Patent Publication Number: US-7724837-B2

Title: Efficient modulation of RF signals

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 371 from PCT International Application No. PCT/EP2004/000047, filed on Jan. 7, 2004, which claims priority from European Patent Application Serial No. 03003830.1 filed Feb. 20, 2003, the disclosures and contents of which are incorporated by reference herein in their entireties. The above-referenced PCT International Application was published in the English language and has International Publication No. WO 2004/075395. 
   FIELD AND BACKGROUND OF THE INVENTION 
   The underlying invention generally relates to the field of modulator architectures of digital communication devices which are used for the processing of modulated RF signals, especially to a power amplifier circuit of a transmitting block for controlling the output power of an RF signal to be transmitted. 
   The wireless communications area has experienced a remarkable growth, and, as a result, the demand for high-efficient power control circuitries applied to wireless communications devices has increased dramatically. Because in a portable wireless environment all circuits are drawing power from a small battery, one of the most important aspects of circuitdesign. is to optimize the power consumption. Additionally, as wireless communication devices must be realized as low-cost products, the cost of the applied circuits must be reduced as well. 
   The basic structure of a conventional wireless communication system consisting of a QAM transmitter and a QAM receiver, respectively, is depicted in  FIGS. 1   a+b . On the transmitter side  100   a  of the system, which is shown in  FIG. 1   a , the basic operations are as follows: The digital data is first encoded (i k , q k ), and then, after being submitted to an digital-to-analog converter  102 , the complex-valued orthogonal in-phase (I) and quadrature (Q) channels are combined by means of a quadrature modulator  104 . The resulting signal combination is up-converted to the RF carrier frequency by means of an internal mixing stage. Then, after having passed a filtering stage, the RF signal drives a power amplifier  106 , whose output signal x(t) is fed to a transmission (TX) antenna  108 . The antenna radiates the signal into the air, and the transmission is complete. The receiver side  100   b , which is depicted in  FIG. 1   b , is simply the inverse, although slightly different components have to be used. First, the received RF signal x(t) is filtered to select the RF band of interest. After that, it is fed to a low noise amplifier  112  (LNA). The signal is then usually filtered and either directly down-converted from the passband to the baseband (in case of a homodyne receiver) or mixed to one or more intermediate frequency (IF) stages (in case a heterodyne or super-heterodyne receiver is employed). Thereby, the final mixing stage separates the signal into its I and Q components. Once at baseband, the I and Q signals are submitted to an analog-to-digital (A/D) converter  120  before they are further processed. 
   The power amplifier (PA) is the component of the system that takes the signal to be transmitted and amplifies it to the necessary level needed to drive the antenna for a particular power output level. In most wireless communications systems, the PA is the largest power consumer, usually because the amount of power that needs to be sent to the antenna (the power output) is itself very large. This does not include the total power that is consumed within the PA, just the amount that is required to drive the antenna. The total power consumed by the PA is necessarily greater than the power output, as there will always be some power consumed in the active devices and the peripheral circuitry. Since the power output specification itself is often larger than the power consumption of the rest of the blocks in the RF system and the power consumption of such a PA will be higher than the specified power output, the PA is decidedly the major power consumer of the system. 
   The signal to be transmitted is often transmitted by applying the output of the PA to a load device, be it a real circuit element or an antenna or similar device. Because the levels of power required to reliably transmit the signal are often quite high, there is a lot of power consumed within the PA. In many wireless applications, the amount of power consumed by this amplifier is not critical; as long as the signal being transmitted is of adequate power, that is good enough. However, in a situation where there is only a limited amount of energy available, which is not sufficient for the transmission procedure, the power consumed by all devices must be minimized, so as to maximize the length of time for which that energy is available. 
   The number of different classes of power amplifiers which are used today is too numerous to be counted, and they range from entirely linear to entirely non-linear, as well as from quite simple to inordinately complex. In PA terminology, a “linear” power amplifier is one which has a linear relationship between its input and output. Although a PA may comprise transistors operating in a nonlinear fashion (e.g. in case a FET switches between cutoff and saturation), it can still be considered linear. While nonlinear PAs feature a comparatively high efficiency, their nonlinearity causes the output signal to spread (due to intermodulation products, especially if there is a lot of phase noise in the local oscillator which will cause spreading of the input to the PA). 
   A power amplifier can consist of several serial stages. Each stage is usually more powerful than the previous one. As most of the quiescent current is drawn by the high power stages, which are not required for the low output power levels needed for wireless communication, means for bypassing high power stages when they are not required can lead to a significant reduction of energy consumption. 
   Since wireless telephones operate on battery power, it is also desirable that their transmitters operate as efficiently as possible to conserve power and extend battery life. Ideally for W-CDMA systems, such as those governed by the UMTS standard, power amplifier stages should be capable of efficient, linear operation in their required dynamic range. However, the prior art has not yet come close to the ideal, and many wireless telephones are having poor power management now. During low power transmissions, power is wasted by cascaded amplifier stages that are not needed. Consequently, attempts have been made to bypass unused stages. 
   Under normal operating conditions, conventional wireless transceivers devices use an APC circuit to control the output power of their amplification stages. The APC circuit found in most RF transceivers has an external connection that is intended to be connected to a linear power amplifier. After having detected the power of the modulated RF signal at the output of the final power amplifier, said signal is converted to a DC voltage and fed back to a variable-gain intermediate frequency (IF) stage in order to keep the final output power constant over a long period of time. As the APC voltage generation is done very early, the gain drift, which is caused by thermal drift, operating voltage deviation, etc., is not compensated by the circuit. Another option is to derive the ALC voltage from the drive power of the final amplifier and feed it to the external APC input of the RF transceiver. The theory is that when the power amplifier becomes overdriven, it will produce a negative voltage that is fed back into the transceiver&#39;s APC circuitry. This voltage acts as a gain control in the transmit stages of the transceiver which, in turn, automatically lowers the drive power (the transceivers output power) and limits distortion from the overdriven amplifier. 
     FIG. 2   a  shows a schematic block diagram for a conventional automatic power control (APC) loop according to the state of the art, which is used for stabilizing the signal level at the output port of an analog circuitry realizing an RF signal generator. Thereby, said circuit can also be used for executing an amplitude modulation. It comprises an frequency synthesizing unit (FSU), a power divider (e.g. a directional coupler), which feeds the reflected wave of the modulated RF output signal to a wideband detector diode, and an amplification stage whose output signal is fed to an electronically controllable attenuator (e.g. an amplitude modulator stage realized by means of PIN diodes). In case said RF signal generator is used for sweep-frequency applications, an external detector is usually applied in order to keep the signal level at the input port of a tested RF unit constant. Thereby, it should be noted that the output impedance of the FSU is changed due to the APC loop. 
