Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03C—MODULATION
  • H03C5/00—Amplitude modulation and angle modulation produced simultaneously or at will by the same modulating signal

Definitions

• the present invention relates to a method for amplifying and modulating the amplitude of a radiofrequency signal, a device for implementing the method, as well as a generator comprising such a device for generating a radiofrequency signal comprising a phase or frequency modulation component and an amplitude modulation component, a radiofrequency transmitter incorporating such a generator, as well as a mobile station and a fixed station of a radiocommunication system comprising such a transmitter.
• It relates to the field of techniques for amplifying radio frequency signals comprising, on the one hand, a phase or frequency modulation component, and, on the other hand, an amplitude modulation component, and suitable for radio transmission via antenna or cable.
• the invention finds applications in radiofrequency transmitters, in particular of mobile stations or fixed stations of a radiocommunication system, for example a private professional radiocommunication system (PMR system, from the English “Private Mobile Radiocommunication”).
• PMR system from the English “Private Mobile Radiocommunication”
• radiofrequency signal emitted comprises both a phase or frequency modulation component, and an amplitude modulation component.
• the addition of an amplitude modulation component generally improves the bit rate characteristics for a given channel width.
• the output stage of the transmitter includes a radio frequency power amplifier which, in order to obtain a high power output (which is particularly required when using the transmitter in a portable radio equipment), must operate in an operating area close to saturation.
• a power amplifier present in such an operating zone amplification non-linearities comprising non-linearities in amplitude and non-linearities in phase.
• these non-linearities are often designated by amplitude / amplitude conversions (or AM / AM conversions) or amplitude / phase conversions (or AM / PM conversions) respectively.
• These non-linearities generate a distortion of the transmitted signal, which degrades the performance of the transmitter in terms of quality of the emission, this loss of quality generally leading to a harmful broadening of the spectrum.
• the EER technique is very old since it was applied in the 1950s for the amplification of single sideband (SSB) radio frequency signals.
• SSB single sideband
• FIG. 1 gives the simplified diagram of a generator of a radiofrequency signal based on this technique.
• the modulation of the radiofrequency signal G at the output of the generator is broken down into a phase or frequency modulation component on the one hand, and an amplitude modulation component on the other hand. These two components are generated in baseband.
• a phase modulation component B is supplied, as a phase or frequency modulation signal, at the input of phase or frequency modulation means MOD, comprising for example a phase modulator, which ensure the transposition towards the field of radio frequencies.
• the MOD means include a phase locked loop (phase feedback loop).
• phase modulation means exhibit extremely low broadband noise characteristics due to the high spectral purity that the phase feedback loop achieves.
• the signal E at the output of the MOD modulation means is a signal of substantially constant amplitude modulated in phase. This signal is then amplified using a PA radio frequency amplifier.
• An amplitude modulation component C is supplied, as amplitude modulation signal, via circuits not shown in FIG. 1, on a gain control input of the amplifier PA, to control the gain of this amplifier. amplifier.
• This mechanism allows the amplitude modulation component to be reintroduced into the amplified radio frequency signal, without additional injection of noise.
• the PA amplifier can be a component comprising a gain control input or an assembly of components comprising a gain control input.
• the amplitude modulation component is superimposed on the phase modulation component to obtain the desired radiofrequency signal G at the output of the PA amplifier, these two components using different paths to reach the output of the PA amplifier.
• a radiofrequency transmitter based on the OPLEER technique is described, for example, in French patent application FR 2 716 589. This transmitter comprises means for correcting AM / AM conversions and means for correcting AM / PM conversions of the radiofrequency power amplifier PA, in the form, respectively, of an amplitude control loop and of a phase control loop of the output signal which are nested.
• FIG. 2 shows means for correcting the AM / AM conversions of the radiofrequency power amplifier PA, for a generator of the type represented in FIG. 1, which are described in the document FR 2 716 589 cited above.
• the analog servo loop comprises a COMP comparator amplifier whose first input receives the amplitude modulation signal C, whose second input receives a signal L, and whose output delivers an amplitude control signal F. The latter is applied to a control input PA amplifier gain.
• the output of the amplifier COMP is looped to its second input via an impedance such as a capacitor C in order to avoid parasitic oscillations of the signal F.
• the signal L is an analog signal representative of the power of the output signal G.
