Abstract:
A technique for the suppression of spurious Direct Digital Synthesis (“DDS”) signals includes a method and an apparatus that equally and oppositely dithers a pair of complementary input digital words from which a pair of analog signals are direct digital synthesized and then mixes the generated analogy signals.

Description:
The earlier effective filing date of co-pending U.S. Provisional Application Ser. No. 60/991,551, entitled, “Spurious DDS Signal Suppression”, filed Nov. 30, 2007, in the name of the inventor Dana W. Kintigh is hereby claimed for all common subject matter under 35 U.S.C. §120. This application is also hereby incorporated by reference for all purposes as if set forth verbatim herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to applications for Direct Digital Synthesis (“DDS”), and, more particularly, the suppression of spurious DDS signals. 
     2. Description of the Related Art 
     Conventional DDS&#39;s generate discrete sidebands as spurious signals combined with their desired output frequency. These are problematic for sensor and radar applications which require sources of clean frequencies without spurious signals. This can be the limiting factor in the performance of advanced sensor and radar systems. Existing systems must either live with this performance impact or use complex, costly radio frequency (“RF”) circuits with limited frequency agility to generate critical source signals. In current DDS applications that filter the DDS output, the high spurious level requires that the output frequency pass through a complex mix, divide and filter process to reduce the spurious signal levels to a usable level. This level is more than 20 dB below the present capability of conventional DDS&#39;s. 
     More particularly, filtering the DDS spurious energy is possible for some very limited applications—i.e., fixed, non-adjustable frequencies. The required precision and narrowness of the needed filters is extremely challenging and greatly increases the cost of the sub-system. These restrictions normally make this approach impractical for most applications. High performance systems generally cannot use DDS&#39;s directly to generate critical signals. Indirect methods are used to generate frequencies with low spurious signals but are limited by the additional circuit complexity needed to individually produce each additional frequency used by the system. 
     The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above. 
     SUMMARY OF THE INVENTION 
     The present invention includes a method and apparatus for the suppression of spurious DDS signals. 
     In a first aspect, the present invention is a method, comprising: receiving a pair of complementary input digital words; equally and oppositely dithering the complementary input digital words; direct digital synthesizing a pair of analog signals from the dithered digital words; and mixing the generated analog signals. 
     In a second aspect, the invention is a method, comprising: pseudo-randomly generating a digital offset; receiving a pair of complementary input digital words; subtracting the digital offset from a first one of the input digital words and adding the digital offset to a second one of the input digital words to create a pair of offset digital words; direct digital synthesizing a pair of analog signals from the offset digital words; and mixing the generated analog signals. 
     In a third aspect, the invention is an apparatus, comprising: a pseudo-random number generator capable of generating a digital offset; an adder capable of adding the digital offset to a first input digital word; a first direct digital synthesis device capable of generating a first analog signal from the first offset digital word; a subtractor capable of subtracting the digital offset from a second digital word, the second digital word being complementary to the first second digital word; a second direct digital synthesis device capable of generating a second analog signal using from the second offset digital word; and a mixer capable of mixing the first and second analog signals 
     In a fourth aspect, the invention is an apparatus, comprising: means for equally and oppositely dithering a pair of complementary input digital words; a first direct digital synthesis device capable of generating a first analog signal from a first one of the dithered input digital words; a second direct digital synthesis device capable of generating a second analog signal using from a second one of the dithered digital words; and means for mixing the first and second analog signals. 
     In a fifth aspect, the invention is an apparatus, comprising: means for pseudo-randomly generating a digital offset; means for subtracting the digital offset from a first one of a pair of received input digital words and adding the digital offset to a second one of the input digital words to create a pair of offset digital words; means for direct digital synthesizing a pair of analog signals from the offset digital words; and means for mixing the generated analog signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
         FIG. 1  depicts in a block diagram one particular embodiment of an apparatus in accordance with the present invention; 
         FIG. 2  is a block diagram of one particular DDS as provided by the vendor and as would be employed in the apparatus of  FIG. 1 ; and 
         FIG. 3  illustrates one particular embodiment of a method performed in accordance with one aspect of the present invention. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, the drawings illustrate specific embodiments herein described in detail by way of example. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort, even if complex and time-consuming, would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The invention uses two or more direct digital synthesizers (DDS&#39;s), each producing a random or pseudo-random, time varying, fractional portion of the final desired frequency in such a way as to spread the spurious signal energy to minimize the impact on the system performance without degrading the desired signal. It is estimated that the peak spurious signals can be reduced 20 dB or more by this method. 
       FIG. 1  illustrates one particular embodiment  100  of an apparatus constructed and operated in accordance with the present invention. The structure and operation of the apparatus  100  revolves around the DDS&#39;s  103 ,  104 . The DDS&#39;s are, preferably, matched, although this is not necessary to the practice of the invention. Any suitable DDS known to the art may be employed. One particular embodiment employs a pair of AD9910 DDS&#39;s commercially available off the shelf from Analog Devices, Inc., who can be reached at One Technology Way, P.O. Box 9106, Norwood, Mass. 02062-9106; phone 1-800-262-5643, or over the World Wide Web of the Internet at www.analog.com. Selected characteristics of the AD9910 provided by the vendor are set forth in Table 1 below.  FIG. 2  is a block diagram of an AD9910  103 , as provided by the vendor and as would be employed in the apparatus of  FIG. 1 . 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Selected Characteristics of Analog Devices AD9910 DDS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1000 MHz Master fclk 
                 14 bit Resolution 
               
