Patent Publication Number: US-8115519-B2

Title: Phase accumulator generating reference phase for phase coherent direct digital synthesis outputs

Description:
TECHNICAL FIELD 
     The invention relates to direct digital synthesis, and more particularly, to a phase accumulator generating a reference phase for providing phase coherent direct digital synthesis outputs that are locked to the reference phase. 
     BACKGROUND 
     Direct digital synthesis (DDS) has been used to produce analog waveforms, such as a sine wave, of various frequencies and profiles. The ability to produce the waveforms accurately and to control the waveforms may be important for various applications and industries such as communications and electronic warfare. DDS devices generate a time-varying signal in digital form and then perform a digital-to-analog conversion. DDS devices generally provide fast switching between output frequencies, fine frequency resolution, and operation over a broad spectrum of frequencies. 
     A DDS device may generally include a phase accumulator, a phase-to-amplitude converter, and a digital-to-analog converter (DAC). The frequency of a DDS output generally depends on a reference clock frequency and a binary number input to the phase accumulator (referred to as a frequency control word). The phase accumulator computes phase values representing phase angles and the phase-to-amplitude converter outputs digital amplitude values corresponding to the phase values (e.g., a sine of the phase angle). The DAC converts the digital amplitude values into a corresponding analog amplitude (e.g., voltage or current), thereby producing an analog sinusoidal output signal. 
     Although conventional DDS devices provide accurate analog waveforms and allow the frequency to be changed, conventional DDS devices are associated with shortcomings. 
     SUMMARY 
     Consistent with one embodiment, a direct digital synthesis (DDS) device includes a polyphase accumulator configured to generate a plurality of digital phase signals in response to a clock signal and a frequency control word. Each of the digital phase signals represents phase data corresponding to phase points on a sinusoidal waveform. The phase data is based on multiples of the frequency control word and an incrementing reference phase such that the digital phase signals remain locked to the incrementing reference phase when the frequency control word changes. The DDS device also includes a plurality of phase-to-amplitude converters coupled to the polyphase accumulator and configured to produce a plurality of digital amplitude signals in response to the digital phase signals, respectively. The digital amplitude signals represent digital amplitude data corresponding to the phase points on the sinusoidal waveform. The DDS device further includes a multiplexer coupled to the phase-to-amplitude converters and configured to multiplex the digital amplitude signals from the phase-to-amplitude converters to produce a multiplexed digital amplitude signal and a digital-to-analog converter coupled to the multiplexer and configured to convert the digital amplitude data represented by the multiplexed digital amplitude signal into an analog output to produce an analog sinusoidal signal. 
     Consistent with another embodiment, a DDS device includes a phase accumulator configured to produce an incrementing reference phase in response to a clock signal and configured to multiply the incrementing reference phase by a frequency control word to produce a digital phase signal representing phase data corresponding to phase points on a sinusoidal waveform. The digital phase signal remains locked to the incrementing reference phase when the frequency control word changes. The DDS device also includes a phase-to-amplitude converter coupled to the phase accumulator and configured to receive the digital phase signal and to provide a digital amplitude signal in response to the digital phase signal. The digital amplitude signal represents digital amplitude data corresponding to the phase points on the sinusoidal waveform. The DDS device further includes a digital-to-analog converter coupled to the phase-to-amplitude converter and configured to receive the digital amplitude signal and to convert the digital amplitude data represented by the digital amplitude signal into an analog output to produce an analog sinusoidal signal. 
     Consistent with a further embodiment, a method is provided for direct digital synthesis. The method includes: producing at least one digital phase signal in response to a clock signal and a frequency control word, the digital phase signal representing phase data corresponding to phase points on a sinusoidal waveform, the phase data being based on multiples of the frequency control word and an incrementing reference phase such that the digital phase signal remains locked to the incrementing reference phase when the frequency control word changes; converting the phase data represented by the digital phase signal into digital amplitude data to produce at least one digital amplitude signal in response to the at least one digital phase signal, the digital amplitude data corresponding to the phase points on the sinusoidal waveform; and converting the digital amplitude data represented by the digital amplitude signal into an analog output to produce an analog sinusoidal signal. 
     The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of a direct digital synthesis (DDS) device including a polyphase accumulator generating a reference phase to provide phase coherent outputs, consistent with an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating sinusoidal waveforms of different frequencies with phase coherency, consistent with an embodiment of the present invention. 
