WO2007069193A2

WO2007069193A2 - Polar modulation system

Info

Publication number: WO2007069193A2
Application number: PCT/IB2006/054763
Authority: WO
Inventors: Gerben W Jong; Jan Vromans
Original assignee: Nxp B.V.
Priority date: 2005-12-15
Filing date: 2006-12-12
Publication date: 2007-06-21
Also published as: WO2007069193A3; EP1969797A2; CN101375574A

Abstract

A system and a method for polar modulation achieved via generation of a multi-level pulse width modulated signal. The system in overview comprises a base-band pre-distortion element (115) producing a pre-distorted signal carrying the amplitude information of the base-band signal, two comparators (140,150) providing two inter-related pulse width modulated signals and a combination and amplification stage (160) that provides a third pulse width modulated radio frequency signal carrying the amplitude and the phase of the base-band signal. The first and second comparators compare a radio -frequenc signal carrying the phase information of the base-band signal with the pre-distorted signal and an inverted copy of the pre-distortion signal respectively.

Description

Polar modulation system

The present invention relates to polar modulation systems and in particular to polar modulation systems wherein polar modulation is achieved via generation of a multilevel pulse width modulated signal. The invention further relates to a method for producing polar modulation.

Polar modulation has proven an effective and efficient method to provide multi-mode and multi-band operation. In polar modulation the carrier amplitude and the phase are modulated independently. This approach allows different wireless standards, e.g. CDMA, TDMA, GSM or GPRS, to be implemented using the same modulator architecture and being digitally switched at a transmitter when necessary.

Rather than an RF carrier being modulated using the traditional In-phase, I, and Quadrature, Q, scalar components of the base-band signal, polar modulation explicitly modulates the amplitude of the RF carrier with the amplitude of the complex base-band signal, A, and may explicitly modulate the phase of the RF carrier with the angle, φ, of the complex base-band signal.

A known method of implementing polar modulation consists in modulating the supply voltage of an RF Power amplifier by applying the amplitude of the complex baseband signal, A, directly to the power supply, and wherein the power amplifier amplifies a constant envelope phase modulated signal carrying the phase information of the complex base-band signal. In order to be power efficient the supply voltage modulation should be done via an efficient DC-DC converter. A drawback of such an implementation may be that achieving a high modulation bandwidth and avoiding switching ripples is complicated in an efficient DC-DC converter. Another known method of implementing polar modulation can be achieved by applying a two-level Pulse Width Modulated signal (PWM) to a switching power amplifier. In this method the PWM signal has to be generated by comparing the amplitude and phase- modulated carrier signal with a triangular or a saw-tooth signal. A requirement for the triangular or saw-tooth signal is that its frequency is at least twice the carrier frequency. The method requires a high switching frequency for the power amplifier, inducing an over- sampling, and that the triangular or saw-tooth signal is highly linear. Furthermore the carrier signal needs to be linearly amplitude modulated prior to the pulse width conversion takes place.

The inventor of the present invention has appreciated that an improved polar modulation system is of benefit, and has in consequence devised the present invention.

The present invention seeks to provide an improved system that enables polar modulation in an effective and efficient way, wherein the switching frequency of the power amplifier equals the RF-carrier frequency requiring no over-sampling. Preferably, the invention alleviates, mitigates or eliminates one or more of the above or other disadvantages singly or in any combination.

Accordingly there is provided, in a first aspect, a polar modulation system for producing an RF output signal modulated by a base-band signal, the system comprising: a base-band pre-distortion element for receiving a signal representing the amplitude of the base-band signal and producing a pre-distorted signal carrying the amplitude information of the base-band signal; a first comparator coupled to compare a radio-frequency signal carrying the phase information of the base-band signal with the pre-distorted signal, providing a first pulse width modulated radio frequency signal; a second comparator coupled to compare the radio-frequency signal carrying the phase information of the base-band signal with the pre-distorted signal, providing a second pulse width modulated radio frequency signal wherein either the pre-distorted signal or the radio -frequency signal carrying the phase information (120) is inverted when being fed to the comparator; and a combination and amplification stage that combines the first pulse width modulated radio frequency signal and second pulse width modulated radio frequency signal and is adapted to provide a third pulse width modulated radio frequency signal carrying the amplitude and the phase of the base-band signal.