               
                 32 bit Tuning Word Width 
                 20 mA FS I out  (mA nom) 
               
               
                 0.5V Compliance Range (V) 
                 REFCLK Multiplier 
               
               
                 On-Board Comparator 
                 Parallel, Serial I/O Interface 
               
               
                 GSPS internal clock speed  
                 Integrated 1 GSPS, 14-bit  
               
               
                 (up to 400 MHz analog output) 
                 digital-to-analog converter  
               
               
                   
                 (“DAC”) 
               
               
                 32-bit tuning word 
                 Phase noise ≦−125 dBc/Hz @ 1  
               
               
                   
                 kHz offset (400 MHz carrier) 
               
               
                 Excellent dynamic performance  
                 Serial input/output (I/O) control 
               
               
                 with &gt;80 dB narrow-band SFDR 
                   
               
               
                 Automatic linear or arbitrary  
                 8 frequency and phase offset profiles 
               
               
                 frequency, phase, and amplitude  
                   
               
               
                 sweep capability 
                   
               
               
                 1.8 V and 3.3 V power supplies 
                 Software and hardware controlled  
               
               
                   
                 power-down 
               
               
                 100-lead TQFP_EP package 
                 Integrated 1024 word × 32-bit RAM 
               
               
                   
               
             
          
         
       
     
     As will be appreciated by those skilled in the art, each DDS  103 ,  104  will generate an analog output  121 ,  122  whose frequency is determined by a binary, digital word  109 ,  110  input to the memory of the DDS. The input digital words  109 ,  110  are sometimes called “digital control signals”. They may be generated by and received from digital oscillators (not shown) or any other technique well known to the art. The generation of digital control signals, such as the input digital words  109 ,  110 , is well known in the art and any suitable technique may be employed. 
     In the present invention, the input is split between the two DDS&#39;s  103 ,  104  and dithered with a digital offset Δ. Thus, the first and second input digital words  109 ,  110  are “complementary” in the sense that they represent two frequencies F 1 , F 2  of the analog output  121 ,  122  that, when summed, equal the output frequency F out  of the sum signal  112 . Thus, if one wishes to achieve an output signal where F out =100 MHz, one may chose the input digital words  109 ,  110  so that F 1 =80 MHz and F 2 =20 MHz. The two input frequencies F 1 , F 2  represented by the input digital words  109 ,  110  can have a ratio of eight or greater to allow easy filtering although this is not required. 
     The present invention is, in theory, not limited by the magnitude F out . However, the present invention may predominantly be employed with frequencies from radio frequency through microwave, including very high frequency (“VHF”) and ultra high frequency (“UHF”). 
     The digital offset Δ can be determined, for example, by the pseudo-random number generator (“PRNG”)  106 . The digital offset Δ can be a random function or can be a specialized set of numbers to provide a lower spurious signal level. Those in the art will appreciate that pseudo-random number generators are also frequently referred to as “random number generators” because, for all practical purposes, the number is random. However, “random number generators” determine the seed in a deterministic fashion, and so are actually “pseudo-random number generators”. The random function by which the pseudo-random generator  106  is seeded is typically be tailored to Doppler bin size and Pulse Repetition Interval (“PRI”). In the illustrated embodiment, the typical Doppler bin size is 200 Hz and the PRI is several milliseconds. 
     More particularly, the spurious signals of a conventional DDS are a function of the output frequency, phase accumulator design, and number of bits in the digital-to-analog converter (“DAC”). For a single DDS, spurious signals are ‘fixed’ in frequency. In the present invention, the two input frequencies represented by the input digital words  109 ,  110  are dithered equally and oppositely so that the desired sum signal  112  is generated without degradation:
 
 F   out =( F   1 +Δ)+( F   2 −Δ)= F   1   +F   2 ,
 
where Δ is the digital offset by which the frequencies F 1 , F 2  are dithered. The spurious signals of F 1 , F 2  move in frequency ‘space’.
 
     The apparatus  100  includes an adder  115  and a subtractor  118  which actually dither the first and second input digital words  109 ,  110  with the digital offset Δ. The adder  115  adds the digital offset Δ to the first digital input word  109  to create the first offset digital word  124 . The subtractor  118  subtracts the digital offset Δ from the second input digital word  110  to create the second offset digital word  125 . Thus, the PRNG  106 , adder  115 , and subtractor  118  comprise, in the illustrated embodiment, a means for equally and oppositely dithering the input digital words  109 ,  110 . However, it is by way of example and illustration, but one means for doing so, and alternative embodiments may employ alternative means. It is the first and second offset digital words  124 ,  125  from which the DDS&#39;s  103 ,  104  generate the analog outputs  121 ,  122 . 
     The analog outputs  121 ,  122  are then mixed (or “multiplied”) by the mixer  127  to generate the sum signal  112  with the frequency F out . The illustrated embodiment also applies an optional bandpass filter  130  to the sum signal  112  to generate a filtered output  133 . However, alternative embodiments may forego this filtering or add additional processing and/or conditioning to the sum signal  112  prior to its use. 
     Turning now to  FIG. 3 , in operation, the apparatus  100 , shown in  FIG. 1 , performs a method  300 . The method begins with pseudo-randomly generating (at  310 ) a digital offset Δ and receiving (at  320 ) a pair of complementary input digital words  109 ,  110 . The digital offset Δ is then subtracted (at  330 ) the digital offset Δ from a first one of the input digital words  110  and adding the digital offset Δ to a second one of the input digital words  109  to create a pair of offset digital words  124 ,  125 . The method  300  then direct digital synthesizes (at  340 ) a pair of analog signals  121 ,  122  from the offset digital words  124 ,  125 . The analog signals  121 ,  122  are then summed (at  350 ). 
     The present invention admits variation in implementation. For example, the present invention can be applied to the phase and amplitude adjustment port of the DDS. Indeed, all three of these options can be employed at the same time in some embodiments. Another area admitting variation is the final form of a given implementation. For example, the apparatus  100  may be incorporated into a mixed application specific integrated circuit (“ASIC”) or a field programmable gate array (“FPGA”) with an external DAC or a standalone DDS. 
     The present invention therefore offers the ability to achieve better spurious performance at low cost. It can generate adjustable frequencies with low spurious signals with a circuit that has reduced complexity. The reduction of spurious levels is greater than for other methods. A large portion of this circuit is digital which also reduces variability and production costs. 
     This concludes the detailed description. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.