Patent Publication Number: US-2007115157-A1

Title: Method and system for generation of double-sided pulse wave modulation signal

Description:
FIELD OF THE INVENTION  
      The present invention generally relates to the field of integrated circuits, and more specifically to, generation of pulse width modulation (PWM) signals.  
     BACKGROUND OF THE INVENTION  
      Pulse width modulation (PWM) signals are increasingly being used in various applications of switching amplifiers. Most high efficiency switching amplifiers are based on the principle of PWM, which is used in a variety of applications, including digital audio amplifiers and control applications including motor controllers. Typically, a pulse code modulation (PCM) data stream is converted to a PWM signal to achieve high efficiency and accuracy. A double-sided PWM signal is a pulse width modulated signal that has both its leading edge and trailing edge modulated by an information source.  
      Several methods are known in the art for generating a PWM signal from a PCM data stream. Implementation of these methods for generation of the double-sided PWM signal generally require the signal processing operations to be running at a frequency that is twice of a switching frequency of the PWM signal. In order to accommodate the bandwidth requirements of common modulated RF signals, the switching frequencies of the PWM signal are generally chosen to be above 100 MHz. Therefore, the signal processing operations involved for generation of PWM signals require a sample frequency above 200 MHz. It is very difficult to realize the algorithms used in generation of double-sided PWM signal using moderate technology with such a high sample frequency requirement. In order to generate digital pulse signals at radio frequency, the PWM signals are first generated at baseband before being modulated to radio frequency using digital clock signals. The clock signals normally consist of one in-phase (I) clock and one quardrature (Q) clock. The phases of these clocks differ from each other by 90 degrees. Therefore, if single sided PWM signals are formed at the baseband, their transition edges cannot be aligned with both clock signals. The mis-alignment generates extra transition edges when modulating the baseband PWM signals to radio frequency, multiplying the digital clock signals with the baseband PWM signals. These extra transition edges will cause distortions to the signals at radio frequency. Double sided PWM signals can avoid the occurrence of such distortion by aligning baseband in-phase (I) PWM signal and quardrature (Q) PWM signal with their corresponding clock signals.  
      In one of the known methods for generation of double sided PWM signals, first an in-phase and a quadrature phase components of a PWM signal are generated in baseband. Then, a pair of clock signals is generated with a desired carrier frequency to modulate the baseband PWM signals to radio frequency digital pulse signals. The signal processing operations in this method are executed at twice the switching frequency of the PWM signals. As described earlier, a high sample frequency requirement makes it very difficult to implement a real time PWM system with radio frequency output. 
    
    
     BRIEF DESCRIPTION OF FIGURES  
      The present invention is illustrated by way of example and not limitation in the accompanying figures, in which like references indicate similar elements, and in which:  
       FIG. 1  representatively illustrates a block diagram of a system for generating a double-sided PWM signal, in accordance with various embodiments of the present invention.  
       FIG. 2  representatively illustrates a schematic block diagram of a circuitry for generating a double-sided pulse width modulation (PWM) signal, in accordance with an embodiment of the present invention.  
       FIG. 3  represents a flowchart depicting a method for generating a double-sided PWM signal, in accordance with various embodiments of the present invention.  
       FIG. 4  represents a flowchart depicting a method for generating a double-sided PWM signal, in accordance with another embodiment of the present invention.  
       FIG. 5  illustrates a spectrum analyzer screenshot that displays an output of the circuitry for generating a double-sided PWM signal. 
    
    
     DETAILED DESCRIPTION OF FIGURES  
      Before describing in detail the particular method and system for generation of double-sided pulse width modulation signal in accordance with various embodiments of the present invention, it should be observed that the present invention resides primarily in combinations of method steps and apparatus components related to generation of double-sided pulse width modulation signal. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the present invention so as not to obscure the disclosure with details that will be readily.  
      In this document, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms ‘comprises’, ‘comprising’, or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by ‘comprises . . . a’ does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.  
      A ‘set’ as used in this document, means a non-empty set (i.e., comprising at least one member). The term ‘another’, as used herein, is defined as at least a second or more. The term ‘including’ as used herein, is defined as comprising.  
      Various embodiments of the present invention provide a method for generating a double-sided pulse width modulation (PWM) signal. The method includes generating a leading edge PWM signal and a trailing edge PWM signal from a pulse code modulation (PCM) data stream. The method further includes combining the leading edge PWM signal and the trailing edge PWM signal for generating the double-sided PWM signal.  
      Various embodiments of the present invention also provide a system for generating a double-sided PWM signal. The system includes at least one modulation conversion circuitry for generating a leading edge PWM signal and a trailing edge PWM signal from a PCM data stream. The system further includes a combining circuitry capable of combining the leading edge PWM signal and the trailing edge PWM signal to form the double-sided PWM signal.  