The base-band pre-distortion element may include one or more pre-distortion blocks wherein each of the blocks mapping the amplitude of the base-band signal to specific requirements of the RF output signal, e.g. modifications on the duty cycle or modifications on the comparator-level used in the first and second comparators. The switching frequencies at the first and second comparator outputs equal the frequency of the RF output signal typically locked to one reference RF-oscillator. The phase modulation can be either achieved by modulating the reference RF-oscillator with the derivative of the phase or by placing a phase shifter with modulation capabilities behind the RF-oscillator. The combination and amplification stage is able to combine the first and second pulse width modulated signals in a way that a third pulse width modulated radio frequency signal is generated carrying the amplitude and the phase of the base-band signal. The combination and amplification stage is furthermore able to provide the third pulse width modulated signal with the power level required to ensure a correct transmission of the modulated signal according to the specific applications, e.g. different power levels may be required if the modulator should be used in a wireless LAN, a BlueTooth application, or a wireless mobile application.

The invention is particularly but not exclusively advantageous for a number of reasons. Since this polar modulation system provides amplitude modulation in itself no additional linear amplitude modulator is required. As the center positions of the pulses of the third pulse width modulated signal are equal to those of the corresponding peaks of the sinusoidal RF-oscillator signal, no AM to PM conversion is introduced in the comparing and combining process. Furthermore the switching frequency at the first and second comparator outputs equals the frequency of the RF output signal avoiding any requirement for over- sampling at the first or second comparators. The RF-oscillator generates a sinusoidal output signal and therefore the requirement for more complex signals, e.g. highly linear triangular signal, saw tooth signal, is avoided.

The optional features as defined in claim 2 are advantageous since by generating a third pulse width modulated radio frequency signal with three levels significantly reduces the content of low frequency spurious in comparison with the generation of a two-level pulse width modulated signal. Moreover all even harmonics and their corresponding sidebands are absent or almost absent. Furthermore ensuring that the first and second pulse width modulated signals are binary signals reduces significantly the complexity of a combination stage generating a three-level pulse width modulated signal.

The optional features as defined in claims 3 and 4 disclose possible alternative embodiments according to the elements comprising the combination and amplification stage. The combination and amplification stage as defined in claim 3 comprises a single power amplifier containing a decoder and a three-level switching output stage, this implementation is specifically designed for a three-level application and minimizes the number of amplifiers required. The switching character of the output stage leads to a low power dissipation in this output stage and therefore to a high efficiency. The three-level character of the output stage leads to a cleaner spectrum (no even harmonics and their corresponding sidebands) in comparison with the two-level case. The combination and amplification stage as defined in claim 4 comprises a plurality of power amplifiers, a plurality of quarter- lambda transmission lines and a combiner, this implementation is specifically designed for the use of on-the-shelf components. In this case each power-amplifier only has to process a binary signal leading to a less complex implementation compared to the three-level power-amplifier.

The optional features as defined in claim 5 are advantageous since the combination and amplification stage may be situated remotely from the first and second comparators.

The optional features as defined in claim 6 are advantageous since the bandpass filtering stage will provide a filtered version of the amplified pulse width modulated signal reducing its frequency content thereby suppressing unwanted frequency components which otherwise might disturb other radio systems. The optional features as defined in claim 7 are advantageous since the bandpass filtering stage will provide a filtered version of the amplified pulse width modulated signal suppressing significantly the higher harmonics.

The optional features as defined in claims 8 and 9 disclose other possible differences in the characteristics of the third pulse width modulated radio frequency signal. In claim 8 the fundamental frequencies of the third pulse width modulated radio frequency signal contain the RF-carrier frequency. This implies that no over-sampling and neither sub- sampling is applied. In claim 9 the fundamental frequencies of the third pulse width modulated radio frequency signal are other than the RF-carrier frequency. The fundamental frequencies according to claim 9 may be frequencies below or above the RF-carrier frequency providing tunability by control of the pre-distortion element. This situation corresponds to sub-sampling or over-sampling respectively. Sub-sampling is advantageous because in that case the switching frequency of the combination and amplification stage is chosen to be lower than the RF-carrier frequency leading to an easier implementation and a higher efficiency. The optional features as defined in claims 10 and 11 disclose possible differences in the characteristics of the radio -frequency signal carrying the phase information of the base-band signal. In claim 10 the radio-frequency signal carrying the phase information of the base-band signal is a pure sinusoidal signal, ensuring a minimal bandwidth requirement for the input stages of the first and second comparator. In claim 11 the radio- frequency signal carrying the phase information of the base-band signal is a nonlinearly distorted sinusoidal signal, relaxing the linearity requirements of the local oscillator.