         FIG. 3  is a schematic diagram of a DDS device including a polyphase accumulator generating a reference phase to provide phase coherent outputs, consistent with another embodiment of the present invention. 
         FIG. 4  is a schematic diagram illustrating operation of a polyphase accumulator that may be used in the DDS device shown in  FIG. 3 . 
         FIG. 5  is a schematic block diagram of a monophase DDS device with a phase accumulator generating a reference phase to provide phase coherent outputs, consistent with a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As previously explained, conventional DDS devices are associated with shortcomings. One subtle and non-trivial such shortcoming is that conventional DDS devices do not provide phase coherency when the frequency changes. For example, when a frequency changes from a first frequency to a second frequency and back to the first frequency, the phase of the output at the first frequency does not necessarily or consistently match the phase of the output when the first frequency was initially applied. For certain applications, such as electronic warfare systems and various commercial communication applications, phase coherency is desired. 
     A phase accumulator, consistent with embodiments described herein, generates phase data for a direct digital synthesis (DDS) device based on a reference phase to provide analog sinusoidal outputs that are locked to the reference phase and thus phase coherent. The frequency of a sinusoidal DDS output may be controlled by changing a frequency control word (FCW) provided to the phase accumulator without affecting the incrementing reference phase. The sinusoidal DDS output is based on a multiple of the FCW and the reference phase and thus remains locked to the reference phase, providing phase coherency even when the FCW changes to change the frequency, as will be described in greater detail below. 
     As used herein, “phase coherent” or “phase coherency” refers to the phase of the sinusoidal DDS output remaining substantially fixed or unchanged (i.e., within acceptable system tolerances) for a number of different frequencies and/or frequency changes. For example, the frequency of the DDS output may be switched from any frequency to any other (or to multiple other frequencies) and return in-phase to the original frequency. As used herein, “sinusoidal” refers to a succession of waves similar to a sine wave but does not require a waveform that exactly corresponds to a sine wave. For instance, the output sinusoidal waveform may be in the form of a sine wave, square wave, triangle wave, or other desired waveform shape. 
     Referring to  FIG. 1 , a DDS device  100  with a phase coherent output, consistent with one embodiment of the present invention, includes a polyphase accumulator  110  with a parallel architecture to generate parallel outputs of phase data in the form of digital phase signals  119 - 1  to  119 - n  locked to a reference phase. The phase data generally represents phase points on a sinusoidal waveform (e.g., phase angles), and the DDS device  100  determines amplitudes corresponding to the phase points to produce a sinusoidal output signal. The polyphase accumulator  110  generates phase data that increments to step through the phase points for a complete cycle (e.g., the angular phase range of 0° to 360° or 0 to 2π radians) of the sinusoidal waveform. 
     A frequency control word (FCW)  111  controls the frequency of the sinusoidal output signal because the size of the phase increment of the phase data is based on the FCW  111 . In other words, changing the FCW  111  changes the size of the phase increment of the accumulator  110 , which changes the speed at which the accumulator steps through the phase points. The phase data on each of the parallel outputs is also based on a multiple of an incrementing reference phase and thus locked to the reference phase, allowing the polyphase accumulator  110  to provide a phase coherent output when the frequency is changed, as described in greater detail below. 
     According to the exemplary parallel architecture, the DDS device  100  also generally includes a plurality of phase-to-amplitude converters  120 - 1  to  120 - n  that convert the phase data represented by the respective digital phase signals  119 - 1  to  119 - n  into digital amplitude data in the form of respective digital amplitude signals  121 - 1  to  121 - n . A multiplexer  130  multiplexes the digital amplitude signals  121 - 1  to  121 - n  into a multiplexed digital amplitude signal  131 . A digital-to-analog converter (DAC)  140  converts the digital amplitude data represented by the multiplexed digital amplitude signal  131  into an analog output to produce an analog sinusoidal output signal  141 . The DDS device  100  may further include a clock signal generator  150  to provide one or more clock reference signals  151 - 153  to the components of the DDS device  100 . The clock signal generator  150  may generate the clock reference signals  151 - 153  at different rates based on an external system clock signal provided to the DDS device  100 . Using a parallelized architecture to generate and convert the phase data enables the front end of the DDS device  100  to be operated at higher speeds with a lower clock frequency. 
