Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Patent
1994-08-11
1997-05-13
Williams, Howard L.
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
H03M 300
Patent
active
056297017
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a method for cascading at least two sigma-delta modulators, wherein an error of one modulator in the cascade is quantized by the next modulator in the cascade; the quantized error is differentiated; and the differentiated error is subtracted from the quantized output signal of said one modulator.
BACKGROUND OF THE INVENTION
In sigma-delta modulators (also known as delta-sigma modulators), the signal is quantized by using only a small number of quantization levels (2-256, which corresponds to an A/D converter with a resolution of 1 to 8 bits) at a high rate, usually 32-512 times the signal frequency. The ratio between the Nyquist sampling frequency (two times the useful signal band) and the used high sampling frequency is also called oversampling ratio (M). A quantizer is a combination of an A/D and a D/A converter, in which an analog signal is converted by the A/D converter to a discrete digital value which is then immediately converted back to an analog voltage (value) by the D/A converter. A quantizing error (e.sub.k) is the difference voltage (value) between the analog input voltage and the analog output voltage, and the quantizing noise is the spectrum (Q.sub.e) of the quantizing error; in the case of the sigma-delta modulator, the quantizing noise can be regarded as white noise. The effective value (E) of the white noise in a one-bit quantizer is (q/2).sup.2 with a one-bit, where q is the spacing between the quantization levels. A sigma-delta modulator configuration is such that the quantizer error transfer function (NTF) to the output of the modulator is different from the signal transfer function (STF) from the input to the output of the modulator. The object is to provide a quantizing error transfer function NTF with the highest possible attenuation within a desired passband while the signal transfer function STF is as uniform as possible over the whole passband. The STF and NTF are interdependent in a manner determined by the used modulator structure. The order of the modulator is the order of the NTF function, or the number of integrators in the modulator. By increasing the order of the modulator the amount of quantizing noise in the passband can be decreased. Another way of decreasing the amount of quantizing noise in the passband is to increase the oversampling ratio however, an increase in the oversampling ratio increases the sampling frequency which in turn is limited by the components used in the implementation. Therefore the only way to improve the ratio (S/N.sub.q) between the signal (S) and the quantizing noise (N.sub.q) in the passband is to increase the order of the modulator or to improve the NTF so that the attenuation in the passband is increased while the order of the modulator and the oversampling ratio remain unchanged.
A conventional sigma-delta modulator with directly series-connected integrators is, however, difficult to implement due to the oscillation caused by the feedback loop. Therefore higher-order sigma-delta modulators have been formed by cascading two or more stable lower-order sigma-delta modulators. The quantizing error of the first modulator in the cascade connection (the difference between the input and output signals of the quantizer) is applied to the second modulator in the cascade, and the amount of quantizing noise over the signal band can be decreased by suitably interconnecting the outputs of the blocks. A 16-bit Oversampling A-to-D Conversion Using Triple-Integration Noise Shaping, IEEE Journal of Solid State Circuits, Vol. SC-22, No. 6, December 1987, p. 921 to 929, describes the cascading of first-order sigma-delta modulators by the so-called MASH technique. FI Patent 80548 describes the cascade connections of second-order multiple-feedback modulators.
SUMMARY OF THE INVENTION
The object of the present invention is to cascade two sigma-delta modulator blocks to achieve a better SNRQ than what was possible in the prior art modulator system of the same order and with the same oversampling ratio.
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"A 16-bit Oversampling A-to-D Conversion Technology Using Triple-Integration Noise Shaping", Matsuya et al, pp. 921-929, IEEE Journal of Solid State Circuits, vol. SC.22, Dec. 1987.
Ritoniemi et al., A Fifth order sigma-delta modulator for audio A/D converter, IEE International Conference on Analogue to Digital and Digital to Analogue Conversion, pp. 153-158, Swansea, UK Sep. 1991.
Karema Teppo
Ritoniemi Tapani
Tenhunen Hannu
Ritoniemi Tapani
Williams Howard L.
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