Patent Publication Number: US-9843345-B2

Title: Transmitter with quantization noise compensation

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/899,757, filed 22 May 2013, which is a continuation of U.S. patent application Ser. No. 13/320,372, filed 14 Nov. 2011, now U.S. Pat. No. 8,472,557, which is a U.S. National Stage Application (35 U.S.C. §371 Application) of International Application No. PCT/SE2009/050552, filed 18 May 2009, the disclosures of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention discloses a transmitter with an increased degree of efficiency. 
     BACKGROUND 
     A transmitter will normally comprise at least one amplifier, the efficiency of which can be enhanced by means of pulsed modulation, i.e. mapping of the modulated communication signal to the characteristics of a pulse train, such as, for example, duration, position or density. In such a transmitter, the pulses are used as input to the amplifier. The purpose of driving the amplifier with a pulse train is to make the amplifier operate more of the time at its high efficiency operating points. By using a pulse train, the power amplifier is only operated in one of two highly efficient regions; deep compression or completely off. 
     However, pulse modulation schemes such as the one described above will introduce undesired signal distortion caused by quantization noise when generating the pulses, which needs to be handled by means of a bandpass filter, a so called reconstruction filter. Due to the nature of the quantization noise, narrowband filters are needed in order to reconstruct the signal before transmitting. These types of filters have a large insertion loss at microwave frequencies, which leads to increased power dissipation in the filter, thus reducing the power efficiency of the transmitter. 
     Document WO 2006/110590 discloses a power control module which receives a dynamic power control signal, and generates a differential bias signal which is proportional to the dynamic power control signal. 
     SUMMARY 
     As previously described, there exists a need for a solution which can suppress quantization noise in a transmitter which uses pulses as input to an amplifier in the transmitter. In particular, the solution should be able to suppress such noise in or around a Radio Frequent, RF, carrier, since that is where the noise can cause the most “damage”. 
     Such a solution is disclosed by means of the present solution in that it discloses a transmitter which includes a pulse encoder for creating pulses from the amplitude of an input signal to the transmitter, a compensation signal generator for cancelling quantization noise caused by the pulse encoder, a mixer or an I/Q modulator for mixing an output of the pulse encoder with the phase of an input signal to the transmitter, and an amplifier for creating an output signal from the transmitter. 
     In the transmitter of the invention, a control signal for controlling a function of the amplifier comprises an output signal from the compensation signal generator, and an input signal to the amplifier comprises an output from said mixer or I/Q modulator. 
     In the transmitter, a control signal for controlling a function of the amplifier comprises an output signal from the compensation signal generator, and an input signal to the amplifier comprises an output from the said mixer or I/Q modulator having been modulated to a desired frequency. The transmitter additionally comprises a band pass filter at the output of the amplifier. In the transmitter, the control signal is at each instant chosen such that the output of the amplifier with the compensation filter is a linear copy of the input signal to the transmitter. The function in the amplifier which the control signal is used for controlling being the maximum output amplitude of the amplifier or the impedance of an output matching network comprised in the amplifier. 
     As will be shown in the following detailed description, the transmitter of the invention is advantageous when it comes to reducing quantization noise from the pulse encoder. 
     In one embodiment, the function in the amplifier which the control signal is used for controlling is maximum output amplitude of the amplifier, so called “amplitude modulation”. In one embodiment, the function in the amplifier which the control signal is used for controlling is the impedance of an output matching network comprised in the amplifier, so called “load modulation”. 
     In one embodiment, the transmitter of the invention additionally comprises a delay circuit for delaying the output of the mixer before it is used as input to the amplifier, in order to compensate for delays in the compensation signal generator. 
     In one embodiment, the transmitter of the invention additionally comprises a digital to analogue converter connected at the output of the delay circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail in the following, with reference to the appended drawings, in which 
         FIG. 1  shows a problem which is addressed by means of the invention, 
         FIG. 2  shows a prior art solution to the problem, 
         FIG. 3  shows a first embodiment of the invention, 
         FIG. 4  shows a second embodiment of the invention, 
         FIG. 5  shows a result obtained by means of the invention, and 
         FIG. 6  shows a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a problem which is addressed by means of the invention: a signal x[n], for example in the form of an analogue signal, which can be written as x[n]=A[n]*e jφ(n) , is used as input signal to a transmitter which comprises an amplifier, usually a power amplifier. 