     In the exemplary embodiment, the polyphase accumulator  110  generates the phase data on parallel outputs by determining a coarse phase component and a plurality of fine phase components added to the coarse phase component. The coarse phase component is based on the reference phase and both the coarse phase component and the fine phase components are based on the FCW  111 . In particular, the polyphase accumulator  110  includes a master accumulator  112  that produces a coarse phase signal  117  representing the coarse phase component of the phase data and a plurality of polyphase offset generators  118 - 1  to  118 - n  that produce and add the fine phase components to the coarse phase component to produce the respective digital phase signals  119 - 1  to  119 - n.    
     The master accumulator  112  increments by a coarse phase increment according to a rate of the clock signal  151  to generate an incrementing reference phase. In one embodiment, a coarse phase increment is equal to the number (n) of parallel paths (i.e., the number of polyphase offset generators  118 - 1  to  118 - n ). The master accumulator  112  then multiplies the incrementing reference phase by the FCW  111  to generate the coarse phase signal  117 . Each of the polyphase offset generators  118 - 1  to  118 - n  multiply the FCW  111  by a fixed fine phase offset to produce fine phase components and add the fine phase components to the coarse phase component represented by the coarse phase signal  117  to produce the digital phase signals  119 - 1  to  119 - n . Thus, the digital phase signals  119 - 1  to  119 - n  may all be based on multiples of the same incrementing reference phase and remain locked to that reference phase even when the FCW  111  changes to change the frequency. 
       FIG. 2  illustrates the phase coherency of a DDS device, consistent with embodiments described herein, using exemplary waveforms. The reference waveform  210  represents a waveform generated with a reference frequency corresponding to the incrementing reference phase (i.e., without being multiplied by frequency control word). The FCW A waveform  220  represents a waveform generated with a first frequency using a first frequency control word (FCW A) and the FCW B waveform  230  represents a waveform generated with a second frequency using a second frequency control word (FCW B). The output waveform  240  represents a waveform output from a DDS device with the frequency changing by changing the frequency control word from FCW A to FCW B and back again. Although the frequency of the output waveform  240  changes, the output waveform  240  is based off of multiples of the reference frequency of the reference waveform  210  and thus the phase is coherent for the different output frequencies. As shown, the phase of the output waveform  240  does not change relative to the reference waveform  210  and is thus locked to the reference phase. In other words, the phase of the output waveform  240  remains coherent even after switching the frequency (i.e., between FCW A and FCW B). For illustrative purposes, only two frequencies are shown in  FIG. 2 ; however, the phase coherency property extends to multiple frequencies (i.e., FCW A to FCW B to FCW C, then coherently back to FCW A, then coherently back to FCW C, etc.). 
     Referring to  FIG. 3 , another embodiment of a DDS device  300  including a polyphase accumulator  310  is shown and described in greater detail. In this embodiment, the polyphase accumulator  310  includes a master accumulator  312  coupled to thirty-two (32) polyphase offset generators  318 . The offset generators  318  and phase-to-amplitude converters  320  may be implemented in respective parallel circuits  328 . As mentioned above, the parallel architecture allows the accumulator  310  to operate with a high speed at a lower clock rate. In the DDS device  300  with thirty-two (32) parallel circuits  328 , for example, the polyphase accumulator  310  may be operated at 800 MHz. Although the exemplary embodiment shows thirty-two (32) parallel circuits  328 , fewer or greater numbers of parallel circuits may also be used (e.g., 16 or 64). 
     The master accumulator  312  may include a counter  314  that increments by the coarse phase increment (e.g., a binary number) with each clock cycle of a reference clock signal  351  to produce an incrementing reference phase  313 . In the illustrated embodiment, the coarse phase increment is thirty-two (32) (e.g., in binary form) corresponding to the number of polyphase offset generators  318 . The counter  314  increments by the coarse phase increment until the polyphase accumulator  310  steps through the phase points within one complete cycle of the sinusoidal waveform and then resets to zero. Thus, the incrementing reference phase  313  may repeatedly step through multiple cycles of phase points corresponding to phase angles within a sinusoidal waveform. The master accumulator  312  multiplies a frequency control word (FCW)  311  (e.g., a binary number) by the incrementing reference phase  313  to produce the coarse phase component represented by the coarse phase signal  317 . Although the exemplary embodiment shows one master accumulator  312  providing the coarse phase signal  317  to the plurality of polyphase offset generators  318 , other embodiments may include multiple master accumulators providing identical copies of the coarse phase signals to one or more offset generators. 