   BRIEF DESCRIPTION OF THE STATE OF THE ART 
   In the U.S. Pat. No. 5,661,434 a high-efficiency multiple-power-level amplifier circuit for reducing power consumption during low-power operations is disclosed. Thereby, a plurality of power amplifier stages are cascaded to provide multiple levels of amplification. At least one power amplification stage includes a signal switching network in order to allow one or any combination of power amplifiers to be switched off when lower power operations are desired. The switched off power amplifiers are biased such that substantially no current is drawn from the power source. 
   A system and method for amplification of a communication signal is disclosed in the patent application publication US 2002/0193085 A1. This system and method comprises the features of the preambles of the independent claims. 
   According to US 2002/0193085 A1 bandwidth reduction of amplitude and/or phase components of relatively wide bandwidth composite signal. In an example, an EER amplifier system for CDMA signal amplification includes an amplitude bandwidth reduction module included in an amplitude signal component path and a phase bandwidth reduction module is included in a phase signal component path, for controlling an RF amplifier. The phase bandwidth reduction module may reduce the phase component bandwith of the input signal by, for example, generating a non-linear relationship between phase signal amplitude and input signal amplitude. The amplitude bandwidth reduction module may reduce the amplitude component bandwidth of the input signal by, for example, generating a non-linear relationship between the supply voltage to the RF amplifier and an input signal amplitude. 
   The European patent application EP 1 229 642 A1 pertains to a power amplifier circuit of a mobile transmitter unit for amplifying an input RF signal comprising an amplification path for amplifying an input RF-signal, a bypass path for bypassing the amplification path and a control terminal for controlling the operation mode of the power amplifier circuit between an amplifying mode and a bypassing mode. Thus, the input RF-signal is either amplified by the amplification path in the amplifying mode or bypassed by the bypass path in the bypassing mode. The bypass path is designed in such a way that it substantially does not affect the output RF transmission signal in the amplifying mode and that there is no need for a serial switch in the signal path of the bypass path. However, EP 1 229 642 A1 describes only the final stage of the transmitter unit and the possibility to bypass the final power amplifier stage in order to save DC energy when transmitting low-power signals. 
   A process for reducing the power consumption of a wireless transmitter and/or receiver, in particular a cellular mobile telephone with a corresponding transmitter and receiver unit, is disclosed in the US patent No. 2002/0010010 A1. Thereby, power consumption is reduced by using fractional-division phase-locked loops (PLLs) receiving a frequency reference from a fairly inaccurate quartz oscillator. Power consumption is also reduced by switching the output of the oscillator onto the input of the processing stage where the transmission/reception stage is inactive. The fractional-division PLLs can then be deactivated. 
   In the US patent 2002/0018534 A1 a multiplier arrangement (MUXER) is disclosed which is adapted to generate the components of a high-frequency phase vector from analog phase information and high-frequency local oscillator (LO) signals as well as to synthesize the phase vector from said components within a summing means. It is furthermore adapted to provide said phase vector as a vector which is making an excursion alongside the contours of a square within the complex plane during a first category of predetermined transitions of a phase signal on which said analog phase information is dependent. A signal modulator including such a multiplier arrangement as well as a transmitter are described as well. 
   Moreover, in the U.S. Pat. No. 5,003,270 an improved circuit for controlling the output power level of a radio frequency (RF) amplifier is disclosed. Thereby, the circuit samples the output of an RF amplifier. These samples are amplitude-modulated at a preselected modulation frequency and passed to a detector. In response to the amplitude-modulated signal, the detector produces a signal whose amplitude is indicative of the respective RF amplifier&#39;s output power and whose frequency is the modulation frequency. The amplitude of the signal produced by the detector is then compared to a reference amplitude, and the difference (the error signal) is used to control the gain of the RF amplifier. 
   A method and apparatus for linearizing an efficient class D/E power amplifier using delta modulation and a delta-modulated power supply, respectively, are disclosed in the two U.S. Pat. Nos. 5,847,602 and 5,973,556. Thereby, a delta-modulated magnitude amplifier is used to amplify the magnitude component of an RF power amplifier which employs envelope elimination and restoration. The delta-modulated amplifier introduces a smaller amount of non-linearity than traditional approaches, which are based upon pulse-width modulation. The disclosed technique can be implemented by using switched-capacitor circuits in a standard MOS technology with only two external components, e.g. an inductor and a capacitor. Thus, the disclosed technique allows the implementation of an efficient and yet linear RF power amplifier using low-cost MOS technology. 
   The invention disclosed in the U.S. Pat. No. 6,377,784 describes a high-efficiency power control of a high-efficiency (e.g. hard-limiting or switch-mode) power amplifier which is used to achieve a desired amplitude modulation. In one embodiment, the spread between a maximum frequency of the desired modulation and the operating frequency of a switch-mode DC/DC converter is reduced by following the switch-mode converter with an active linear regulator. Said regulator is designed to control the operating voltage of the power amplifier with sufficient bandwidth to faithfully reproduce the desired amplitude modulation waveform. The linear regulator is further designed to reject variations on its input voltage, even while the output voltage is changed in response to an applied control signal. This rejection occurs even though the variations on the input voltage are of commensurate or even lower frequency than that of the controlled output variation. In this connection, amplitude modulation may be achieved by either directly or effectively varying the operating voltage of the power amplifier while simultaneously achieving high efficiency in the conversion of primary DC power to the amplitude-modulated output signal. High efficiency is enhanced by allowing the switch-mode DC/DC converter to also vary its output voltage such that the voltage drop across the linear regulator is kept at a low and relatively constant level. Time-division multiple access (TDMA) bursting capability may be combined with efficient amplitude modulation, with control of these functions being combined. In addition, the variation of average output power level in accordance with commands from a communications system may also be combined within the same structure. 
   OBJECT OF THE UNDERLYING INVENTION 
   In view of the state of the art mentioned above, it is the object of the underlying invention to provide a particular efficient and flexible technique for modulating radio frequency (RF) signals. 
   The invention particularly deals with an optimization of the different amplification stages of a modulator. 
   The aforementioned object is achieved by means of the features in the independent patent claims. Advantageous features are defined in the dependent patent claims. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention, method for I/Q modulation is proposed, wherein
         the in-phase (I) and quadrature (Q) component of a complex-valued analog baseband input signal (x LP (t)) are converted to signals representing the magnitude (|x LP (t)|) and phase (∠x LP (t)) of the I/Q modulation states (x LP (t)), respectively and   the envelope (|x LP (t)|) and phase signals (∠x LP (t)) are combined to generate a modulated RF output transmission signal (x(t)).       