     Prior to being used as input to the amplifier in the transmitter, the amplitude component A[n] of the input signal x[n], is passed through a pulse encoder which “maps” the signal onto an N-bit representation (or in a general case to some discrete signal levels) in order to use the power amplifier more efficiently. The N-bit representation is usually a 1-bit representation, and creates pulses as the output from the pulse encoder. 
     However, the pulse encoder also introduces an error term, commonly labelled as quantization noise, which, in the frequency domain, is primarily centred around the RF-carrier about to be transmitted. 
     The input signal to the transmitter, x[n]=A[n]*e(n) jφ(n) , is shown as x[n] in  FIG. 1 , and the output signal from the amplifier is shown as y[n] in  FIG. 1 . As can clearly be seen in  FIG. 1 , the output also comprises quantization noise, shown as q[n], centred around the output frequency, so that the output signal can be written as (A[n]+q[n])*e jφ(n) . 
     In  FIG. 2 , a prior art solution for reducing the quantization noise q[n] is indicated: a bandpass filter, the characteristic of which is shown as “Bandpass” in  FIG. 2 , can be applied at the output of the amplifier, which will result in an output signal y[n] which is shown in  FIG. 2 . However, as is also evident from  FIG. 2 , a residue q[n] of the quantization noise will still remain in the filtered output signal, since a bandpass filter will not be able to have a filter characteristic which is sufficiently narrowband to remove all of the quantization noise, particularly not the quantization noise which is in the immediate vicinity of the desired signal, i.e. the modulated RF-carrier. 
     Thus, as also stated previously, it is an objective of the invention to obtain a solution to the problem of quantization noise in the output signal of a transmitter with an amplifier which has a pulse-train as its input signal. 
     In the following, the invention will be described by means of three embodiments of transmitters. 
       FIG. 3  shows a first embodiment  300  of a transmitter which is based on the principles of the invention. The transmitter  300  is arranged to receive a complex-valued input signal, here denoted as x[n]=A[n]*e jφ(n) . The input signal is arranged to be fed to a signal component separator  305  which separates the complex valued input signal into phase and amplitude components. The amplitude component is denoted A[n] and the phase component is denoted as e jφ(n)  or exp (jφ[n]). 
     The signal separator  305  delivers said amplitude component A[n] to a pulse encoder  310 , i.e. a component which generates pulses as its output in response to the amplitude of the input signal A[n]. As mentioned previously, the pulse encoder  310  will also generate signal distortion in the form of so called quantization noise, so that the output signal from the encoder  310  can be written as A[n]+q[n], where q[n] represents the quantization noise. 
     The output signal A[n]+q[n] from the pulse encoder  310  is mixed with the phase signal e jφ(n)  from the signal component separator  305  in a mixer  307 , so that the output from the mixer  307 , here denoted as S[n], can be written as S[n]=(A[n]+q[n])*e jφ(n) , which can also be written as X[n]+q[n]*e jφ(n) . The signal S[n] is then mixed in a mixer  340  with a radio frequency signal, an RF carrier, from a local oscillator, an LO, (not shown in  FIG. 3 ), in order to modulate the signal z[n] to a desired RF frequency. In some embodiments, the mixer  307  can be replaced by a so called I/Q-modulator. 
     The modulated signal is then used as input to an amplifier  320 , in order to amplify the output signal from the transmitter to a desired level. Thus, the input signal to the amplifier  320  comprises an output from the mixer  307  which has been modulated to a desired frequency. This is also the case for the other embodiments of the transmitter of the invention which will be described in this text. 
     According to the invention, and as shown in  FIG. 3 , the amplifier  320  in the transmitter circuit  300  is arranged to receive as a control signal, C A , the output signal from a compensation signal generator  315 . The control signal C A  controls a function of the amplifier, in this case the amplification of the amplifier  320 , by means of controlling the maximum output signal amplitude of said amplifier. This can also be expressed by saying that since the input signal to the amplifier is either on or off due to the use of the pulse encoder, the resulting amplifier output amplitude for the “on-state” is governed by the control signal C A . 
     Turning now to a more detailed description of the compensation signal generator  315 , the input to this component in the embodiment of the invention shown in  FIG. 3  is two signals, one of which is the amplitude signal A[n] from the signal component separator  305 , and the other is the output signal from the pulse encoder  310 , i.e. A[n]+q[n]. 
     In other words, one of the input signals to the compensation signal generator  315  is the “pure” amplitude signal A[n], and the other input signal is the output from the pulse encoder  310 , i.e. A[n]+q[n]. As explained previously, the output from the compensation signal generator  315 , here denoted as C A [n], is used as control signal to the amplifier  320 . Thus, the output signal from the compensation signal generator  315  is used to control a function of the amplifier  320 , in this case the amplification of the modulated input signal to the amplifier  320 . 
     A more exact description of the nature and function of the compensation signal generator  315  is as follows: the compensation signal C A  should at every instant be chosen such that the output signal of the amplifier is a linear copy of the input signal to the transmitter taking the reconstruction filter into consideration. This can be expressed as:
 