       FIG. 1  representatively illustrates a block diagram of a system for generating a double-sided PWM signal, in accordance with various embodiments of the present invention. The system takes a pulse code modulation (PCM) data stream  102  as an input and delivers a double-sided pulse width modulation (PWM) signal  110  as an output of the circuitry.  FIG. 1  includes a PCM input block  104 , a modulation conversion circuitry  106  and a combining circuitry  108 . The PCM data stream  102  can be a received audio signal, a received television audio signal, an Internet audio signal, or any other type of input signal. These input signals may be digital signals and converting them to radio frequency (RF) PWM signals is helpful for processing these input signals. In an embodiment of the present invention, the circuitry of  FIG. 1  can be implemented in an integrated circuit. The PCM data stream  102  is input to the PCM input block  104  of the circuitry for generation of double-sided PWM signals. The output of the PCM input block  104  acts as an input to the modulation conversion circuitry  106 . The modulation circuitry  106  is capable of generating a leading edge PWM signal and a trailing edge PWM signal from the PCM data stream  102 . The output of the modulation conversion circuitry  106  is provided to the combining circuitry  108 . The combining circuitry  108  combines the leading edge PWM signal and the trailing edge PWM signal to form a double-sided PWM signal  110 . Therefore, the double-sided PWM signal  110  can be viewed as the combination of two single-sided PWM waveforms, one single-sided PWM waveform with its leading edge being modulated and the other single-sided PWM waveform with its trailing edge being modulated. In an embodiment of the present invention, the double-sided PWM signal  110  can be input to a mixer or an output stage of the switching amplifier.  
       FIG. 2  representatively illustrates a schematic block diagram of a circuitry for generating a double-sided pulse width modulation (PWM) signal, in accordance with an embodiment of the present invention.  FIG. 2  shows a PCM input block  102 , an interpolator  202 , a re-sampling block  204 , a first quantizer  206 , a second quantizer  208 , a first modulation conversion circuitry  210 , a second modulation conversion circuitry  212 , a combining circuitry  108  and an output block  214 . The PCM data stream  102  is input to the PCM input block  104  of the circuitry for generation of the double-sided PWM signal  110 . The PCM input block  104  provides the PCM data stream  102  to the interpolator  202 . The interpolator  202  is used for changing the sampling rate of the PCM data stream  102  by an integer factor L. The interpolator  202  also increases the sampling rate in order to prepare the PCM data stream  102  to sampling frequency suitable for PWM generation. An exemplary sampling frequency suitable for PWM generation can be taken to be 40 MHz. The re-sampling block  204  re-samples the up-sampled PCM data stream. Further, the re-sampling block  204  creates a first and a second PCM data stream from the up-sampled PCM data stream. The first and the second PCM data streams are non-overlapping in that data elements from the up-sampled PCM data stream will exist on only the first or the second PCM data streams. Re-ampling block  204  creates the first and the second PCM data streams by extracting an odd numbered PCM data stream and an even numbered PCM data stream from the PCM data stream  102 . In other words, if the PCM data stream is denoted by {X n }, where n is any integer, the odd numbered PCM data stream includes data from {X 2k−1 } and the even numbered PCM data stream includes data from {X 2k }, where k is any integer. In an embodiment of the present invention, the sampling frequency of the odd numbered PCM data stream and the sampling frequency of the even numbered PCM data stream are equal to half of the sampling frequency of the PCM data stream  102 . The first quantizer  206  quantizes the odd numbered PCM data stream that is output by the re-sampling block  204 . Similarly, the second quantizer  208  quantizes the even numbered PCM data stream that is output by the re-sampling block  204 . The odd numbered PCM data stream from the first quantizer  206  is provided as an input to the first modulation conversion circuitry  210 . The first modulation conversion circuitry  210  converts the odd numbered, PCM data stream to a leading edge PWM signal. The even numbered PCM data stream from the second quantizer  208  is provided as an input to the second modulation conversion circuitry  212 . The second modulation conversion circuitry  212  converts the even numbered PCM data stream to a trailing edge PWM signal. It should be appreciated that the roles of the odd numbered PCM data stream and the even numbered PCM data stream are interchangeable. Therefore, the odd numbered PCM data stream can be provided to the second modulation conversion circuitry  212  and be converted to the trailing edge PWM signal. Similarly, the even numbered PCM data stream can be provided to the first modulation conversion circuitry  210  and be converted to the leading edge PWM signal. The leading edge PWM signal and the trailing edge PWM signal are then provided to the combining circuitry  108  that combines them to form the double-sided PWM signal  110 . The double-sided PWM signal  110  is a pulse width modulated signal that has both its leading edge and trailing edge modulated by an information source. The output of the combining circuitry  108  can be fed to the output block of a switching amplifier. In an embodiment of the present invention, the output block  214  of the circuitry can be a mixer of a switching amplifier system.  