   Thereby slow high-level components and fast low-level components of the signal representing the magnitude (|x LP (t)|) of an I/Q modulation state (x LP (t)) are amplified in separate stages. The slow high-level components of the signal representing the magnitude (|x LP (t)|) of an I/Q modulation state (x LP (t)) can be amplified using a DC/DC converter. 
   Slow high-level components and fast low-level components of the signal representing the magnitude (|x LP (t)|) of an I/Q modulation state (x LP (t)) can be combined (x MC (t)) by supplying the low-level components to a power amplifier whose gain is controlled depending on the amplified high-level components. 
   The combined envelope (|x LP (t)|) representing signal (x MC (t)) can be controlling the gain of at least one further power amplifier supplied with an RF signal (x P (t)) modulated with the phase information (∠x LP (t)) of the I/Q modulation states (x LP (t)). 
   A signal (x MC (t)) representing the magnitude (|X LP (t)|) of the I/Q modulation states can control the gain of several of serially connected power amplifiers, wherein the most upstream one ( 212 ) of the cascade of power amplifiers is supplied with an RF signal (x P (t)) carrying the phase information (∠x LP (t)) of the I/Q modulation states (x LP (t)). 
   At least one of the cascade of power amplifiers can be selectively bypassed. 
   The power supply (V bat ) of the bypassed power amplifier can be selectively switched off. 
   The signal (x MC (t)) representing the magnitude (|x LP (t)|) of the I/Q modulation states (x LP (t)) and controlling the gain of one of said serially connected power amplifiers can be selectively dampened. 
   The high-level components of the signal (x MC (t)) representing the magnitude (|x LP (t)|) of an I/Q modulation state (x LP (t)) are represented by a ramp signal supplied from a baseband block to an amplifier. 
   According to a further aspect of the present invention a method for selectively carrying out a frequency or I/Q modulation is proposed, wherein
         the in-phase (I) and quadrature (Q) component of a complex-valued analog baseband input signal (x LP (t)) is converted to signals representing the magnitude (|x LP (t)|) and phase (∠x LP (t)) of the I/Q modulation states (x LP (t)) respectively and   the envelope (|x LP (t)|) and phase signals (∠x LP (t)) are combined to generate a modulated RF output transmission signal (x(t)),       