The optional feature as defined in claim 12 is advantageous as using the polar modulation system described in claim 1 will allow for the transmitter to adapt as needed to different transmission standards, e.g. CDMA, TDMA, GSM or GPRS. The high efficiency of the polar modulation system described in claim 1 is beneficial for long battery lifetime in portable applications.

In a second aspect of the invention is provided a method for generating an RF output signal modulated by a base-band signal, comprising the steps of: - producing a pre-distorted signal carrying the amplitude information of the base-band signal; comparing a radio-frequency signal carrying the phase information of the base-band signal with the pre-distorted signal, providing a first pulse width modulated radio frequency signal; - comparing the radio-frequency signal carrying the phase information of the base-band signal with the pre-distorted signal, providing a second pulse width modulated radio frequency signal, wherein either the pre-distorted signal or the radio-frequency signal carrying the phase information (120) is inverted when being fed to a comparing function; and providing a third pulse width modulated radio frequency signal carrying the amplitude and the phase of the base-band signal based on the combination and amplification of the first and second pulse width modulated radio frequency signals.

In a third aspect of the invention is provided computer readable code for implementing the method of the second aspect.

The present invention will be now explained, by the way of example only, with reference to the accompanying Figures wherein:

Fig. 1 is a detailed diagram illustrating the elements of a polar modulation system for producing an RF output signal modulated by a base-band signal according to one embodiment of the invention;

Fig. 2 is a block diagram illustrating the elements of a possible implementation of the combination and amplification stage containing a three-level switching output stage according to one possible embodiment of the invention; Figs. 3a, 3b, 3c and 3d depict some characteristics of the different signals involved in the creation of an RF output signal modulated by a base-band signal via polar modulation;

Fig. 4 is a block diagram illustrating the elements of an alternative implementation of the combination and amplification stage, based on multiple power amplifiers and quarter-lambda transmission lines, according to another possible embodiment of the invention.

The present invention provides polar modulation systems wherein polar modulation is achieved via generation of a multi-level pulse width modulated signal and wherein the multi-level pulse width modulated signal is generated by comparing an RF- oscillator output-signal carrying the phase information of a base-band signal with a pre- distorted signal related to the amplitude of the base-band signal. A block diagram illustrating elements required in a system for achieving polar modulation via generation of a multi-level pulse width modulated signal is shown in Figure 1 according to one embodiment of the invention. A system for achieving polar modulation via generation of a multi-level pulse width modulated signal 100 comprises a base-band pre- distortion element 115, an inverting linear amplifier with unity gain 130, a first comparator 140, a second comparator 150, a combination and amplification stage 160, a band-pass filter 170 and an antenna 180. The first comparator 140 has a positive input 148 and a negative input 142, the second comparator 150 has a positive input 158 and a negative input 152. The base-band pre-distortion element 115 receives a signal 110 representing the amplitude information of a complex base-band signal and provides a pre-distorted signal, which is an altered version of the amplitude of the complex base-band signal, to the inverting linear amplifier with unity gain 130 and to the negative input 142 of the first comparator 140. The pre-distorted signal provided by the base-band pre-distortion element 115 is negated in 130 and this negated signal is provided to the negative input 152 of the second comparator 150. An RF sinusoidal signal is generated by the RF-oscillator 125 and modulated in phase by the phase information 120 of the base-band signal. The phase modulation may be achieved by modulating the RF oscillator 125 with the derivative of the phase. The phase modulation may also be achieved by placing a phase shifter with modulation capabilities behind the RF- Oscillator. The RF phase modulated signal is input to the positive input of the first comparator 148 and the positive input of the second comparator 158. The switching frequency at the first and second comparator outputs equals the RF-carrier frequency provided by the local oscillator and no over-sampling is required. The outputs of the first and second comparator are a first and second inter-related binary pulse width modulated signals that are input to the combination and amplification stage 160. In a quasi- static approximation, when the amplitude of the base-band signal varies very slowly compared to the RF-cycle, the duty cycles of the first pulse width modulated signal, dcA, and the duty cycle of the second pulse width modulated signal, dcβ, are related as follows: dcA=(l-dCβ) where both duty cycles are a function of time. The duty cycle of the first pulse width modulated signal, dcA, is 50% or smaller while the duty cycle of the second pulse width modulated signal, dcβ, is 50% or larger. Furthermore when the first pulse width modulated signal has a value of one it is required that the second pulse width modulated signal has also a value of one, it is also required that when the second pulse width modulated signal has a value of zero the first pulse width modulated signal has a value of zero. The combination and amplification stage 160 provides an amplified three-level pulse width modulated signal combining the pulse width information provided by the output signals of the first 140 and second 150 comparators. The amplified three-level pulse width modulated signal is filtered at the band-pass filter 170 and the filtered signal is sent via an antenna 180. The band-pass filter may reduce the bandwidth of the pulse width modulated signal so that the RF signal at the output of the band-pass filter mainly contains its first harmonic and the corresponding sidebands, having significantly suppressed the higher harmonics of the amplified pulse width modulated signal.