 A[n]e   jφ[n] =Σ k=0   L   f[k−n]·C   A   [n] ·( A[n]+q[n ]) e   jφ[n]   (1)
 
where f[k−n] is the impulse response from a filter used at the output of the transmitter, a so called reconstruction filter.
 
     The invention&#39;s use of the output signal from the compensation signal generator  315  as control signal to the amplifier  320  is advantageous since an extra degree of freedom is introduced in obtaining the final output signal given a reconstruction filter, which will make it possible to maximize the power efficiency of the transmitter. The RF input signal to the amplifier can be maintained in a so called “deep saturation mode”, thereby ensuring optimum efficiency, while the control signal to the amplifier will set the correct signal level at the output of the amplifier. 
       FIG. 4  shows a second embodiment  400  of a transmitter of the invention. Components which have the same basic function as in embodiment  300  in  FIG. 3  have retained their reference numbers from  FIG. 3 . 
     Apart from the components of the transmitter  300  of  FIG. 3 , the transmitter  400  shown in  FIG. 4  comprises a delay circuit  430 , in order ensure that the signal which is used as input to the amplifier  320  is “synchronized with” the signal which is used to control the amplifier  320 . Thus, the delay of the delay circuit  430  should suitably correspond to the “processing time” of the compensation signal generator  315 , i.e. the delay introduced by the compensation signal generator  315 . 
     The output of the delay circuit, in  FIG. 4  denoted as z[n], is put through a digital to analogue converter DAC  335 , and is then mixed in the mixer  340  with a Radio Frequency signal, an RF signal, from a Local Oscillator, an LO, (not shown in  FIG. 4 ), in order to modulate the signal z[n] to a desired RF frequency. 
     As shown in  FIG. 4 , the transmitter circuit  400  in this embodiment comprises a bandpass filter  425 , a so called reconstruction filter as mentioned above, at the output of the amplifier  320 , in order to remove undesired components in the output signal of the amplifier. 
       FIG. 5  shows a result obtained by means of the invention: the input signal x[n] to the circuit  400  of  FIG. 4  is shown in  FIG. 5 , as well as the output signal y[n] from the circuit  400 . As can be seen, the output signal y[n] has a significantly reduced content of quantization noise, q[n], which, as a reference, is also shown in  FIG. 5  by means of dashed lines. Thus, the invention reduces the quantization noise in the output signal in a sharp area around the “desired” output signal x[n], which is a clear improvement as compared to the solution with a narrow bandpass filter at the output of the circuit. Remaining quantization noise is shown as q′[n], and as can be seen, q′[n] is located outside of the spectrum of the output signal y[n]. 
     The suppression obtained of the quantization noise by means of the invention is also shown in  FIG. 5  by an arrow “A”, and as mentioned, the “out of band” quantization noise in the output signal is shown in  FIG. 5  as q′[n]. 
       FIG. 6  shows a further example of an embodiment of the invention. Components which have the same basic function as those in the embodiments  300  in  FIGS. 3 and 400  in  FIG. 4  have retained their reference numbers, and as can be seen from  FIG. 6 , the embodiment  600  is in many ways similar to the embodiments  300  and  400  of  FIGS. 3 and 4 . Thus, the embodiment  600  comprises a signal component separator  305 , a pulse encoder  310 , an amplifier  320  and a compensation signal generator  315 , the output, C A , of which is used as a control signal to the amplifier  320 . 
     The compensation signal generator  315  of the embodiment  600  has two input signals, one of which is the input signal x[n] and the other of which is the output from the mixer  307 , again denoted as S[n]. Thus, the two input signals to the compensation signal generator  315  in  FIG. 5  are x[n] and S[n]=x[n]+q[n]*e jφ(n) . The use of the input signal x[n] as one of the input signals to the compensation signal generator  315  is achieved by means of “splitting” the input signal x[n] before the component separator  305 , so that one “branch” of x[n] is connected to the compensation signal generator  315 , and the other “branch” of x[n] is used as input signal to the signal component separator  305 . 
     The output S[n] from the mixer  307  is, in the embodiment of  FIG. 5 , also split, so that one “branch” of S[n] is used as input to the compensation signal generator  315 , as mentioned above, and one “branch” of S[n] is used as input to a first delay circuit  630 . 
     As opposed to the embodiments shown and described previously, the compensation signal generator  315  of the embodiment  600  produces both a first and a second compensation signal, here denoted as C A [n] and C RF [n]. The first compensation signal, C A [n], is used as a control signal to the amplifier  320 , after having been passed through a second delay circuit  631 . The second delay circuit  631  serves to align (in time) the input signals to the different components. 
     The output from the first delay circuit  630  is added to the second compensation signal from the circuit  315 , C RF [n] in an adder  607 , with the sum output from the adder  607 , denoted in  FIG. 6  as z′, being passed through a digital to analogue converter  435 , following which it is arranged to be modulated by an external LO to a desired RF frequency, and then used as input signal to the amplifier  320 . The purpose of the second compensation signal C RF  is to create further possibilities to fine tune the efficiency of the circuit  600 , corresponding to a certain output signal from the amplifier. 
     The combination of the compensation signals C A  and C RF  should be chosen as follows:
 
 A=[n]e   jφ[n] =Σ k=0   L   f[k−n]·C   A   [n ]·( A[n]+q[n]−C   RF   [n] ) e   jφ[n]   (2)
 
     The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims. For example, the control signal C A [n] can also, instead of being used as a means of controlling the output amplitude of the amplifier by means of supply modulation, be used for controlling the impedance of the an output matching network of the amplifier in the transmitter, which is referred to as “load modulation”. In such an embodiment, the matching network is suitably comprised in the amplifier.