   Thereby the power (p x (t)) of the modulated RF output transmission signal (x(t)) is selectively fed back to provide an automatic power-controlled (APC) loop. 
   The APC loop can be opened in case of I/Q modulation as soon as the power (p x (t)) of the modulated RF output transmission signal (x(t)) reaches a defined nominal value (P 0 ). 
   According to a still further aspect of the present invention a I/Q modulator is proposed designed to carry out a method as set forth above. 
   According to a still further aspect of the present invention a I/Q modulator is proposed, comprising
         a unit ( 208 ) for converting the in-phase (I) and quadrature (Q) component of a complex-valued analog, baseband input signal (x LP (t)) to signals representing the magnitude (|x LP (t)|) and phase (∠x LP (t)) of the I/Q modulation states (x LP (t)), respectively, and   a unit ( 212 ) for combining the magnitude (|x LP (t)|) and phase signals (∠x LP (t)) to generate a modulated RF output transmission signal (x(t)),       

   The I/Q modulator can comprise separate stages for amplifying slow high-level components and fast low-level components of the signal representing the magnitude (|x LP (t)|) of an I/Q modulation state (x LP (t)), respectively. 
   A DC/DC converter can amplify the slow high-level components of the signal (x MC (t)) representing the magnitude (|x LP (t)|) of an I/Q modulation state (x LP (t)). 
   A power amplifier can be present supplied with the fast low-level components and gain-controlled depending on the amplified slow high-level components in order to combine the slow high-level components and fast low-level components of the signal (x MC (t)) representing the magnitude (|x LP (t)|) of an I/Q modulation state (x LP (t)). 
   At least one further power amplifier can be present being supplied with an RF signal (x P (t)) modulated with the phase information (∠X LP (t)) of the I/Q modulation states (x LP (t)) and being gain-controlled by the combined envelope (|x LP (t)|) representing signal (x MC (t)). 
   Several serially connected power amplifiers can be provided being gain-controlled by a signal (x MC (t)) representing the magnitude (|x LP (t)|) of the I/Q modulation states (x LP (t)), wherein the most upstream one of the cascade of power amplifiers is supplied with an RF signal (x P (t)) carrying the phase information (∠x LP (t)) of the I/Q modulation states (x LP (t)). 
   A switch can be provided to selectively bypass at least one of the cascade of power amplifiers. 
   A switch can be provided to selectively switch off the power supply (V bat ) of the bypassed power amplifier. 
   A switch (S 1 ) to selectively dampen the signal (x MC (t)) representing the magnitude (|x LP (t)) of the I/Q modulation states (x LP (t)) and controlling the gain of one of said serially connected power amplifiers. 
   The power amplification circuit according to the present invention can advantageously be used in wireless communication devices which are capable of transmitting wireless signals according to the GSM, EDGE or UMTS standard. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantageous features, aspects, and advantages of the invention will become evident from the following description, the appended claims, and the accompanying drawings. Thereby, 
       FIG. 1   a  presents a schematic block diagram of a conventional QAM transmitter using an I/Q modulator according to the state of the art, 
       FIG. 1   b  presents a schematic block diagram of a conventional QAM receiver using an I/Q demodulator according to the state of the art, 
       FIG. 2   a  shows a schematic block diagram for a conventional automatic level control (ALC) loop, which is used for stabilizing the signal level at the output port of an analog circuitry realizing a measurement generator, and 
       FIG. 2   b  depicts a schematic block diagram of a QAM transmitter with an I/Q modulator. 
   