     In this embodiment, the polyphase accumulator  310  may also include an adjustable phase offset to change the phase of the output signal. In particular, the master accumulator  312  may add a phase offset value (e.g., a binary number) to the coarse phase component. Locking the phase data to the reference phase allows phase coherency for different frequency/phase offset pairs in addition to phase coherency for different frequencies. 
     Each polyphase offset generator  318  multiplies the FCW  311  by a respective static offset value ranging from 0 to 31 to produce respective fine phase components represented by fine phase signals  315 . Each polyphase offset generator  318  then adds the coarse phase component to the respective fine phase components to produce the respective phase data output represented by digital phase signal  319 . The master accumulator  312  and the polyphase offset generators  318  may be implemented using circuitry (e.g., adders, multipliers and registers) capable of adding, multiplying and storing digital values, such as the type known by those skilled in the art for use in phase accumulators. 
     Each phase-to-amplitude converter  320  receives a respective digital phase signal  319  and converts the phase data into digital amplitude data represented by a digital amplitude signal  321 . Each phase-to-amplitude converter  320  may be implemented using a sine (or cosine or other periodic function) lookup table with the respective output of the polyphase accumulator  310  serving as the address to the lookup table. Each address in the lookup table may correspond, for example, to a phase point on the sinusoidal waveform from 0° to 360° (or from 0 to 2π radians) and the lookup table may contain the corresponding digital amplitude data capable of reproducing a complete cycle of the sinusoidal waveform. In other words, one embodiment of the phase-to-amplitude converter  320  maps the phase data to a digital amplitude word. The phase-to-amplitude converter  320  may be implemented using any circuitry capable of converting the phase data into the digital amplitude data such as one or more ROM (Read Only Memory) lookup tables. Although the illustrated embodiment shows the parallel circuit  328  with a single phase-to-amplitude converter  320  receiving a single digital phase signal  319 , one or more of the parallel circuits  328  may include multiple phase-to-amplitude converters  320  (e.g., multiple ROMs) receiving multiple copies of the digital phase signal  319 . 
     The DDS device  300  may also using dithering techniques to improve spurious free dynamic range (SFDR) of the DDS device  300 . In one embodiment, for example, a pseudorandom binary sequence (PRBS) generator  322  may be coupled to each of the phase-to-amplitude converters  320  to add noise (e.g., in the form of random bits) to the digital amplitude data output from the phase-to-amplitude converters  320  before the digital amplitude data is converted to an analog output. In response to a dither enable signal  323 , for example, each PRBS generator  322  may generate a PRBS signal  325  including the random bits that are added to the respective digital amplitude signal  321  output from the respective phase-to-amplitude converter  320 . The amplitude dither may result in fewer bits in the digital amplitude signals  321 , which allows power savings in the multiplexer  330  and DAC  340 . 
     In the exemplary embodiment, a multiplexer  330  receives the parallel digital amplitude signals  321  and combines the signals  321  into a multiplexed digital amplitude signal  331 . The multiplexer  330  may be implemented using any circuitry capable of multiplexing a plurality of multiple bit digital signals into a single multiplexed digital signal, such as a 32:1 digital multiplexer. 
     In the exemplary embodiment, a DAC  340  receives the multiplexed digital amplitude signal  331  and converts the digital amplitude data into an analog output to produce an analog sinusoidal output signal  341 . The analog sinusoidal output signal  341  may be provided on one or more outputs  342 ,  344 . The DAC  340  may be implemented using any circuitry capable of converting a digital input into an analog output (e.g., current or voltage). The DAC  340  may include, for example, a hybrid DAC such as a 6 bit RTZ DAC that uses a hybrid thermo-coding and R2R architecture. Because the 32 outputs are multiplexed, the clock generator  350  provides a clock signal  353  to the DAC  350  at a higher clock rate (e.g., 25.6 GHz) than the clock signal  351  (e.g., 800 MHz) provided to the front end of the DDS device  310 . 