      In an alternate embodiment of the invention, a PCM memory can store at least one of the odd numbered PCM data stream and the even numbered PCM data stream that are output by the re-sampling block  204 . The odd numbered PCM data stream and the even numbered PCM data stream can then be converted to the leading edge PWM signal and the trailing edge PWM signal (or vice versa) by retrieving the odd numbered PCM data stream or the even numbered PCM data stream from the PCM memory. The PCM memory can be implemented, for example, in the form of a Random Access Memory (RAM). In this embodiment, the leading edge PWM signal and the trailing edge PWM signal can be generated by a single modulation conversion circuitry. A PWM memory can store at least one of the trailing edge PWM signal and the leading edge PWM signal. A single combining circuitry can then combine the leading edge PWM signal and the trailing edge PWM signal to form the double-sided PWM signal  110 . In an embodiment of the present invention, the leading edge PWM signal or the trailing edge PWM signal can be retrieved from the PWM memory and can then be combined to form the double-sided PWM signal  110 . The PWM memory can be implemented, for example, in the form of a Random Access Memory (RAM).  
       FIG. 3  represents a flowchart depicting a method for generating a double-sided PWM signal, in accordance with various embodiments of the present invention. At step  302 , a leading edge PWM signal is generated from the PCM data stream  102 . A PCM data stream is received at the PCM input block  104  of the circuitry for generation of the double-sided PWM signal. The modulation conversion circuitry  106  as described in  FIG. 1  can be used to convert the PCM data stream to a leading edge PWM signal. At step  304 , a trailing edge PWM signal is generated from the PCM data stream. In an embodiment of the invention, both steps  302  and  304  are performed in parallel. The modulation conversion circuitry  106  as described in  FIG. 1  can be used to convert the PCM data stream to the trailing edge PWM signal. In an embodiment of the present invention, the switching frequency of the leading edge PWM signal is equal to half of the sampling frequency of the PCM data stream  102 . In another embodiment of the present invention, the switching frequency of the trailing edge PWM signal is also equal to half of the sampling frequency of the PCM data stream  102 . At step  306 , the leading edge PWM signal and the trailing edge PWM signal are combined to form the double-sided PWM signal. The double-sided PWM signal is a pulse width modulated signal that has both its leading edge and trailing edge modulated by the information source.  
       FIG. 4  represents a flowchart depicting a method for generating a double-sided PWM signal, in accordance with another embodiment of the present invention. At step  402 , a PCM stream is received at the circuitry for generation of a double-sided PWM signals. The PCM data stream can be received at the PCM input block  104  of the circuitry, as described in  FIG. 2 . At step  404 , the PCM data stream is interpolated for up-sampling the PCM data stream. The interpolator  202 , as described earlier in  FIG. 2  can be used to interpolate the PCM data stream  102 . At step  406 , the PCM data stream is re-sampled. The re-sampling block  204  can re-sample the PCM data stream. The sampling rate of the re-sampling block  204  is twice as a switching frequency of the PWM data stream. At step  408 , an even numbered PCM data stream and an odd numbered PCM data stream are extracted from the PCM data stream. The re-sampling block  204  can extract the odd numbered PCM data stream and the even numbered PCM data stream from the PCM data stream. At step  410 , the odd numbered PCM data stream and the even numbered PCM data stream are quantized. The first quantizer  206  can quantize the PCM data stream used to modulate a leading edge of a PWM signal and the second quantizer  208  can quantize the PCM data stream used to modulate a trailing edge of a PWM signal. At step  412 , a leading edge PWM signal is generated from the odd numbered PCM data stream. The first modulation conversion circuitry  210  can generate the leading edge PWM signal from the odd numbered PCM data stream. At step  414 , a trailing edge PWM signal is generated from the even numbered PCM data stream. The second modulation conversion circuitry  212  can generate the trailing edge PWM signal from the even numbered PCM data stream. It should be appreciated that the roles of the odd numbered PCM data stream and the even numbered data stream can be exchanged in steps  412  and  414 . This means that the leading edge PWM signal can be generated from the even numbered PCM data stream, while the trailing edge PWM signal can be generated from the odd numbered PCM data stream. At step  416 , the leading edge PWM signal and the trailing edge PWM signal are combined to form the double-sided PWM signal. The double-sided PWM signal is a pulse width modulated signal that has both its leading edge and trailing edge modulated by the information source.  
       FIG. 5  illustrates a spectrum analyzer graph that displays an exemplary output of the system for generating a double-sided PWM signal, in accordance with various embodiments of the present invention. The spectrum analyzer graph plots magnitude spectra of a double-sided PWM signal with respect to frequency. The spectrum analyzer graph shown in  FIG. 5  corresponds to a double-sided PWM signal for which the carrier frequency is 800 MHz and the switching frequency is 100 MHz. Further, the bandwidth of the bandpass signal is 20 MHz.  
      Various embodiments of the present invention, as described above, generate a double-sided pulse width modulated (PWM) signal. The method and system, as described above, generate the double sided PWM signal at a switching frequency that is half of a sampling frequency of the input PCM data stream. The system for generating a double-sided pulse width modulated (PWM) signal, as described above, reduces power consumption and complexity of an amplifier system.  
      It will be appreciated the method and system for generation of double-sided pulse width modulation signal in accordance with the present invention described herein may comprise one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method for generation of double-sided pulse width modulation signal described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method for generation of double-sided pulse width modulation signal. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits ASICs, in which each function or some combinations of certain functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein.  
      It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.  
      In the foregoing specification, the invention and its benefits and advantages have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.