Abstract:
Circuitry for providing non-uniform analog-to-digital (“A/D”) signal conversion for wideband signals is provided. The circuitry of the invention is optimized for wideband signals because it does not sacrifice the small-scale resolution of high-probability signal amplitudes while preventing the clipping of low-probability signal amplitudes. The circuitry includes a nonlinear amplifier and an A/D converter that may be uniform or non-uniform. The digital output of the A/D converter may be further processed by circuitry that has an output function that is the inverse of that of the nonlinear amplifier, so as to maintain linear A/D conversion.

Description:
[0001]    This application is a continuation U.S. patent application Ser. No. 11/602,676, filed Nov. 20, 2006, which is a continuation of U.S. patent application Ser. No. 11/001,927, filed Dec. 1, 2004 (now U.S. Pat. No. 7,158,061), which claims the benefit of U.S. Provisional Application No. 60/592,291, filed Jul. 28, 2004, each of which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to analog-to-digital (“A/D”) signal conversion in wideband digital communication. Specifically, the invention relates to improving the dynamic range of A/D converters (“ADCs”). 
         [0003]    Currently, in many areas of high-speed digital communication, analog signals are processed and converted into digital data by being downconverted, filtered, and amplified to a particular analog baseband frequency prior to A/D conversion. As a result, such signals are confined to certain frequency boundaries and therefore make ADCs with uniform quantization (i.e., equal granularity or step sizes between increments) an acceptable solution for such applications. 
         [0004]    However, the recent advent of digital cable television and cellular phone technologies such as CDMA (Carrier Division Multiple Access) has brought about an increase in the use of wideband digital signals. These signals, which may typically be approximated by a Gaussian probability distribution function, are generally small in amplitude with extremely large amplitude spikes. As a result, the dynamic range of the previous uniform ADCs are no longer adequate, since they would result in either the sacrificing of small-scale resolution or else would clip the large-amplitude signals due to their limited range. 
         [0005]    It would therefore be desirable to design an ADC with non-uniform granularity for wideband signals that require a large dynamic range. It would further be desirable to design an ADC with non-uniform granularity, without sacrificing small-scale resolution or clipping the large amplitude signal components. Finally, it would be desirable to design an ADC with non-uniform granularity and without a costly increase in the amount of hardware required to implement the ADC. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention, circuitry for performing non-uniform analog-to-digital signal conversion for accommodating wideband signals such as those previously described is provided. The circuitry may be implemented using a linear ADC. The ADC is preceded by a nonlinear amplifier that may demonstrate, for example, a relatively smaller gain for input signals with comparatively small amplitudes and a relatively larger gain for input signals having comparatively larger amplitudes. The combined effect of the nonlinear amplifier and the ADC is to provide analog-to-digital conversion that exhibits small quantization noise for small-amplitude (i.e., high probability) signals and larger quantization noise for large-amplitude (i.e., low probability) signals, for instance. The resultant digital signals from the ADC may be subsequently processed by inverse transfer function circuitry in order to preserve a linear relationship between the input and output signals. 
         [0007]    The circuitry of the present invention demonstrates increased small-scale resolution without clipping the large-scale signal components or requiring substantial additional circuitry. The improvement in the signal-to-noise (“SNR”) ratio of the non-uniform ADC compared to a uniform ADC for these wideband signal inputs is substantial. For example, for an input consisting of 10 QAM (Quadrature Amplitude Modulation) channels in which one channel has 10 dB less power than the other channels (typical of cable television and wireless communication), the SNR of a 7-bit ADC that is preceded by a nonlinear amplifier implementing a square-root function may be 10 dB better than that of a normal uniform 7-bit ADC. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a simplified block diagram of an illustrative embodiment of A/D conversion circuitry in accordance with the present invention; and 
           [0009]      FIG. 2  is an illustrative circuit diagram of a nonlinear amplifier portion of the A/D conversion circuitry in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]      FIG. 1  shows an illustrative block diagram of the circuitry of the present invention. The circuitry in  FIG. 1  includes a first nonlinear amplifier  10 , ADC  12 , and inverse transfer function circuitry  14 . Analog input signals are first transmitted to nonlinear amplifier  10 . Nonlinear amplifier  10  amplifies the input signals according to a nonlinear output function f(x). The amplified signals are then passed on to ADC  12 . ADC  12  digitizes the amplified incoming analog signals, and subsequently outputs the converted digital signals to inverse transfer function circuitry  14 . Inverse transfer function circuitry  14  has an output function that is the inverse (i.e., f −1 (x)) of that of nonlinear amplifier  10 . Circuitry  14  performs a nonlinear mapping to the received digital signals that results in the desired linear relationship between the analog input and the final digital output signals. 