• the analog servo loop further comprises coupling means, such as a radio frequency coupler 4, making it possible to take part of the energy from the output signal G and delivering an image signal H image of the output signal G.
• coupling means such as a radio frequency coupler 4
• the DET detector makes it possible to extract from the signal H the amplitude modulation component of the output signal G, by applying rectification and low-pass filtering to the signal H so that the voltage amplitude of the signal L, conventionally expressed in decibel-Volt (dBV), which is a function of the instantaneous power of the signal H, conventionally expressed in decibel (dBm).
• dBV decibel-Volt
• the signal L is therefore representative of the amplitude modulation component actually present in the output signal G.
• the L signal and the amplitude modulation signal C are very close to each other, and differ only by the effect of AM / AM conversions in the PA amplifier.
• the signal L is compared to the amplitude modulation signal C by the comparator amplifier COMP, which produces the amplitude control signal F as a function of their difference.
• the operating point can move towards the saturation zone of l power amplifier.
• the analog servo loop may no longer allow an excursion of the control signal of amplitude F sufficient to obtain an output signal G having the required amplitude variation.
• the corresponding spectrum is shown on the graph in Figure 3d. As can be seen by comparing these figures with Figures 3a and 3b, the clipping distortion of the amplitude then leads to a widening of the spectrum of the output signal G.
• the invention aims to overcome the aforementioned drawback of the prior art, and for this purpose proposes a method of modulating the amplitude of a radiofrequency signal using a radiofrequency power amplifier with variable gain, which allows the saturation of the PA amplifier to be detected and the operating point of the amplifier to be lowered so as to decrease the average power at the amplifier output when the latter saturates.
• the invention provides a method of modulating the amplitude of a radiofrequency signal using a radiofrequency power amplifier, comprising the steps consisting in: a) controlling the power of the output signal of the amplifier by generating an error signal between an amplitude modulation signal and a signal representative of the power of the amplifier output signal, from which an amplifier gain control signal is generated; b) monitoring any saturation of the amplifier by comparing the relative phases of the error signal and the amplitude modulation signal; c) in the event of saturation of the amplifier, generate a correction signal enabling the operating point of the amplifier to be lowered.
• the invention further provides a device for implementing the method, comprising:
• the invention applies in particular to the restitution of the amplitude modulation component by controlling the gain of a radiofrequency power amplifier in a generator based on the EER technique or on the OPLEER technique, without however its scope is limited to this application.
• the invention indeed provides a generator of a radiofrequency signal having a phase modulation component and an amplitude modulation component, comprising:
• phase or frequency modulation means comprising an input which receives the phase or frequency modulation signal, and an output which delivers a radiofrequency signal of substantially constant amplitude modulated in phase or frequency;
• the invention also proposes a radiofrequency transmitter, for example a mobile station or fixed station transmitter of a PMR system, comprising such a generator.
• a mobile station and a fixed station of a radiocommunication system for example a PMR system, which include such a transmitter, as well as a radiocommunication system, for example a PMR system, comprising at least one such mobile station. and at least one such fixed station.
• - in Figure 6 the diagram of a generator of a radio frequency signal comprising a device according to the invention
• - in Figure 7 a graph illustrating the effects of a displacement of the operating point of the radiofrequency power amplifier of a device according to the invention
• - in Figures 8a to 8f graphs illustrating the comparison of the relative phases of the error signals and of the phase modulation signal according to the invention
• the device according to the invention is shown diagrammatically in FIG. 6, in its application to a generator of a radiofrequency signal comprising a phase or frequency modulation component and an amplitude modulation component and suitable for transmission radioelectric via antenna or cable, which is based on the OPLEER technique.
• the device according to the invention is indeed incorporated advantageously, but not necessarily, into such a generator.
• the OPLEER technique which was presented in the introduction with reference to the diagram in FIG. 2, is particularly suitable for transposition to radio frequencies and the amplification of power of baseband signals having a low amplitude modulation depth, that is to say a small variation in amplitude.
• FIG. 4a An example of such a baseband signal is shown in the graphs of Figures 4a, 4b and 4c. These figures respectively give the constellation, the spectrum, and the amplitude variation (centered around a zero mean value) of the signal.
• This signal is for example obtained by phase and quadrature filtering (I and Q filtering) of a width modulated signal pulse (modulation called CPM, from the English “Codeutere Modulation”). It is a good compromise between spectral occupancy and sensitivity to noise.