In an alternative implementation of the embodiment described above the RF phase modulated signal is input to an inverting linear amplifier with unity gain, providing an inverted RF phase modulated signal. The inverted RF phase modulated signal is provided to the negative input 152 of the second comparator 150. The pre-distorted signal obtained from the base-band pre-distortion element 115 is directly provided to the positive input 158 of the second comparator 150.

Systems generating pulse width modulated signals with more than three levels are also envisaged with the objective of generating signals with even cleaner frequency spectra. In alternative implementations of the embodiment described above the bandpass filter may also be included in the combination and amplification stage as an LC-tank. The combination and amplification stage, the band-pass filter and/or the antenna may be physically separated from the comparators and/or the base-band pre-distortion element. The base-band pre-distortion element may be implemented in the analogue or in the digital domain.

In another embodiment of the invention the radio -frequency signal carrying the phase information of the base-band signal is a nonlinearly distorted sinusoidal signal, which is generally easier to generate than a pure sinusoidal signal. This implementation will therefore relax the specifications and requirements of the local RF-oscillator 125.

In another embodiment of the invention the combination and amplification stage is based on a decoder and a three-state switching output stage. A block diagram illustrating the elements of a possible implementation of the combination and amplification stage containing a decoder 220 and a three-state switching output stage 240 is shown in

Figure 2. A first pulse width modulated signal 205, output from the first comparator 140, and a second pulse width modulated signal 210, output from the second comparator 150, are coupled to the input of the decoder 220. The decoder translates a "one" level from the first pulse width modulated signal to a "high level" signal 238 at the output of the decoder. The decoder translates a "zero" level from the first pulse width modulated signal and a "one" level from the second pulse width modulated signal to a "middle level" signal 236 at the output of the decoder. The decoder translates a "zero" level from the second pulse width modulated signal to a "low level" signal 234 at the output of the decoder. The three-state switching stage 240 contains a high-level power supply 250, Vsup, a middle-level power supply 260, Vmid, and a low-level power supply 270, Vlow. The three power supplies Vsup, Vmid and Vlow are related by equation (1), and in that the middle-level power supply 260, Vmid, has a voltage equal or approximately equal to half of the sum of the voltage of the high-level power supply 250, Vsup, and the voltage of the low-level power supply 270, Vlow. The three-state switching stage 240 furthermore contains three switches 280 controlled by the "high level" signal 238, "middle level" signal 236 and "low level" signal 234 and providing a three-level pulse width modulated signal with Vsup, Vmid or Vlow accordingly. The output signals from the three switches are combined in the output node 290 of the power amplifier providing the amplified three-level pulse width modulated signal.

V sup > Vmid > Vlow (1)

In another embodiment of the invention the amplitude 110, _^4, and the phase information 120, φ, of the base-band signal are obtained from the in-phase, /, and quadrature, Q, components of the base-band signal. The relation between the amplitude and the in-phase and quadrature components of the base-band signal being provided by equation (2).

The relation between the phase and the in-phase and quadrature components of the base-bans signal being provided by equation (3).

φ = arctan(β//) (3)

where all the variables /, Q, A and φ, are (in general) a function of time t.