         [0011]      FIG. 2  is an illustrative circuit diagram of nonlinear amplifier  10  in accordance with the present invention. In particular, nonlinear amplifier  20  in FIG.  2  includes amplifier  22 , resistors  24  and  26 , and matched diodes  28  and  30 . Amplifier  22  may be for instance an operational amplifier (“op-amp”). Diodes  28  and  30  are connected in parallel facing opposite directions and are matched to each other so as to cause nonlinear amplifier  20  to produce an output that has zero-input symmetry. In other words, the output of nonlinear amplifier  20  is dependent only on the magnitude of the input. 
         [0012]    The gain of nonlinear amplifier  20  is determined by the electrical characteristics of resistors  24  and  26  and of diodes  28  and  30 . Specifically, the amount of current I across diodes  28  and  30  for a given voltage drop V across the diodes is (the same equation applies to both diodes): 
         [0000]        I=I   1 ×((exp( V/V   0 )−1) 
         [0000]    where I 1  and V 0  are constant values that depend on the characteristic of each corresponding diode. Plugging this expression into the equation for the impedance Z d  for each diode yields: 
         [0000]        Z   d   =V/I=V/ ( I   1 ×((exp( V/V   0 )−1) 
         [0013]    The diode impedance Z d,28  for diode  28 —the diode with the positive conductivity in the circuit arrangement shown in FIG.  2 —may subsequently be used in determining the output function of nonlinear amplifier  20  (the contribution of diode  30  to the output function is negligible compared to that of diode  28  and therefore may be ignored). Specifically, the output function of nonlinear amplifier  20  expressed in terms of the R 1  (the resistance of resistor  24 ), R F  (the resistance of feedback resistor  26 ) and Z d,28  is given by the equation: 
         [0000]        f ( x )= x ( t )×( R   f   ×Z   d,28 /( R   f   +Z   d,28 ))/ R   1    
         [0014]    It is thus seen from the above expressions that when the input voltage signal is small (i.e., for small amplitude signals), the impedance of diode  28  is much larger than Rf, and as a result the gain of nonlinear amplifier  20  is approximately Rf/R1. On the other hand, when the input signal is large, the impedance of diode  28  is much smaller than Rf, and as a result the gain of nonlinear amplifier  20  is logarithmically related to the input signal. Therefore, the overall effect of nonlinear amplifier  20  is to limit the amplifier gain for small-amplitude input signals and to compress the amplifier gain for large-amplitude input signals. As a result, when the amplified signals are transmitted to the ADC and quantized, the quantization step that is applied to the small-amplitude input signals is effectively smaller than the quantization step applied to the large-amplitude input signals, thereby allowing a uniform ADC to achieve non-uniform granularity at little additional cost in hardware. The non-uniform granularity created by the nonlinear amplifier helps to reduce the quantization noise created by a fixed-resolution ADC and substantially improves the SNR of the ADC. 
         [0015]    After the analog input signals have been processed by the nonlinear amplifier  10 , the nonlinear data is digitized by ADC  12  and subsequently sent to inverse transfer function circuitry  14 . The purpose of inverse transfer function circuitry  14  is to remap the nonlinear digital data so that the final digital output of the ADC circuitry is a linear function of the analog input. Therefore, if the nonlinear amplifier  14  implemented the output function shown above with respect to  FIG. 2 , inverse transfer function circuitry  14  would implement an output function given by the following inverse expression: 
         [0000]        f   −1 ( x )= x ( t )×(( R   f   +Z   d )/ R   f   ×Z   d ) R   1    
         [0000]    Although an exemplary circuit implementation exhibiting this particular output function is not shown, different circuit implementations for such a function will be readily apparent to one of ordinary skill in the art. For example, similar to nonlinear amplifier  10 , inverse transfer function circuitry  14  may also utilize an op-amp along with matching diodes and linear resistors. 
         [0016]    It will be understood from the foregoing that although the present invention has been described herein with respect to a particular type of nonlinear amplifier and ADC, other variations of nonlinear amplifiers and ADCs may be used without departing from the scope of the invention. For example, alternative nonlinear amplifiers that implement different output functions or use different circuit components may be employed. The specific output functions of the nonlinear amplifier—which thereby affects the granularity of the ADC—and of the corresponding inverse transfer function circuitry may be altered according to the probability density functions of the particular input signals of a given application. Furthermore, other types of ADCs besides uniform, linear ADCs may be used. For example, different types of nonlinear ADCs (e.g., ADCs with a stepwise output function) may be used in accordance with the present invention in order to further improve the dynamic range of the circuitry. Alternatively, a nonlinear ADC may be realized by not using inverse transfer function circuitry following a linear ADC. Depending on the particular application, the ADC may be implemented using a flash ADC, a sigma-delta ADC, or any other type of ADC that is known to one of ordinary skill in the art. 
         [0017]    It will be understood, therefore, that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention, and that the present invention is limited only by the claims that follow.