       FIG. 4  illustrates the operation of the polyphase accumulator  310  in greater detail including mathematical expressions representing the digital data outputs at various points in the accumulator  310 . In an exemplary embodiment, the polyphase accumulator  310  may be implemented as a modulo accumulator that increments until the accumulator overflows and resets to zero, which allows the accumulator  310  to step through the phase points for a complete cycle of a sinusoidal waveform and then repeat the cycle. The number of phase points within the cycle of the accumulator  310  may be represented by the modulo operation %2 p  and depends upon the width p of the accumulator (e.g., in bits). In the illustrated embodiment, the width of the polyphase accumulator  310  is 14 bits and the accumulator  310  may step through 2 14  possible phase points before it overflows. Other widths are also possible depending upon the desired resolution of the accumulator (i.e., the number of discrete phase points). 
     In the mathematical expressions, N represents the number of incremental steps, p represents the width of the accumulator in bits, FCW represents the frequency control word, PHI represents the phase offset, and %2 p  represents the modulo operation. If p=14, FCW=1 and PHI=0, for example, the incrementing reference phase  313  output from the counter  314  and the coarse phase signal  317  output from the master accumulator  312  may include data represented as 0, 32, 64, 96, . . . , 16381, 93, 189, . . . ; the first digital phase signal  319 - 1  may include data represented as 0, 32, 64, 96, . . . , 16352, 0, 32, . . . ; and the thirty-second digital phase signal  319 - 32  may include data represented as 31, 63, 95, 127, . . . , 16383, 31, 63, . . . . The digital phase signals  319 - 2  to  319 - 31  between the first and thirty-second digital phase signals are similar but with different fine phase offsets. 
     Thus, a larger FCW  311  results in a larger increment of the coarse phase component (N*FCW*32+PHI) represented by the coarse phase signal  317  and a larger fine phase component (0*FCW to 31*FCW) represented by each of the fine phase signals  315 - 1  to  315 - 32 , and a larger phase increment in the resulting phase data represented by the digital phase signals  319 - 1  to  319 - 32 . The larger phase increment in the phase data thus causes the polyphase accumulator  310  to step more quickly through the phase points within a cycle of the sinusoidal waveform. Also, each of the phase data outputs represented by the digital phase signals  319 - 1  to  319 - 32  is based on a multiple of the incrementing reference phase (N*32) even when the FCW  311  is changed and thus is locked to the reference phase to provide phase coherency. The exemplary polyphase accumulator  310  also achieves this phase coherency without sacrificing fast switching time or frequency agility. 
     Referring to  FIG. 5 , a further embodiment of a DDS device  500  with a phase coherent output is shown and described. In this embodiment, the DDS device  500  includes a phase accumulator  510  including a counter  514  that increments by one (1) to produce the incrementing reference phase  513 . The phase accumulator  510  multiplies the incrementing reference phase  513  by a frequency control word (FCW)  511  to produce a digital phase signal  519  representing the phase data corresponding to phase points on a sinusoidal waveform. 
     The DDS device  500  also includes a phase-to-amplitude converter  520 , such as a ROM lookup table, that converts the phase data represented by the digital phase signal  519  into digital amplitude data, for example, as described above. The DDS device  500  further includes a DAC  540  that converts the digital amplitude data into an analog output to produce an analog sinusoidal output signal  542 . A clock generator  550  generates clock signals  551 ,  552  used by the DDS device  500 . As discussed above, the FCW  511  determines the frequency of the output signal  541  and the digital phase signal  519  is based on a multiple of the incrementing reference phase  513  such that the output signal  541  remains locked to the reference phase  513  even when the frequency is changed by changing the FCW  511 . 
     The circuitry used to implement the components and/or functions of the DDS devices described herein may be implemented using integrated circuits, for example, including CMOS logic. The disclosed techniques can be implemented in any number of configurations, including integrated system-on-chip or chip set configurations as well as on printed circuit boards configured with various discrete DDS system components. In addition, any number of suitable process technologies and material systems can be used in fabricating the DDS system. For example, for higher speed applications (e.g., in excess of GHz), the DDS system can be implemented, for instance, in heterojunction bipolar transistor (HBT) and/or high electron mobility transistor (HEMT) technology. However, if such high speeds are not necessary (e.g., less than 2 GHz), then other technologies such as complementary metal oxide semiconductor (CMOS) technology can be utilized. Example materials from which the system can be implemented include III-V material systems such as gallium arsenide (GaAs) or indium phosphide (InP), or other suitable semiconductor material systems. 
     In other embodiments, one or more components or functions of the DDS devices described herein may be implemented using software, firmware or any combination thereof, for example, stored on or in a storage medium and executed by a processor. 
     The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.