Adaptive sigma-delta data converter for mobile terminals

Details Adaptive sigma-delta data converter for mobile terminals Adaptive sigma-delta data converter for mobile terminals

2001-10-01

2003-06-10

Jeanglaude, Jean Bruner (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Differential encoder and/or decoder

C341S155000

Reexamination Certificate

active

06577258

ABSTRACT:

TECHNICAL FIELD
These teachings relate generally to sigma-delta (SD) modulators of a type used in data converters and, more specifically, relate to the use of SD-based converters in equipment that places different demands on the performance and operation of the SD converter. Even more specifically, these teachings are directed to the data conversion of audio and RF signals at a digital baseband interface for enabling further processing either for RF transmission or for digital signal processing.
BACKGROUND
SD modulators used in analog-to-digital converters (ADCs) are well known in the art. Reference may be had, by example, to S. R. Norsworthy et al., “Delta-Sigma Data Converters”, IEEE Press, NY, 1997, and to J. G. Proakis et al., Digital Signal Processing” Third Edition, Prentice-Hall, 1996.
Currently several air interface standards for mobile communications devices, such as cellular telephones, are in wide use, and the signal bandwidth and dynamic range are different in each standard. In one possible data conversion solution each of the RF-baseband (analog-to-digital (AD) and digital-to-analog (DA)) interfaces uses dedicated RF AD/DA converters with certain optimum numbers of bits and sampling rates for each targeted system and radio architecture. Furthermore, when one mobile communication device is required to support more than one standard, then more than one RF DA/AD converter will typically be required, e.g., one for each supported standard. This, however, results in an increased hardware cost and overhead.
Additionally, the conversion performance is typically designed according to predetermined worst-case signal conditions. This results in an overhead in performance under most usage conditions, which in turn can result in increased power consumption. For battery powered devices, such as typical cellular telephones and personal communicators, any increase in power consumption is detrimental to the goal of extending the talk and standby times.
A single-bit second-order sigma-delta modulator with a digital decimation filter is widely used for audio and RF converters. The virtues of the single-bit sigma-delta technique include a high achievable linearity and a wide dynamic range. However, these benefits come at the cost of a required high over-sampling ratio (OSR) and, therefore, increased current consumption. Due to stability reasons the highest practical order of the sigma-delta modulator is limited to two, and any possibilities for providing configurability by changing the order of the sigma-delta modulator are limited. Thus, the only practical parameter for configuring or adapting the sigma-delta modulator is the over-sampling ratio. However, in order to obtain a wide dynamic range a high over-sampling ratio is required.
As is known, oversampling analog-to-digital converters two parts: an analog modulator and a digital filter. The analog modulator receives an analog signal and produces a serial data stream having a bit rate which is much greater than the Nyquist sampling frequency. The quantization noise of the analog modulator is shaped to minimize the noise in the passband of interest, at the expense of higher noise outside of this passband. This is as opposed to distributing the noise evenly between DC and the modulator sampling frequency. The digital filter portion of the ADC is operable to filter and decimate the modulator representation of the analog input. Since the modulator quantization noise is shaped, the digital filter must filter this out-of-band quantization noise and reduce the output word frequency to the desired final sample frequency. Decimation is a well-known technique that is utilized in most oversampling ADCs.
Typically, oversampling ADCs utilize a fixed decimation filter architecture (usually SINC filters in combination with FIR filters) to realize a desired filter transfer function, and the decimation filter is arranged to reduce the sampling frequency of a digital information signal step-by-step such that no aliasing occurs. Various structures and embodiments of decimation filters are known. The common practice is to use one or several SINC filters in the first stage(s) due to their simplicity and efficiency, and to use FIR filters in the final stages. The order of the SINC filter is typically one order higher than the sigma-delta modulator in order to filter out the spectrally shaped quantization noise. The decimation ratio of the SINC filter is usually chosen to be one-fourth of the total decimation ratio (or oversampling ratio OSR) due to the consideration of the attenuation at the upper end of the passband. The FIR filter performs the final decimation to the desired sample frequency, and defines and equalizes for the final desired frequency response.
FIG. 1
shows a typical sigma-delta ADC topology, where the output of the analog sigma-delta modulator
10
is connected to an input of a decimation filter
12
. The modulator
10
typically employs a loop filter (
10
A) that feeds a quantizer
10
B, and an analog feedback path that includes a DAC
10
C. The quantizer
10
B typically is a 1-bit quantizer for linearity purposes and is of second order for stability reasons. The modulator
10
over-samples at a sampling rate of Fs, and the resulting data stream is gradually stage-by-stage decimated by the over-sampling ratio (OSR) to a lower rate (Fs/OSR), but higher resolution, signal (N bits) in the decimation filter
12
. In a typical case the decimation filter
12
includes a data register
12
A, a multiply and accumulate (MAC) unit
12
B and a coefficients register
12
C.
Instead of 1-bit quantization, a 2-bit or higher resolution quantization is beneficial for at least two reasons: the dynamic range is increased by at least 6 dB /per additional bit, and higher order modulators can be used without incurring stability problems.
Multi-bit sigma-delta techniques thus provide an additional degree of freedom for the configuration and adaptation of a sigma-delta data converter. Configuration and adaptation by changing the number of bits is an effective means for setting the converter performance at a level required in certain conditions. This is also an effective technique for use with low over-sampling ratios, when changing the order of the sigma-delta modulator is not appropriate. The use of cascaded sigma-delta modulators offers a possibility for increasing the order of the sigma-delta modulator and for configuring the modulator to simultaneously meet the requirements of different standards, such as the second generation (2G) digital cellular standard and the third generation (3G) digital cellular standard. Reference with regard to cascaded modulators and to changing the modulator order can be had to U.S. Pat. No. 6,087,969 “Sigma-delta Modulator and Method for Digitizing a Signal”, by Stockstad et al. However, when the over-sampling ratio is low the benefits of modifying the order of the sigma-delta modulator are only marginally effective in the most critical cases.
As was noted, the dynamic range is increased 6dB for each additional bit, and can be increased even further if the modulator coefficients are configured simultaneously. Therefore, in a performance-wise optimum configuration the modulator coefficients are changed each time the number of quantization levels (bits) is altered.
SUMMARY OF THE PREFERRED EMBODIMENTS
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
These teachings are directed to an adaptive sigma delta modulator that avoids the use of several RF (and audio) SD converters in a mobile communication device that is intended to operate in accordance with several air interface standards and under various signal conditions. The use of these teachings achieves this goal by providing a single sigma-delta modulator-based AD and or DA converter that is constructed so as to be adaptable to different system requirements by changing at least the number of bits used in the SD modulator. The loop filter parameters of the SD modulat

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