Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Patent
1997-01-14
1998-09-08
Gaffin, Jeffrey A.
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
341118, H03M 300
Patent
active
058050930
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an oversampled high-order modulator, especially a sigma-delta modulator.
BACKGROUND OF THE INVENTION
In analog-to-digital converters (A/D) and digital-to-analog converters (D/A) utilizing noise shaping technique based on oversampling and sigma-delta modulation a signal is quantized into one or several bits at an oversampling frequency in a sigma-delta modulator. The resulting quantization noise is shaped in such a way that the proportion of the quantization noise remaining within the frequency band of the actual signal band is as small as possible, and the noise is transferred outside the signal band. The performance of the noise shaping is mainly dependent on the order and oversampling ratio of the noise shaper, i.e. the sigma-delta modulator. A high-order modulator attenuates the quantization noise on the signal band more than a low-order modulator. Thus, a high-order modulator provides a better signal-to-noise ratio on the signal band, but as a result there is accordingly more quantization noise energy outside the signal band. This quantization noise outside the band is easy to remove by filtering, however.
Using high-order modulators would thus be considerably advantageous, since they enhance the signal-to-noise ratio. However, their use is limited by problems in stability.
As is well known, a low-order, i.e. a first-order modulator, is stable irrespective of the level of the input signal, and so is a second-order sigma-delta modulator. There are, however, severe stability problems associated with high-order sigma-delta modulators having an order of three or higher. The stability of the high-order modulators is dependent on the amplitude of an incoming signal. When the modulator assumes an unstable mode of operation, the output voltages/output values of its integrators increase abruptly, and the bit stream from the output of the modulator begins to oscillate independently of the input signal. The high-order modulator does not necessarily return to the linear operation range even though the input signal would return to the stable operation range.
A known arrangement aiming at maintaining the stability of the modulator carefully limits the voltage range of the integrators within the stable operation range. The problem with limiting the voltage range of the integrators is the practical implementation, since additional circuits causing interference should be avoided in the first stage of the modulator, in which the sensitivity of the modulator is at its highest.
U.S. Pat. No. 5,012,244 discloses a stabilization circuit of a high-order sigma-delta modulator, comprising a comparator which resets at least one integrator in the modulator upon detecting that the modulator operates at or near an unstable condition. Finnish Patent 88,765 discloses a similar stabilization circuit, where only the last integrators of the modulator are reset, however, i.e. the order of the modulator is lowered. Such resetting of the integrators is an effective way of restoring the modulator from the unstable mode, but there is also significant interference in the operation of the modulator.
DISCLOSURE OF THE INVENTION
An object of the invention is such a stabilization of the modulator that requires no resetting of the integrators.
The present invention relates to an oversampled high-order modulator, especially a sigma-delta modulator, comprising cascaded integrators in a number corresponding to the order of the modulator, a quantizer, and a negative feedback from the quantized output of the modulator to the input of at least one integrator, a means for detecting the unstable mode of the modulator, and means for restoring the modulator to stable operation, characterized in that the means for restoring the modulator to stable operation comprise means for temporarily changing the value of said feedback to a direction which will restore the stable operation.
The present invention is based on the discovery that a high-order sigma-delta modulator can also be stabilized in such a way th
REFERENCES:
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patent: 5311181 (1994-05-01), Ferguson, Jr. et al.
patent: 5311183 (1994-05-01), Ferguson, Jr. et al.
patent: 5376892 (1994-12-01), Gata
patent: 5623263 (1997-04-01), Kuo et al.
Heikkila Juha
Lipasti Lauri
Atmel Corporation
Gaffin Jeffrey A.
Kost Jason L. W.
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