   DETAILED DESCRIPTION OF THE UNDERLYING INVENTION 
   In the following, one embodiment of the underlying invention as depicted in  FIG. 2   b  shall be explained in detail. The meaning of the symbols designated with reference signs in  FIGS. 1   a  to  2   b  can be taken from an annexed table. 
   In the shown I/Q modulator  200   b  the in-phase (I) and quadrature (Q) component of a complex-valued analog baseband input signal x LP (t) are supplied from a baseband input  228   a  and then converted to signals representing the magnitude |x LP (t)| and phase ∠x LP (t) of the I/Q modulation states x LP (t), respectively, in a processing block  208 . 
   Note that the baseband input may already consist of a signal representing the magnitude |x LP (t)| and phase ∠x LP (t) of the I/Q modulation states x LP (t), respectively, in which alternative case the conversion block  208  is not necessary. 
   The amplitude |x LP (t)| and phase signals ∠x LP (t) are separately processed and then combined in a series of amplification stages  212 ,  214 , to generate a modulated RF output transmission signal x(t). 
   The baseband and/or the conversion unit  208  are thereby designed to split the signal representing the magnitude |x LP (t)| of an I/Q modulation state x LP (t) in a first branch  224   b  for slow high-level components and a second branch  224   a  for fast low-level components. According to the invention, these two branches are processed separately. F.e. slow high-level components  222  represented in a ramp signal  226  and fast low-level components  224  of the signal representing the magnitude |x LP (t)| of an I/Q modulation state x LP (t) are amplified in separate stages  204 ,  206 . 
   The slow (narrowband) high-level components  222  of the signal representing the magnitude |x LP (t)| of an I/Q modulation state x LP (t) are f.e included in a ramp signal (this is known as such from the UMTS and EDGE specifications) fed to an OP amplifier  202  (which second inverted input is optionally supplied with a feedback signal as will be explained later) and finally amplified using a DC/DC converter  204  supplied with a battery DC voltage  201 . 
   The slow (narrowband) high-level components  222  and fast (wideband) low-level components  224   a  of the signal representing the magnitude |x LP (t)| of an I/Q modulation state x LP (t) are combined by supplying the low-level components  224   a  to a power amplifier  206  whose gain is controlled by the output of the DC/DC converter  204  and thus depending on the amplified high-level components  222 . 
   The phase modulation of the analog phase signal (∠x LP (t)) out put by the conversion block  208  (or directly by the baseband) is performed by an up-conversion mixing stage  210  comprising a frequency-synthesizing unit  210   b  (FSU) equipped with a voltage-controlled oscillator  210   a  (VCO), whose output frequency is controlled by an external frequency control circuit  216 , and a phase-locked loop  210   c  (PLL) which is used for compensating phase instabilities of the VCO  210   a.    
   The combined envelope-representing signal x MC (t) output by the wide-band amplifier  206  is selectively dampened (see resistor R and switch S 1 ) controls the gain of at least one further power amplifier  212  supplied with the output signal of the up-conversion mixing stage  210 , i.e. the RF signal x P (t) modulated with the phase information ∠x LP (t) of the I/Q modulation states x LP (t). Thus, the modulated RF transmission signal x(t) is obtained by a multiplication of the combined amplitude representing component x MC (t) and the phase-modulated RF carrier x P (t). 
   According to a further embodiment of the invention, a signal x MC (t) representing the magnitude |x LP (t)| of the I/Q modulation states controls the gain of several of serially connected power amplifiers  212 ,  214 , wherein the most upstream one ( 212 ) of the cascade of power amplifiers  212 ,  214  is supplied with an RF signal x P (t) carrying the phase information ∠x LP (t) of the I/Q modulation states x LP (t). Thereby, at least one  214  of the cascade of power amplifiers  212 ,  214  can be selectively bypassed by means of a switch S 3 . In the bypass case, the power supply V bat  of the bypassed power amplifier  214  can be selectively switched off by a switch S 2 . With other words, the switch S 3  is closed by a control unit (not shown) when the switch S 2  is opened (and otherwise). 
   The power amplifier circuit according to the underlying invention works as follows: In the amplifying mode (high power mode), an input RF signal is amplified by the amplification path  212 ,  214 , while the bypass path  215  in this mode does not influence the amplification path  3  operation. In the bypassing mode (low power mode) with closed switch S 3 , an input RF signal passes the power amplifier circuit output through the bypass path  215  without considerable changes in signal power. In this mode, the final power amplifier  214  does not affect the bypass path operation at all. As a result, when operating in the broadband spectrum of the required output signal, a significant decrease in the average consumed current of the amplification means  4  can be achieved. 