• the total amplitude variation does not exceed ⁇ 1 dB, with a ratio between peak power and rms power (from the English "Root-Mean-Square") less than 1 dB.
• the generator comprises means for generating a phase or frequency modulation signal B and an amplitude modulation signal C.
• phase modulation component which is obtained by frequency conversion from the I and Q components of the corresponding baseband signal.
• This component is thus determined by the phase or frequency modulation signal B, which is an analog signal.
• the phase modulation component can also be obtained by direct digital generation, in which case signal B would be a digital signal.
• amplitude modulation component which is obtained by rectifying the corresponding signal in baseband. This component is then determined by the amplitude modulation signal C, which is an analog signal. Note however that this component can also be obtained by direct digital generation in which case the signal C would be a digital signal.
• the signals B and C are supplied to the generator by upstream coding means.
• phase or frequency modulation means MOD the generator comprises a phase locked loop (phase feedback loop), as described in the document FR 2 716 589 cited in the introduction.
• the device advantageously comprises a FET transistor (from the English "Field Effect Transistor", which means field effect transistor) having characteristics such as those represented on the graph of FIG. 5.
• An amplifier of this type has the advantage, on the one hand, of having a curve of the output power Po which is linear over a wide interval of the voltage gain control, and, on the other hand, to have a total efficiency Rt which reaches its maximum value for a control voltage corresponding to an operating point sufficiently set back with respect to saturation. In this way the amplification of a signal of the type illustrated in FIGS. 4a, 4b and 4c can be done without the general efficiency of the amplification being reduced compared to the amplification of a signal with constant amplitude. at or beyond saturation point.
• the device according to the invention further comprises a comparator amplifier COMP, operating as an error amplifier. It is for example an operational amplifier.
• the amplifier COMP produces an error signal U between the amplitude modulation signal C and a signal L which is representative of the power of the output signal G of the amplifier PA.
• the signal L is produced by a detector DET from a signal H taken at the output of the amplifier PA by a coupler 4.
• the inverting input of the amplifier COMP which receives the signal L (here via a summing amplifier 7 to which we will return later), is connected to its output by means of two capacitors C2 and C3 in series. These capacitors make it possible to avoid parasitic oscillations of the output of the amplifier COMP.
• the signal U can be applied to a gain control input of the amplifier PA.
• the aforementioned elements of the device according to the invention then form means for controlling the power of the output signal G of the amplifier PA, known per se, and comparable to the corresponding elements of the generator shown in FIG. 2.
• means for controlling the power of the output signal G of the device according to the invention further comprise a summing amplifier 5, operating as an analog adder.
• the amplifier 5 produces a control signal F of the gain of the amplifier PA which is carried on the gain control input of the amplifier PA.
• the signal F is produced by the amplifier 5 by adding the error signal U on the one hand, and the amplitude modulation signal C or a signal P on the other hand, which signal P being obtained from said signal C by applying to the latter a pre-distortion by means of a predistortion module PD.
• the advantage of the PD pre-distortion module is as follows.
• the signal P at the output of the pre-distortion module PD is a signal which is in phase with the signal C. It corresponds to the voltage control which must be applied to the gain control input of the amplifier PA under nominal conditions so that the power of the output signal G corresponds to the desired value.
• the graph in FIG. 7 shows the characteristic of the amplifier PA, that is to say the curve of the output power Po as a function of the control voltage Vp applied to the gain control input. This graph illustrates the role of the means for controlling the amplitude of the output signal G.
• the aforementioned nominal conditions correspond to an operating point FPO. Under these nominal conditions, to obtain an output signal G with a given amplitude variation, represented in FIG. 7 by a curve 20, it is necessary to apply to the gain control input of the amplifier PA a control signal F having a given amplitude variation, represented in FIG. 7 by a curve 10.
• the following remarks can be made, by observing the relative phases of the error signal U on the one hand and of the amplitude modulation signal with pre-distortion (signal P ) or without pre-distortion (signal C) on the other hand.
• the amplitude variation of the amplitude modulation signal is greater than the value normally required at the PFO operating point.
• the error signal U then has the effect of reducing the amplitude control signal F, ensuring a power of the output signal G conforms to the desired value.
• the effective operating point is in a lower position than that of the PFO operating point under nominal conditions.
