signal IN. If, in this embodiment, a choice has been made to split up the input signal LN into two components LNI and LNQ, it may also be envisaged to use only a single delta/sigma module that directly receives the input signal LN, while the multiplexer MUX thus has only two signal inputs which form the only two reference inputs of the first frequency converter FCl and are intended to receive only a single differential signal (0; 180) coming from the first oscillator OSC1. The structure of the first frequency converter FCl, and thus that of the whole transmission circuit, will thus be simplified, but the resolution of the phase modulation performed by the first frequency converter FCl will be reduced by half. In the particular embodiment represented by Fig. 1, the second frequency converter FC2 is formed by a phase-locked loop which includes a phase detector PD that has a first input connected to the output of the first frequency converter FCl, a second input, and an output intended to deliver a signal representative of a phase difference between signals present on the first and the second input. The phase-locked loop further includes a second oscillator OSC2, which has a tunable input connected to the output of the phase detector PD, and an output that forms the output of the second frequency converter FC2. A low-pass filter LPF is inserted between the phase detector PD and the second oscillator OSC2. The phase-locked loop finally includes a mixer MIX, which has a first input connected to the output of the second oscillator OSC2, a second input connected to an output of a third oscillator OSC3, and an output connected to the second input of the phase detector PD. Other embodiments, for example, based on a mixer, may be envisaged for constructing the second frequency converter FC2. However, the particular embodiment of the second frequency converter FC2 described by Fig. 1 is advantageous in that the phase-locked loop generates a radio frequency signal that has an outstanding spectral purity. Moreover, the frequency of the output signal of the transmission circuit may be tuned by modifying that of the output signal of the third oscillator OSC3. None the less, a certain number of constraints are the result of the structure described above: first of all, the difference between the frequencies of the output signals of the second and the third oscillator OSC2 and OSC3 must be equal to the value of the intermediate frequency, and the value of the gain of the phase- locked loop is to be such that it does not jeopardize the stability of said loop. In the following of the description there will be demonstrated that the invention largely enables to mitigate these constraints. Fig. 2 is a Fresnel diagram whose axes I and Q support a circle describing the evolution of a vector that represents the input signal IN of the transmission circuit described previously. According to this Fresnel diagram, the splitting up of the input signal LN into two signals LNI and LNQ in phase quadrature with each other may be interpreted as the projection of the vector LN on the axes I and Q. The signals LNI and LNQ are respectively the cosine and sine of the angle β that represents the phase of the input signal LN. Fig. 3 diagrammatically shows an embodiment of a delta/sigma module SDI included in the first frequency converter according to the invention. The basic structure of such a delta/sigma module SDI is known to the expert and comprises a subtracter SUB intended to receive on a first input the input signal of the delta/sigma module, an integrator LNT intended to integrate the output signal of the subtracter SUB, and a quantizer QUANT intended to deliver on an output a signal equal to -1 or +1 as a function of the value of the output signal of the integrator LNT. The output of the quantizer QUANT forms the output of the delta/sigma module and is further connected to a second input of the subtracter SUB. Such a structure is commonly called a structure of the first order. In other embodiments there may be chosen for realizing the delta/sigma module, a structure of the N* order, formed by N interleaved structures of the first order. In the embodiment described by Fig. 3, the delta/sigma module comprises a module A whose gain is variable. This module A enables to influence the gain of the whole transmission circuit, and thus the gain of the second frequency converter, which enables to simplify thereof the design and mode of operation. Indeed, if in certain operating conditions the gain of the phase-locked loop which forms the second frequency converter becomes, for example, sufficiently high to jeopardize the stability of said loop, it will be sufficient to diminish the gain of the variable gain modules A included in the delta/sigma modules to preserve said stability. This possibility renders the constraints to which the oscillators included in the transmission circuit are subjected considerably more flexible, which constraints may in certain cases form an obstacle for the realization of said oscillators in integrated form. Fig. 4 describes in the form of a timing diagram the evolution plotted against time of the output signal MI of the delta/sigma module described above. To facilitate the comprehension of the invention, the represented case is that shown by Fig. 2 where LNI = cosβ = 0.5 and the variable gain delta/sigma module A works as a follower having a gain equal to 1. The delta/sigma module SDI produces a signal MI oscillating between two values -1 and +1 at a sampling frequency that is higher than that of the input signal LNI, the mean value of the output signal MI being equal to the value of the input signal LNI. Since the second delta/sigma module SDQ has a structure that is identical to that described earlier, but is intended to receive an input signal LNQ in phase quadrature to the input signal LNI of the first delta/sigma module SDQ, it is clear that the pair of signals (MI, MQ) received by the multiplexer MUX on its two selection inputs, connected to the outputs of the first and second delta/sigma modules SDI and SDQ, will alternatively present each of the four possible combinations of the states -1 and +1. The multiplexer will thus alternatively select each of the signals (0, 90, 180, 270) present on its reference inputs. The first frequency converter FCl will thus perform a phase modulation of the output signal of the first oscillator OSCl as a function of the phase β of the input signal LN. Fig. 5 diagrammatically shows a signal transmitter device, for example, a radiotelephone, including a transmission circuit in accordance with a second embodiment of the invention. As far as possible, the elements common to this transmission circuit and that described in Fig. 1 are assigned identical references and will not be described again here. In this particular embodiment, the first frequency converter FCl comprises a sole delta/sigma module SD intended to deliver an intermediate signal LNT of which an average value is equal to the value of the input signal LN. The first frequency converter FCl further includes a module called quadratic module IQM intended to split up the intermediate signal LNT into a first and a second control signal MI and MQ whose phases are in quadrature with each other and represent data carried by the input signal LN. The multiplexer MUX has four signal inputs which form four reference inputs of the first frequency converter FCl and are intended to receive two differential signals (0; 180) and (90; 270) in phase quadrature with each other coming from the first oscillator OSCl, a first and a second control input intended to receive the first and the second control signal MI and MQ respectively, and a signal output forming the output of the first frequency converter FCl. In certain embodiments, the quadratic module IQM could be realized even inside the delta/sigma module SD. This variant is advantageous because of the fact that the first and second control signals MI and MQ are correlated. Thus, the noise generated by the modulation performed by the first frequency converter FCl is solely linked with a phase quantization and is independent of an amplitude quantization. This implies that it is possible to use a limiter LLM on the output of the first frequency converter FCl, which limiter LLM is intended to transform the output signal of the first frequency converter into a square-wave signal by subjecting it to a large amplification which causes a systematic peak limiting. The useful information contained in the output signal of such a limiter will thus solely be represented by the switchings of the output signal of the limiter, and thus be purely digital. This will permit to considerably simplify the structure of the second frequency converter FC2. More particularly, the phase detector PD may in this variant be formed by a simple phase/frequency comparator, whereas an analog multiplier would be necessary, in order to preserve a linear mode of operation, if the signal coming from the first frequency converter were analog. The second frequency converter FC2 can also be designed, in essence, in digital technology, which will simplify the structure and diminish the cost thereof while the performance will be improved. Fig. 6 diagrammatically shows a signal transmitter, for example, a radiotelephone, including a transmission circuit in accordance with another embodiment of the invention. As often as possible, the elements common to this transmission circuit and that described by Fig. 1 are assigned identical references and will not be described again here. In this particular embodiment, the second frequency converter FC2 is formed by a phase-locked loop which comprises a phase detector PD, which has a first input connected to the output of the first frequency converter FCl, a second input, and an output intended to deliver a signal representative of a phase difference between signals present on the first and the second input. The phase-locked loop further includes a second oscillator OSC2, which has a tuning input connected to the output of the phase detector PD and an output that forms the output of the second frequency converter. A low-pass filter LPF is inserted between the phase detector PD and the second oscillator OSC2. The phase-locked loop further includes a third oscillator OSC3 of which an output is connected to the second input of the phase detector PD. The first oscillator OSCl delivers, via a quadratic module QM, two differential signals in phase quadrature with each other, whose frequency is of the same order of magnitude as that of the output signal of the second oscillator OSC2. The phase-locked loop finally includes a double mixer (MIXl, MLX2) having two symmetrical inputs connected, via the quadratic module QM, to the output of the first oscillator OSCl. The double mixer (MIXl, MIX2) has another input connected to the output of the second oscillator OSC2, and two symmetrical outputs connected to four reference inputs of the first frequency converter FC 1 , and intended to receive two differential signals (0; 180) and (90; 270) in phase quadrature with each other, and having the intermediate frequency. This embodiment of the second frequency converter FC2 is advantageous in that the phase- locked loop generates a radio signal that has an outstanding spectral purity. Moreover, the frequency of the output signal of the transmission circuit may be