Abstract:
Methods and apparatus are provided for whitening quantization noise in a delta-sigma modulator using a dither signal. An input signal is quantized using a predictive delta-sigma modulator by quantizing the input signal using a quantizer; adding a dither signal at a first location of the predictive delta-sigma modulator; determining a quantization error associated with the quantizer; removing the dither signal at a second location of the predictive delta-sigma modulator (for example, by subtracting a substantially similar version of the dither signal at the second location); generating an error prediction value using an error predictive filter; and subtracting the error prediction value from the input signal. The dither signal may be a white noise signal and may optionally be generated using a pseudo-random number generator.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related to delta-sigma modulators and, more particularly, to techniques to reduce the correlation between the quantization noise and the input signal in such delta-sigma modulators. 
       BACKGROUND OF THE INVENTION 
       [0002]    Delta-sigma (Δ-Σ) modulators, sometimes referred to as sigma-delta (Σ-Δ) modulators, encode higher resolution signals into lower resolution signals. Delta-sigma modulators have found increasing use in a range of modern electronic components, such as digital-to-analog converters (DACs), analog-to-digital converters (ADCs) and frequency synthesizers. For example, delta-sigma modulators have been proposed or suggested for directly synthesizing RF signals from information carrying digital base band signals. 
         [0003]    Delta-sigma modulators typically quantize an input signal and determine a quantization error by comparing the input signal to the quantized output value. The quantization error is often assumed to be uncorrelated to the input signal. More typically, however, the error signal is correlated to the input. Thus, the delta-sigma modulator may suffer from noise coloration due to the correlation of the quantization noise with the input signal, particularly for low resolution quantizers. 
         [0004]    A need therefore exists for techniques to reduce the correlation between the quantization noise and the input signal, without significantly degrading the signal to noise ratio. 
       SUMMARY OF THE INVENTION 
       [0005]    Generally, methods and apparatus are provided for whitening quantization noise in a delta-sigma modulator using a dither signal. According to one aspect of the invention, an input signal is quantized using a predictive delta-sigma modulator by quantizing the input signal using a quantizer; adding a dither signal at a first location of the predictive delta-sigma modulator; determining a quantization error associated with the quantizer; removing the dither signal at a second location of the predictive delta-sigma modulator (for example, by subtracting a substantially similar version of the dither signal at the second location); generating an error prediction value using an error predictive filter; and subtracting the error prediction value from the input signal. The dither signal may be a white noise signal and may optionally be generated using a pseudo-random number generator. 
         [0006]    A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates an exemplary conventional delta-sigma modulator; 
           [0008]      FIGS. 2A and 2B  illustrate a conventional dithering technique for noise whitening; and 
           [0009]      FIG. 3  illustrates an exemplary delta-sigma modulator in accordance with the dithering techniques of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The present invention provides improved techniques for reducing the correlation between the quantization noise and the input signal in a delta-sigma modulator. 
         [0011]      FIG. 1  illustrates an exemplary conventional delta-sigma modulator  100 . As shown in  FIG. 1 , the exemplary delta-sigma modulator  100  employs a quantizer  110 , such as a one bit quantizer, and an error predictive filter  120 . The input value, u, to the quantizer  110  is compared to the quantized output value, q, by an adder  130  that generates a quantization error, e. The quantization error, e, is processed by the error predictive filter  120  to generate an error prediction value, el, that is stored in a register  140  for one clock cycle and then subtracted from the input signal, r, by an adder  150  that generates the error-compensated input value, u. Generally, error predictive filters  120  employ some knowledge of the input signal to filter the signal, in a known manner. For example, if the error is known to be slowly varying, the error predictive filter  120  can use the same value for subsequent samples. The input signal, r, may be, for example, a 16 bit digital value, and the one bit quantization performed by the quantizer  110  (e.g., the quantization can be based on the polarity of the input signal) provides a coarse analog conversion. The quantization noise, e, associated with the one bit quantizer  110  is primarily out-of-band. As previously indicated, the one bit quantization performed by the exemplary quantizer  110  is inherently linear. 
         [0012]    The quantization error, e(n), is often assumed to be uncorrelated to the input r(n). See, e.g., United States Patent Application entitled “Methods and Apparatus for Direct Synthesis of RF Signals Using Delta-Sigma Modulator,” filed contemporaneously herewith and incorporated by reference herein. More typically, however, the error signal may be correlated to the input. Thus, the delta-sigma modulator  100  may suffer from noise coloration due to the correlation of the quantization noise with the input of the quantizer  110 , particularly for low resolution quantizers, such as the one bit quantizers described herein. In order to reduce the effects of the correlation between the quantization noise and the input, a dithering configuration or a decorrelating configuration (or both) can be employed to reduce the noise correlation without significantly degrading the signal to noise ratio. For a more detailed discussion of a decorrelating configuration, see, and “Methods and Apparatus for Decorrelating Quantization Noise in a Delta-Sigma Modulator,” filed contemporaneously herewith and incorporated by reference herein. 
         [0013]    Typically, the one bit quantization performed by the quantizer  210  would require a high oversampling ratio. For example, audio encoding techniques that employ one bit quantization of audio signals that are on the order of 100 KHz typically oversample the audio signal at a rate of 20 MHz. Such oversampling is not practical in the wireless communication context of the present invention, where the signals are typically on the order of multiple GHz. The desired low oversampling rates of the present invention, however, would typically lead to an unstable encoder. As discussed hereinafter, aspects of the present invention provide techniques for providing a stable encoder based on one bit quantization. 