   The signal x MC (t) representing the magnitude |x LP (t)| of the I/Q modulation states x LP (t) and controlling the gain of one ( 212 ) of said serially connected power amplifiers can be selectively dampened by means of resistor R in case switch S 1  is open. The small “modulation” in the amplifier  212  through the resistor R is added to enable the use of a high efficient Class C amplifier as amplifier  212  to carry out a low distortion collector or source amplitude modulation. 
   Note that in case the bypass mode  215  for the following amplifer  214  is used by closing the switch S 3 , the switch S 1  is also closed (by a control unit not shown) and the small dampened modulation of the amplifier  212  is thus changed to the full value. 
   Now the APC (automatic power control) loop  220  will be explained: The small part of the output signal power p x (t), which is extracted by means of a directional coupler  218 , is detected by an amplitude detector, amplified and selectively (switch S 4 ) fed back to the inverting input of the operation amplifier  202  in order to provide an automatic power-controlled (APC) loop  220 . 
   The APC loop  220  is closed/opened depending on the modulation scheme (frequency modulation as in case of GSM or QAM or higher as in case of UMTS and EDGE) and the operation state of the transmitter as will be explained in the following: 
   In UMTS or EDGE the switch S 4  is open and the ouput signal of the OP amplifier  202  thus contains the amplitude information coming from the baseband and included in the ramp signal  226 . When processing EDGE signals, however, the APC loop can be closed until the signal power reaches a defined nominal value. At this moment the APC loop is opened as otherwise the amplitude information would be detoriated. 
   When transmitting frequency modulated GSM signals, the APC loop is closed and the well-known GSM ramp power controll (power-up and power-down) is carried out. No amplitude modulatin is carried out. 
   A further embodiment of the underlying invention pertains to an I/Q modulator  200 , which comprises a unit  208  for converting the in-phase (I) and quadrature (Q) component of a complex-valued analog baseband input signal x LP (t) to signals representing the magnitude |x LP (t) and phase ∠x LP (t) of the I/Q modulation states x LP (t), respectively, and a unit  212  for combining the amplitude |x LP (t)| and phase signals ∠x LP (t) to generate a modulated RF output transmission signal x(t). Thereby, said I/Q modulator  200  comprises separate stages  204 ,  206  for amplifying slow high-level components  222  and fast low-level components  224  of the signal representing the magnitude |x LP (t)| of an I/Q modulation state x LP (t), respectively. 
   In this connection, the slow high-level components  222  of the signal x MC (t) representing the magnitude |x LP (t)| of an I/Q modulation state x LP (t) are amplified by means of a DC/DC converter  204 . 
   A first power amplifier  206  providing a pre-amplification stage is supplied with the fast low-level components  224  and gain-controlled depending on the pre-amplified slow high-level components  222  in order to combine the slow high-level components  222  and fast low-level components  224  of the signal x MC (t) representing the magnitude |x LP (t)| of an I/Q modulation state x LP (t). The I/Q modulator  200  comprises at least one further power amplifier  212  being supplied with an RF signal x P (t) modulated with the phase information ∠x LP (T) of the I/Q modulation states x LP (t) and being gain-controlled by the combined envelope-representing signal x MC (t). 
   According to another embodiment of the underlying invention, several serially connected power amplifiers  212 ,  214 , which are gain-controlled by a signal x MC (t) representing the magnitude |x LP (t)| of the I/Q modulation states x LP (t), wherein the most upstream one ( 212 ) of the cascade of power amplifiers  212 ,  214  is supplied with an RF signal x P (t) carrying the phase information ∠x LP (t) of the I/Q modulation states x LP (t), are employed. 
   For the sake of efficiency optimization, at least one ( 214 ) of the cascade of power amplifiers  212 ,  214  can selectively be bypassed by a bypass circuit  215 , which is activated when switch S 3  is closed. In the bypass case, the power supply V bat  of the bypassed power amplifier  214  can selectively be switched off by means of another switch (S 2 ). The bypass mode should be used when the needed RF signal power is low and the driver PA  212  is able to produce enough gain. In this connection, switch S 2  helps to reduce energy consumption as some PA chips do not have any power saving mode. According to another embodiment, the signal x MC (t), a switch (S 1 ) is used to selectively dampen the signal x MC (t) representing the magnitude |x LP (t)| of the I/Q modulation states x LP (t) by means of a resistor R and control the gain of one ( 212 ) of said serially connected power amplifiers  212 ,  214 . All switches can be realized by the use of PIN diodes or MOS field effect transistors (FETs). 
   Finally, another embodiment of the invention refers to a wireless transmitter and a mobile communication device, wherein an I/Q modulator  200  as described above is applied. 
   The proposed technique thus allows universal transmitter supporting different standards of cellular communication systems (and also multistandard environments). The transmitter can be set to different high efficient modes for different standards such as f.e. QAM (CDMA, WCDMA or EDGE) and GMSK. 
   