• the error signal U on the one hand and the amplitude modulation signal with pre-distortion (signal P) or without pre-distortion (signal C) on the other hand are in phase, the variation d amplitude of the amplitude modulation signal is greater than the value normally required at the PFO operating point.
• the error signal U then tends to increase the amplitude control signal F.
• the effective operating point is in this case in a higher position than that of the PFO operating point under nominal conditions.
• the error signal U may not possibly be able to reach a sufficient excursion to ensure the amplitude variation of the control signal F to be applied to the 'order entry gain of the power amplifier.
• the effective operating point is then in the saturation zone. In other words, the PA amplifier is saturated.
• the invention suggests to compare the relative phases of the error signal U and of the phase modulation signal C or of the phase modulation signal with pre-distortion P, in order to deduce therefrom a possible saturation of the PA amplifier.
• the device further comprises means for comparing the relative phases of the error signal U and of the amplitude modulation signal C.
• These means comprise means for synchronous detection of the alternative component Vy ⁇ of the error signal U with respect to the AC component Vc ⁇ of the amplitude modulation signal C, which produce a signal J.
• They further comprise means for comparing the signal J to a threshold, which comparison means, in if the PA amplifier is saturated, determined by exceeding the threshold, generate a correction signal W making it possible to lower the operating point of the PA amplifier.
• the synchronous detection means comprise a synchronous detector SD having at least two inputs and one output. A first of these inputs receives the alternating component Vy ⁇ of the error signal U which is taken by the capacitor C2. A second of these inputs receives the AC component VQ ⁇ of the amplitude modulation signal C via a capacitor C1. The output delivers the aforementioned signal J.
• the graphs in FIGS. 8a, 8b and 8c give a representation respectively of the AC component VQ ⁇ of the amplitude modulation signal C, the AC component ⁇ / ⁇ ⁇ of the error signal U, and of the signal
• the signal J at the output of the synchronous detector SD corresponds to the error signal U during the positive half-waves of the amplitude modulation signal C, and conversely to the signal of error U during the negative half-waves of the amplitude modulation signal C.
• a saturation of the amplifier PA is determined when the signal J is positive by exceeding a certain threshold.
• the means for comparing the signal J with a threshold comprise for example a comparator 6, or a Schmitt trigger or the like.
• the signal at the output of comparator 6 is filtered by a smoothing capacitor C4, or an integrating assembly or the like.
• the correction signal W is the signal at the terminals of this smoothing capacitor C4.
• the device further comprises means, such as the summing amplifier 7 presented above operating as an analog adder, for adding the correction signal W to the signal L representative of the power of the output signal G of the amplifier PA.
• the correction signal W which is a continuous signal, has the effect of simulating an increase in the continuous component of the output signal G, so that the means for controlling the amplitude of the output signal G intervene to lower the operating point of this amplifier.
• FIG. 9 an exemplary embodiment of the synchronous detector SD has been shown.
• the detector comprises a switch 90 with two positions.
• the switch 90 delivers on the output 98 of the detector the alternative component V
• the switch 90 delivers on the output 98 this alternative component Vy ⁇ after inversion thereof by means of an inverting amplifier 93.
• the inverting amplifier 93 is for example an assembly comprising an operational amplifier, whose gain is equal to -1. Switching of the switch 90 from one to the other of the positions 91 and
• the diagram in FIG. 10 shows another embodiment of the synchronous detector SD.
• the synchronous detector SD comprises a symmetrical multiplier 100 with two Gilbert cells.
• One and the multiplier comprises four inputs 105, 106, 107 and 108 and one output 109.
• the output 109 of the symmetrical multiplier delivers the signal J.
• the symmetrical multiplier 100 is preceded by an adaptation stage (not shown) similar to that used in quadrature detectors for demodulation of frequency modulation.
• the alternative component Vy ⁇ of the error signal U, received on a first input 103 of the detector SD, is carried on the input 105 and, via an inverting amplifier 101, on the input 106 of the symmetrical multiplier 100.
• the AC component VQ ⁇ of the amplitude modulation signal C, received on a second input 104 of the detector SD is carried on the input 107 and, via another inverting amplifier 102, on the input 108 of the symmetrical multiplier 100.
• the inverting amplifiers 101 and 102 are for example produced by respective assemblies each comprising an operational amplifier, and the gain of which is equal to -1.