         [0014]    According to one aspect of the invention, well-known dithering techniques are employed to reduce noise correlation to the quantizer  110  of the exemplary delta-sigma modulator  100  by introducing a dither signal at one point in the modulator loop and then introducing an inverse of the dither signal at another point in the modulator loop. In this manner, the introduced dither signal is cancelled, while providing the desired noise whitening properties of the dithering technique in the quantizer  110 . Among other benefits, the disclosed dithering techniques reduce spurs in the in-band and out-of-band noise spectrum, and improve the stability of the loop  100  by reducing limit cycles (caused by the non-linear behavior of the coarse quantizer  110  in a closed loop. It is noted that even when noise spurs are out-of-band they can violate emission regulations, such as those specified by the FCC in the United States. 
         [0015]      FIGS. 2A and 2B  illustrate a conventional dithering technique for noise whitening.  FIG. 2A  illustrates a quantization of an input signal, x, by a quantizer  210 , to generate a quantized output signal, y, equal to Q(x).  FIG. 2B  illustrates a quantization of an input signal, x, with dithering by a quantizer  220 . As shown in  FIG. 2B , a dither signal, N, is added to the input signal, x, prior to being quantized by the quantizer  220 . The dithered input signal, x+N, is applied to the quantizer  220 , which generates a quantized output, Q(x+N). The dither signal, N, is then removed from the quantized output, by adding an inverse, −N, of the dither signal. The output signal, y, is equal to Q(x+N)−N. 
         [0016]    Generally, dithering techniques recognize that if the quantization error is de-correlated due to the addition of the dither noise to the input signal, x, then the output signal, y, is equal to Q(x+N)−N, which is equal to (x+N)+E−N, or x+E, where E is the quantization error, e. 
         [0017]    It is noted that by adding the dither signal, N, to the quantizer input and subtracting the dither signal, N, from the quantizer output, the effect of the dither signal N is effectively canceled, and only the benefits of noise whitening of the non-linear quantizer Q remain. The present invention recognizes, however, that this dithering technique cannot be directly applied to the delta-sigma modulator  100  of  FIG. 1  since the quantizer output must remain a quantized value (i.e., −N cannot be added to the quantizer output of in a delta-sigma modulator  100 ). 
         [0018]      FIG. 3  illustrates an exemplary delta-sigma modulator  300  in accordance with the dithering techniques of the present invention. As shown in  FIG. 3 , the exemplary delta-sigma modulator  300  employs a one bit quantizer  110  and an error predictive filter  120 , in a similar manner to the delta-sigma modulator  100  of  FIG. 1 . The input value to the quantizer  110  is compared to the output value, q, by an adder  130  that generates a quantization error, e. The quantization error, e, is processed by the error predictive filter  120  to generate an error prediction value, el, that is stored in a register  140  for one clock cycle and then subtracted from the input signal, r, by an adder  150  that generates an error-compensated input value, u. 
         [0019]    As shown in  FIG. 3 , the exemplary delta-sigma modulator  300  employs well-known dithering techniques to introduce a noise signal, N, such as white noise, using an adder  310 . The noise (dither) signal may be generated, for example, using a pseudo-random number generator. A substantially similar version of the noise signal, N, is then subtracted by an adder  320  to cancel the introduced noise (alternatively, an inverse of the noise signal, N, can be added by the adder  320  to cancel the introduced noise). 
         [0020]    It can be shown that the quanitization error, q, of the exemplary delta-sigma modulator  100  of  FIG. 1  can be expressed as follows: 
         [0000]        Q=R +(1− H ) E.    (1) 
         [0021]    It is noted that in the exemplary notation employed herein, lower case variables, such as q, indicate a time domain, while upper variables, such as Q, indicate a corresponding frequency (or z) domain. Likewise, it can be shown that the quanitization error, q, of the exemplary delta-sigma modulator  300  of  FIG. 3  can be expressed as follows: 
         [0000]        Q=R +(1− H )( E+N ).   (2) 
         [0000]    Significantly, the exemplary delta-sigma modulator  300  of  FIG. 3  with dithering demonstrates an SFDR of approximately 95 dB. 
       Conclusion 
       [0022]    While exemplary embodiments of the present invention have been described with respect to digital logic blocks, as would be apparent to one skilled in the art, various functions may be implemented in the digital domain as processing steps in a software program, in hardware by circuit elements or state machines, or in combination of both software and hardware. Such software may be employed in, for example, a digital signal processor, application specific integrated circuit or micro-controller. Such hardware and software may be embodied within circuits implemented within an integrated circuit. 
         [0023]    Thus, the functions of the present invention can be embodied in the form of methods and apparatuses for practicing those methods. One or more aspects of the present invention can be embodied in the form of program code, for example, whether stored in a storage medium, loaded into and/or executed by a machine, wherein, when the program code is loaded into and executed by a machine, such as a processor, the machine becomes an apparatus for practicing the invention. When implemented on a general-purpose processor, the program code segments combine with the processor to provide a device that operates analogously to specific logic circuits. The invention can also be implemented in one or more of an integrated circuit, a digital signal processor, a microprocessor, and a micro-controller. 
         [0024]    It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.