In an embodiment of the invention the comparator-level produced by the baseband pre-distortion element is related to the duty cycle of the signal amplified by the radio- frequency power amplifier. The band-pass filtering ensures that in essence only the first harmonic content, including its sidebands, of the binary signal is transmitted by the antenna. In general the amplitude, A_g of the first-harmonic content of a binary signal having a duty cycle, dc_g is given by equation (4)

A_g = dc_g • sinc(π • dc_g ) = 1/π • sin(π • dc_g ) (4)

In the described embodiment the system requires the transmission of an RF signal with an amplitude A, therefore the power amplifier should be driven with a binary signal having a duty cycle, dc, as indicated in equation (5).

dc = 1/π ^• arcsin(π ^• A) (5)

A first pre-distortion block 116 of the pre-distortion element 115 carries out the conversion from the required amplitude, A, of the RF signal generated to a required duty cycle, dc, of the signal at the input of the power amplifier. The first comparator 140 compares the sinusoidal phase modulated RF-oscillator signal with the comparator level signal, Cl, provided by the base-band pre-distortion element 115. Slicing a sinusoidal signal, having an amplitude of one, with a comparator level signal, Cl_g, produces a binary signal with duty cycle, dc_g, as indicated in equation (6) 1 arcsin(CL )

In the described embodiment the system requires that the power amplifier should be driven with a binary signal having a duty cycle, dc, as indicated in equation (5), therefore the second pre-distortion block 117 of the pre-distortion element 115 should generate a comparator level, Cl, following equation (7)

Cl = sin(π - (- - dc)) (7)

In another embodiment of the invention the radio -frequency signal carrying the phase information of the base-band signal is a nonlinearly distorted sinusoidal signal. In this embodiment the RF-oscillator may deliver a signal that can be represented by a nonlinear transfer function of a pure sinusoidal signal, f(sin(ω_c£ + φ(/))), wherein the function, f, represents the non- linear distortion transfer function. Slicing a non- linearly distorted sinusoidal signal, as given by the previous expression, with a comparator level signal, Cl_g, produces a binary signal with duty cycle, dc_g, as indicated in equation (8).

dc_g = i -- - arcsin(f^mv (ci_g )) (8) 71

wherein f^nv represents the inverse function of the function, f.

Therefore the second pre-distortion block 117 of the pre-distortion element 115 should generate a comparator level, Cl, following equation (9)

In Figure 3, an example of the application of a polar modulation system according to the previous embodiment is presented. The example showing the characteristics of the different signals involved in the creation RF output signal modulated by a base-band signal via polar modulation. The example is limited, in order to provide for a better understanding, to a base-band amplitude only modulated signal. In Figure 3a the amplitude 310, A, of the base band signal is depicted as a function of time. In Figure 3b the duty cycle signal 320, dc, as generated by a first pre-distortion block 116 of the base-band pre-distortion element 115 signal, is plotted as function of time. In Figure 3c three signals are depicted as a function of time; the RF-oscillator signal carrier 350, the comparator level 330, Cl, as generated by the second pre-distortion block and the inverse of the comparator level 340, -Cl. Every time the RF-oscillator carrier signal is higher that the comparator level, Cl, a "high level" signal 238 is generated at the decoder 220. In the case where the RF-oscillator signal carrier is lower than inverse of the comparator level, -Cl, a "low level" signal 234 is generated at the decoder. When the RF-oscillator carrier is lower than the comparator level, Cl, and higher than the inverse of the comparator level, -Cl, a "middle level" signal 236 is generated at the decoder. The resulting three-level pulse width modulated signal 370 can be observed in Figure 3d. Figure 3d also presents the sinusoidal radio frequency signal 360 obtained at the output of a band-pass filtering stage 170 characterized by in essence allowing only transmission of the first-harmonic content including its sidebands.

In another embodiment of the invention the combination and amplification stage 160 is implemented comprising a plurality of power amplifiers, a plurality of quarter- lambda transmission lines and a combiner, a block diagram illustrating the elements of this alternative implementation of the combination and amplification stage is shown in Figure 4. In this embodiment the output pulse width modulated signal from the first comparator 140 is amplified by a first power amplifier 410 providing a first amplified pulse width modulated signal. The output pulse width modulated signal from the second comparator 150 is amplified by a second power amplifier 420 providing a second amplified pulse width modulated signal. The first and second amplifiers typically present similar characteristics. In this embodiment the first and second power amplifiers have low-ohmic outputs, and first 430 and second 440 quarter- lambda transmission lines are used to transform the voltage-source characteristic of the first and second power-amplifier output into a current-source characteristic, allowing for a simple connection of the transmission-line outputs in parallel at a combiner 450.

In another embodiment of the invention the base-band pre-distortion element 115 is adjusted in order to provide pulse width modulated signals at the outputs of the comparators with fundamental frequencies other than the RF-carrier frequency. This characteristic can be achieved by applying proper amplitude to duty cycle conversion within the first block 116 of the base-band pre-distortion element. Two different cases can be envisioned in this particular embodiment. The first case relates to the situation where the fundamental frequencies of the pulse width modulated signals at the output of the comparators are lower than the RF-carrier frequency, this case can be considered as a sub- sampling case. The second case relates to the situation where the fundamental frequencies of the pulse width modulated signal at the output of the comparators are higher than the RF- carrier frequency, this case can be considered as an over-sampling case.

