Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2001-04-30
2002-12-03
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S116000, C341S118000, C341S120000, C341S143000
Reexamination Certificate
active
06489908
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention pertains to a wireless local loop terminal and corresponding system that incorporate a digital-to-analog (D/A) converter for performing high speed and high-resolution digital-to-analog conversion using an oversampling principle.
BACKGROUND OF THE INVENTION
Digital-to-analog conversion refers to the process of converting discrete digital signals into a continuous-time range of analog signals. The conversion of analog signals to digital signals and vice versa is often used in order to interface real world systems, many of which monitor continuously varying analog signals, with digital systems that read, store, interpret, manipulate and otherwise process the discrete values of sampled analog signals. Real world applications which use digital-to-analog converters (DACs) include, for example, digital audio systems such as compact disc players, digital video players, and various other high performance audio applications, which include conversion of digital signals to analog waveforms at a high resolution.
Sigma-delta modulation (sometimes called “delta-sigma modulation”) provides a high-resolution digital-to-analog conversion solution. Sigma-delta DACs have come into widespread use with the development of signal processing and digital audio technologies and their applications. Sigma-delta modulation incorporates a noise-shaping technique whereby the noise of a quantizer (often 1-bit) operating at a frequency much greater than the bandwidth is moved to high frequencies not of interest in the output signal. A filter after the quantizer removes the out-of-band noise. The resulting system synthesizes a high-resolution data converter, but is constructed from low-resolution building blocks. Since sigma-delta DACs provide for oversampling digital-to-analog conversion through the sampling of signals at very high frequencies (i.e., sampling at rates much greater than the Nyquist rate), high signal-to-noise ratios are achieved. Thus, the combination of oversampling and noise shaping technologies may be implemented using a sigma-delta DAC in order to achieve high resolution without external trimming. There, however, does not exist at present a digital-to-analog conversion solution that provides both high speed and high resolution. A good overview of the theory of sigma-delta modulation is given in “Oversampling Delta-Sigma Data Converters,” by Candy and Temes, IEEE Press, 1992. Examples of D/A converters utilizing delta-sigma modulation are given in U.S. Pat. Nos. 4,901,077; 5,079,551; 5,185,102; 5,313,205; 5,701,106; 5,712,635; 5,786,779; 5,920,273; and 5,952,947. The disclosures of the foregoing references are incorporated herein by reference.
Specifically, sigma-delta DACs commonly include a front-end interpolator which receives digital input samples and increases the sampling rate (typically 64-256 times the input sample rate) of the digital input samples. The sigma-delta modulator receives the higher frequency input samples from the interpolator and converts the samples to a lower resolution (typical one-bit), high frequency bit stream. Rather than spreading quantization noise uniformly over the frequency range from 0 to the sampling Nyquist frequency, the sigma delta modulator shapes the noise so that the majority of the noise falls into the very high frequencies above the Nyquist frequency. Thus, it effectively removes the noise from the lower frequency range which is of interest for the particular applications cited above. Techniques for increasing the sample rate, generally called interpolation, are well understood by those skilled in the art. Most designs will utilize several stages of increase.
An oversampling DAC which utilizes a second order sigma-delta quantizer and an analog low pass filter to convert the data from the sigma-delta quantizer to analog signal is a very effective device for low speed audio applications; yet, inadequate for high speed applications. In addition, it has a relatively high output data transition rate, requiring higher power than is desirable. Moreover, considering oversampling interpolations on the order of n=256 for high sampling rates, such as the 400M samples/sec required for cellular base station applications, extreme clocking speeds (400 MHz×256) become a serious design obstacle.
Thus, there exists a need for a wireless local loop terminal and corresponding system having an improved DAC operable at higher speed than heretofore achievable which exploits the sigma-delta principle in a different way.
SUMMARY OF THE INVENTION
The invention comprises a wireless local loop communications apparatus and corresponding system having an improved DAC operable at higher speed than heretofore achievable which exploits the sigma-delta principle in a different way. More particularly, the invention comprises a wireless local loop terminal and corresponding system that implement a digital-to-analog conversion circuit having a memory for storing delta-sigma bit sequences corresponding to all possible values of a digital input coupled to a plurality of one-bit digital to analog converters. Each of the digital-to-analog converters being clocked by multi-phase clocks such that each phase applied to each one of the digital to analog converter is delayed with respect to a next one by the oversampling period, which is the Nyquist period divided by the number of predetermined interpolated samples. An analog summer is coupled to all the digital-to-analog converters for summing all the outputs from the plurality of digital to analog converters to generate an analog output. Hereby, the digital-to-analog conversion circuit embodied in the wireless communications apparatus and corresponding system emulates a delta-sigma digital-to-analog converter having both high speed and high resolution.
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Panasik Carl M.
Viswanathan Thayamkulangara Ramaswamy
Brady III Wade James
Neerings Ronald O.
Nguyen Linh Van
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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