     
       
         
             
           
             
               TABLE 
             
           
          
             
                 
             
             
               Depicted Features and Their Corresponding Reference Signs 
             
          
         
         
             
             
          
             
               No. 
               Technical Feature or System Component 
             
             
                 
             
             
               100a 
               schematic block diagram of a conventional QAM transmitter 
             
             
                 
               using an I/Q modulator according to the state of the art 
             
             
               100b 
               schematic block diagram of a conventional QAM receiver using 
             
             
                 
               an I/Q demodulator according to the state of the art 
             
             
               102 
               digital-to-analog (D/A) converter in the transmission chain 
             
             
                 
               of the QAM transmitter 100a 
             
             
               104 
               I/Q modulator in the transmission chain of the QAM 
             
             
                 
               transmitter 100a 
             
             
               106 
               power amplifier in the transmission chain of the QAM 
             
             
                 
               transmitter 100a 
             
             
               108 
               transmitting (TX) antenna of the QAM transmitter 100a 
             
             
               110 
               receiving (RX) antenna of the QAM receiver 100b 
             
             
               112 
               low noise amplifier (LNA) in the reception chain of the QAM 
             
             
                 
               receiver 100b 
             
             
               114 
               down-conversion mixer in the reception chain of the QAM 
             
             
                 
               receiver 100b 
             
             
               116 
               band-pass filter (BPF) in the reception chain of the QAM 
             
             
                 
               receiver 100b 
             
             
               118 
               I/Q demodulator in the reception chain of the QAM receiver 100b 
             
             
               120 
               analog-to-digital (A/D) converter in the reception chain 
             
             
                 
               of the QAM receiver 100b 
             
             
               122 
               local oscillator (LO) of the QAM receiver 100b 
             
             
               200a 
               schematic block diagram of a conventional automatic level control 
             
             
                 
               (ALC) loop according to the state of the art, which is used for 
             
             
                 
               stabilizing the signal level at the output port of an analog 
             
             
                 
               circuitry realizing an RF signal generator 
             
             
               200b 
               schematic block diagram of a practical QAM transmitter with an 
             
             
                 
               I/Q modulator, which comprises a power amplifier circuitry 
             
             
                 
               according to the underlying invention 
             
             
               201 
               DC power supply (V bat ) for the I/Q modulator of the QAM 
             
             
                 
               transmitter 200b 
             
             
               201′ 
               frequency synthesizing unit (FSU) of the RF signal generator 200a 
             
             
               202 
               operation amplifier (Op Amp), supplied with the high-level 
             
             
                 
               components 222 of the signal x MC (t) representing the 
             
             
                 
               magnitude |x LP (t)| of an I/Q modulation state x LP (t), 
             
             
                 
               which are represented by a ramp signal 226 supplied from 
             
             
                 
               a baseband block 
             
             
               204 
               DC/DC converter amplifying the slow (narrowband) high-level 
             
             
                 
               components of the signal x MC (t) representing the magnitude 
             
             
                 
               |x LP (t)| of an I/Q modulation state x LP (t) 
             
             
               204′ 
               electronically controllable attenuator of the RF signal 
             
             
                 
               generator 200a (e.g. an amplitude modulator stage realized 
             
             
                 
               by means of PIN diodes) 
             