• the means for controlling the power of the output signal G of the amplifier PA are arranged to produce the control signal F of the gain of the amplifier PA by adding to the amplitude modulation signal C (or the signal P corresponding to the amplitude modulation signal C with predistortion) the error signal U after weighting of the latter by a weighting parameter k, for example a multiplicative parameter.
• a weighting parameter k for example a multiplicative parameter.
• the value of the parameter k increases so as to compensate for the loss of gain of the PA amplifier (reflected by the decrease in the slope of the characteristic of the PA amplifier shown in FIG. 7). In this way, a constant loop gain is ensured, facilitating the production of the low-pass loop filter (not shown), the cut-off frequency of which would otherwise be variable as a function of the average power of the output signal G.
• DET is not used in an area where the detector could have linearity faults and where the PA amplifier does not. It is thus possible to use only one detector for which the detection linearity area is reduced, the predistortion alone ensuring the necessary corrections in the area where the power amplifier is practically linear.
• a step 73 the power of the output signal G of the amplifier PA is controlled by generating, in a sub-step 71 of l step 73, the error signal U between the amplitude modulation signal C and the signal L representative of the power of the output signal G of the amplifier PA.
• a control signal F of the gain of the amplifier PA is then generated from the error signal U.
• the control signal F is generated by adding the error signal U and the amplitude modulation signal C.
• the error signal U is added with the amplitude modulation signal C after a pre-distortion of the latter, that is to say, it is actually added with the signal P delivered by the predistortion module PD.
• a possible saturation of the amplifier PA is monitored by comparing the relative phases of the error signal U and of the amplitude modulation signal C.
• a correction signal W making it possible to lower the operating point of the amplifier PA.
• the saturation of the amplifier PA is monitored, by a substep 74 of step 76 consisting in performing synchronous detection of the AC component Vy ⁇ of the error signal U with respect to the AC component Vç ; ⁇ the amplitude modulation signal C, and by a substep 73 of step 76 consisting in comparing the signal produced by this detection (signal shown in FIGS. 8c and 8f) with a threshold, in order to generate, at step 77, the correction signal W when this threshold is exceeded.
• the correction signal W is added to the signal L representative of the power of the output signal G of the amplifier PA, in order to involve the means for controlling the amplitude of the output signal G to lower the operating point of the PA amplifier and thus leave the saturation zone thereof.
• FIG. 13 gives the diagram of a radiofrequency transmitter according to the invention.
• the transmitter 60 includes a data input 100 for receiving a digital message A containing data to be transmitted.
• the input 100 can be connected to the output of a speech coder or a channel coder.
• the transmitter also comprises composite coding means such as an encoder 200 for generating, from digital message A, a first series of digital values constituting the phase modulation signal B, and a second series of digital values constituting the amplitude modulation signal C.
• signal B and signal C are therefore digital signals.
• the transmitter further comprises, downstream of the encoder 200, a generator 300 of a radiofrequency signal comprising a phase modulation component and an amplitude modulation component, as described above with reference to FIG. 6.
• the transmitter finally comprises a radiofrequency antenna 400, connected to the output of the generator 300.
• This antenna allows the emission of the radiofrequency signal modulated in phase and in amplitude G on the transmission channel.
• the antenna 400 can be replaced by a cable.
• the system 70 comprises a network subsystem, symbolically represented by a cloud 73. It also includes a radio subsystem, comprising mobile stations 71 and / or fixed stations 72.
• the mobile stations 71 are for example portable terminals or Portable.
• the fixed stations 72 are for example base stations, ensuring the radio interface with the mobile stations which are located inside their radio coverage area. As a variant, they may be fixed terminals.
• At least one fixed station 71 and / or at least one mobile station 72 of the system 70 are equipped with a radiofrequency transmitter 60 in accordance with the diagram in FIG. 12.

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• 2001
  • 2001-06-13 FR FR0107746A patent/FR2826205B1/fr not_active Expired - Lifetime
• 2002
  • 2002-06-05 WO PCT/FR2002/001916 patent/WO2002101916A2/fr not_active Ceased
  • 2002-06-05 US US10/480,670 patent/US7024167B2/en not_active Expired - Lifetime
  • 2002-06-05 EP EP02748935A patent/EP1417748A2/de not_active Withdrawn

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