tuned by modifying that of the output signal of the first oscillator OSCl, because the intermediate frequency is fixed and equal to the difference between the frequencies of the output signals of the first and the second oscillator OSCl and OSC2. If, in this embodiment, the choice is made to split up the input signal LN into two components LNI and LNQ, it may also be envisaged to use only a single delta/sigma module which directly receives the input signal LN, the multiplexer MUX thus having only two signal inputs which form the only two reference inputs of the first frequency converter FCl and which multiplexer is intended to receive only a single differential signal (0, 180) coming from a single mixer MX1. The structure of the first frequency converter FCl, and thus that of the whole transmission circuit, will thus be simplified, but the resolution of the phase modulation realized by the first frequency converter FCl will be reduced by half. Besides, in a variant of the embodiment described with reference to Fig. 6, it is possible to utilize a single delta/sigma module, intended to receive the input signal LN from the signal processing unit PU, and to split up the output signal of such a delta/sigma module into two control signals MI and MQ which are in phase quadrature with each other, which will enable to insert a limiter between the output of the first frequency converter and the frequency divider, with the advantages cited above. In the embodiment described here, the signals incoming at the reference inputs of the first frequency converter FCl have already been modulated, because of the mixing realized by the double mixer (MX1, MX2). Thus, the phase modulation realized by the first frequency converter FCl, although apparent in the output signal of the transmission circuit, is not significantly apparent in the output signal of the first frequency converter FCl. This authorizes the insertion of a frequency divider DIV/N between the first and second frequency converters FCl and FC2, which gives an additional degree of liberty for adjusting the values of the frequencies of the output signals of the various oscillators included in the transmission circuit, which simplifies the design and mode of operation thereof. More particularly, the frequency of the output signal of the third oscillator OSC3 will no longer have to be equal to the intermediate frequency. Advantageously, an oscillator already existing in another part of the transmitter(not shown in the Figure), can be used for realizing the third oscillator OSC3, such as, for example, an oscillator whose oscillation frequency is used as a reference for a phase-locked loop included in a receiving circuit present in the device.

Claims: CLAIMS: 1. A transmission circuit of a radio frequency signal, having at least one input intended to receive an input signal that has a phase that is representative of data carried by said signal, comprising: . a first frequency converter, having at least two inputs called reference inputs intended to receive a differential signal coming from a first oscillator, and one output intended to deliver an output signal that has a predetermined frequency called intermediate frequency and a phase that is representative of that of the input signal, and . a second frequency converter having an output that forms the output of the transmission circuit and is intended to deliver the radio frequency signal resulting from a conversion of the frequency of the output signal of the first frequency converter into an adjustable frequency, characterized in that the first frequency converter comprises digital means for selecting one of the reference inputs as a function of the value of the input signal, for connecting said reference input to the output of the first frequency converter. 2. A transmission circuit as claimed in claim 1, characterized in that the first frequency converter comprises: . a delta/sigma module having an output intended to deliver an intermediate signal of which a mean value is equal to the value of the input signal, and . a multiplexer having two signal inputs that form the reference inputs of the frequency converter, one control input connected to the output of the delta/sigma module and one signal output forming the output of the frequency converter. 3. A transmission circuit as claimed in claim 2, characterized in that the first frequency converter further includes a quadratic module intended to split up the intermediate signal into a first and a second control signal whose phases are in quadrature with each other and represent data conveyed by the input signal, the multiplexer having four signal inputs which form four reference inputs of the first frequency converter and are intended to receive two differential signals which are in phase quadrature with each other, coming from the first oscillator, the multiplexer having a first and a second control input intended to receive the first and the second control signal respectively, and a signal output forming the output of the first frequency converter. 4. A transmission circuit according to claim 1, characterized in that it is intended to receive a first and a second intermediate signal whose phases are in quadrature and represent data carried by said signals, and in that the first frequency converter comprises: . a first delta/sigma module having an output intended to deliver a first control signal of which a mean value is equal to the value of the first intermediate signal, . a second delta/sigma module haying an output intended to deliver a second control signal of which a mean value is equal to the value of the second intermediate signal, and . a multiplexer having four signal inputs which form four reference inputs of the frequency converter and are intended to receive two differential signals which are in phase quadrature with each other, coming from the first oscillator, the multiplexer having a first and a second control input connected to the outputs of the first and second delta sigma modules, and a signal output forming the output of the frequency converter. 