             
               206 
               gain-controlled power amplifier (VGA), operated at audio 
             
             
                 
               frequency (AF), which is used for amplifying the fast (wideband) 
             
             
                 
               low-level components 224 of the signal representing the magnitude 
             
             
                 
               |x LP (t)| of an I/Q modulation state x LP (t), respectively, 
             
             
                 
               and whose gain is controlled depending on the amplified slow 
             
             
                 
               (narrowband) high-level components 222 
             
             
               208 
               I/Q-to-magnitude/angle converter for converting the inphase (I) 
             
             
                 
               and quadrature (Q) component of a complex-valued analog 
             
             
                 
               baseband input signal (x LP (t)) to signals representing the 
             
             
                 
               magnitude (|x LP (t)|) and phase (∠x LP (t)) of the I/Q modulation 
             
             
                 
               states (x LP (t)), respectively 
             
             
               210 
               up-conversion mixing stage used for a phase modulation of the 
             
             
                 
               phase information (∠x LP (t)) of the I/Q modulation states (x LP (t), 
             
             
                 
               which comprises a frequency-synthesizing unit (FSU) with a 
             
             
                 
               phase-locked loop (PLL) and a voltage-controlled oscillator (VCO) 
             
             
               210a 
               voltage-controlled oscillator (VCO) of the circuitry 210 
             
             
               210b 
               frequency-synthesizing unit (FSU) with a phase-locked loop (PLL) 
             
             
               212 
               gain-controlled power amplifier (VGA), operated at radio 
             
             
                 
               frequency (RF) and realized as a class C driver PA, supplied with 
             
             
                 
               an RF signal (x p (t)) modulated with the phase information 
             
             
                 
               (∠x LP (t)) of the I/Q modulation states (x LP (t) 
             
             
               214 
               power amplifier, operated at radio frequency (RF) and realized 
             
             
                 
               as a class C, D, E, or F driver PA, which can selectively be 
             
             
                 
               bypassed by means of a bypass circuit, which is activated when 
             
             
                 
               switch S 3  is closed, while, simultaneously, its power 
             
             
                 
               supply V bat  is switched off by means of switch S 2   
             
             
               215 
               bypass path for bypassing the power amplifier 214 
             
             
               216 
               frequency control circuit for the frequency synthesizing unit (FSU) 
             
             
                 
               210b of the upconversion mixing stage 210 
             
             
               218 
               power divider (here: directional coupler), which supplies the APC 
             
             
                 
               loop 220 with the extracted feedback signal (the small part of 
             
             
                 
               the RF signal power) 
             
             
               219 
               wideband detector diode for detecting said feedback signal 
             
             
               219′ 
               external power APC detector 
             
             
               220 
               automatic power control (APC) loop, which guarantees excellent 
             
             
                 
               output power stability over time and temperature variations 
             
             
               222 
               fast (wideband) low-level components of the signal representing 
             
             
                 
               the magnitude |x LP (t)| of an I/Q modulation state x LP (t) 
             
             
               224 
               slow (narrowband) high-level components of the signal 
             
             
                 
               representing the magnitude |x LP (t)| of an 
             
             
                 
               I/Q modulation state x LP (t) 
             
             
               226 
               ramp signal supplied from a baseband block to the amplifier 202 
             
             
               226′ 
               analog baseband circuit for generating an analog baseband signal 
             
             
                 
               to be transmitted 
             
             
               228a 
               baseband input port of the QAM transmitter 
             
             
               228b 
               passband (RF) output port of the QAM transmitter 
             
             
               228c 
               input port for an external powerdetector 219′ 
             
             
               R 
               resistor used for selectively dampening the signal x MC (t) 
             
             
                 
               representing the magnitude |x LP (t)| of the I/Q modulation states 
             
             
                 
               x LP (t), which represents the gain of power amplifier 212, 
             
             
                 
               in case switch S 1  is opened 
             
             
               S 
               switch used for choosing an internal or external APC detector 219′ 
             
             
               S 1   
               switch used for controlling the gain of power amplifier 212 
             
             
               S 2   
               switch used for selectively switching off the power supply 
             
             
                 
               V bat  of the bypassed power amplifier 214 
             
             
               S 3   
               switch used for selectively bypassing at least one (214) of 
             
             
                 
               the series of power amplifiers 212, 214 
             
             
               S 4   
               switch used for opening the APC loop 220 as soon as the power 
             
             
                 
               p x (t) of the modulated RF output transmission signal x(t) 
             
             
                 
               reaches a defined nominal value P 0