The relation presented in equation (4) relating the amplitude, A_g of the N^th- harmonic content of a given binary signal with the duty cycle, dc_g, can be generalized to the integer sub-sampling or integer over-sampling cases as shown in equation (10)

sin(π ^• N ^• dc „ ) Λ = dc - sinc(π - JV - de J = — — (10) π - N wherein

N € {2,3,4...} in the integer sub-sampling case

N G 1% , J/ , V[, ...} in the integer over-sampling case.

In the described embodiment the system requires the transmission of an RF signal with an amplitude A, therefore the power amplifier should be driven with a binary signal having a duty cycle, dc, as indicated in equation (11).

, arcsin(π - N - A) .. .. dc = (11) π - N

The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention can be implemented as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit, or may be physically and functionally distributed between different units and processors.

Claims

CLAIMS:

1. A polar modulation system for producing an RF output signal, containing an RF carrier and modulated by a base-band signal, the system comprising: a base-band pre-distortion element (115) for receiving a signal representing the amplitude (110) of the base-band signal and producing a pre-distorted signal carrying the amplitude information of the base-band signal; a first comparator (140) coupled to compare a radio-frequency signal carrying the phase information (120) of the base-band signal with the pre-distorted signal, providing a first pulse width modulated radio frequency signal; a second comparator (150) coupled to compare the radio -frequency signal carrying the phase information (120) of the base-band signal with the pre-distorted signal, providing a second pulse width modulated radio frequency signal wherein either the pre- distorted signal or the radio-frequency signal carrying the phase information (120) is inverted when being fed to the comparator; and a combination and amplification stage (160) that combines the first pulse width modulated radio frequency signal and second pulse width modulated radio frequency signal and is adapted to provide a third pulse width modulated radio frequency signal carrying the amplitude and the phase of the base-band signal.

2. The system according to claim 1, wherein the first and second pulse width modulated radio frequency signals are binary signals and the third pulse width modulated radio frequency signal is a three-level signal.

3. A system according to claim 2, wherein the combination and amplification stage (160) comprises a single power amplifier containing a decoder (220) and a three-level switching output stage (240).

4. A system according to claim 1, wherein the combination and amplification stage comprises a plurality of power amplifiers (410,420), a plurality of quarter- lambda transmission lines (430,440) and a combiner (450).

5. A system according to claim 1, wherein the combination and amplification stage is situated remotely from the other components of the system.

6. The system according to claim 1, further comprising a band-pass filtering stage (170) for filtering the amplified version of the pulse width modulated signal.

7. The system according to claim 6, wherein the band-pass filtering stage (170) limits the transmitted signal essentially to its first-harmonic and corresponding sidebands.

8. The system according to claim 1, wherein the fundamental frequencies of the third pulse width modulated radio frequency signal contain the RF-carrier frequency.

9. The system according to claim 1, wherein the fundamental frequencies of the third pulse width modulated radio frequency signal are other than the RF-carrier frequency.

10. The system according to claim 1, wherein the radio-frequency signal carrying the phase information of the base-band signal is a sinusoidal signal.

11. The system according to claim 1 , wherein the radio-frequency signal carrying the phase information of the base-band signal is a nonlinearly distorted sinusoidal signal.

12. A transmitter comprising a modulation system according to claim 1.

13. A method for generating an RF output signal, containing an RF carrier and modulated by a base-band signal, comprising the steps of: producing a pre-distorted signal carrying the amplitude information of the base-band signal; comparing a radio-frequency signal carrying the phase information of the base-band signal with the pre-distorted signal, providing a first pulse width modulated radio frequency signal; comparing the radio-frequency signal carrying the phase information of the base-band signal with the pre-distorted signal, providing a second pulse width modulated radio frequency signal, wherein either the pre-distorted signal or the radio-frequency signal carrying the phase information (120) is inverted when being fed to a comparing function; combining the first pulse width modulated radio frequency signal and second pulse width modulated radio frequency signal in order to provide an amplified third pulse width modulated radio frequency signal carrying the amplitude and the phase of the base- band signal.

14. Computer readable code for implementing the method of claim 13.