5. A transmission circuit as claimed in one of the claims 2, 3 or 4, characterized in that the delta/sigma modules have an adjustable gain. 6. A transmission circuit as claimed in claim 1, characterized in that the second frequency converter is formed by a phase-locked loop, comprising: . a phase detector having a first input connected to the output of the first frequency converter, a second input, and an output intended to deliver a signal representative of a phase difference between signals present on the first and second input, . a second oscillator having a tuning input connected to the output of the phase detector, and an output forming the output of the second frequency converter, and . a mixer having a first input connected to the output of the second oscillator, a second input connected to an output of a third oscillator, and an output connected to the second input of the phase detector. 7. A transmission circuit as claimed in claim 1, characterized in that the second frequency converter is formed by a phase-locked loop, comprising: . a phase detector having a first input connected to the output of the first frequency converter, a second input, and an output intended to deliver a signal representative of a phase difference between signals present on the first and the second input, . a second oscillator having a tuning input connected to the output of the phase detector, and an output forming the output of the second frequency converter, . a third oscillator of which an output is connected to the second input of the phase detector, and . a mixer having a first input connected to the output of the first oscillator, a second input connected to the output of the second oscillator, and at least one symmetrical output connected to the reference inputs of the first frequency converter. 8. A radiotelephone apparatus comprising a signal processing unit and an antenna intended to transmit a radio frequency signal, characterized in that it further includes a transmission circuit as claimed in claim 1, intended to receive an input signal from the signal processing unit and to deliver the radio frequency signal to the antenna. 9. A frequency converter having a signal input intended to receive an input signal that has a phase that is representative of data carried by said signal, two reference inputs intended to receive from an oscillator a differential signal that has a predetermined frequency, and an output intended to deliver a signal having the predetermined frequency and a phase representative of that of the input signal, which frequency converter is characterized in that it comprises: . a delta/sigma module having an output intended to deliver an intermediate signal of which the mean value is equal to the value of the input signal for a given time interval, and . a multiplexer having two signal inputs that form the reference inputs of the frequency converter, one control input connected to the output of the delta/sigma module and one signal output forming the output of the frequency converter. 10. A frequency converter as claimed in claim 9, characterized in that it further includes a quadratic module intended to split up the intermediate signal into a first and a second control signal whose phases are in quadrature with each other and represent data conveyed by the input signal, the multiplexer having four signal inputs which form four reference inputs of the first frequency converter and are intended to receive two differential signals which are in phase quadrature with each other, coming from the first oscillator, the multiplexer having a first and a second control input intended to receive the first and the second control signal respectively, and a signal output forming the output of the first frequency converter. 11. A frequency converter having two signal inputs intended to receive a first and a second input signal that are in phase quadrature with each other and are representative of data carried by said signals, four reference inputs intended to receive from an oscillator two differential signals in phase quadrature having a predetermined frequency, and an output intended to deliver a signal that has the predetermined frequency and a phase that is representative of that of the input signals, which frequency converter is characterized in that it comprises: . a first delta/sigma module having an output intended to deliver a first control signal of which a mean value is equal to the value of the first input signal, . a second delta/sigma module having an output intended to deliver a second control signal of which a mean value is equal to the value of the second input signal, and . a multiplexer having four signal inputs which form four reference inputs of the frequency converter and are intended to receive two differential signals which are in phase quadrature with each other, coming from the first oscillator, the multiplexer having a first and a second control input connected to the outputs of the first and second delta/sigma modules, and a signal output forming the output of the frequency converter.

Inventors: PHILIPPE PASCAL, PHILIPPE, PASCAL
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPPE PASCAL, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPPE, PASCAL
Application year: 1999
Main classification: H04B1
IPC codes: H03M3, H04B1, H04L27