Analogue and digital convertors using pulse edge modulators with

Details Analogue and digital convertors using pulse edge modulators with Analogue and digital convertors using pulse edge modulators with

1993-08-25

1996-08-20

Mis, David

Coded data generation or conversion

Analog to or from digital conversion

341144, 341155, 332107, 332109, H03M 106, H03M 112, H03M 166, H03K 708

Patent

active

055482864

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to signal converters, i.e., digital to analogue convertors, and to analogue to digital convertors.
2. Discussion of Prior Art
In one type of digital to analogue convertor, a multi-bit digital signal is employed to control the width of pulses output by a pulse width modulator. The pulses are then integrated or otherwise low pass filtered over a period of time to provide analogue output levels. The pulse rate of the pulse width modulator, and hence the rate at which digital samples are supplied thereto, clearly needs to be well above that of the pass bang of the low pass filter for this to be successful, however; since the pass band at the low pass filter is half the sampling frequency for the signal to be converted, the rate at which digital samples are supplied is therefore generally well in excess of the sampling frequency for the signal to be converted.
Another type of digital to analogue convertor, described for example in GB2183115, employs oversampling--that is, receives digital data samples at a significant multiple of the Nyquist frequency for the signal represented. The accuracy of the signal is then reduced by a quantizer (or more accurately, a re-quantizer) having a small number of levels. The output of the quantizer is fed back and subtracted from the input to the quantizer, and the result (which represents the error due to the quantizer, hereinafter referred to as the quantizer noise) is subtracted from the next sample input to the quantizer.
If the input signal level is constant (that is, the input signal contains no frequencies above zero hertz or DC) the quantizer error for one sample is thus taken account of to some extent in the following sample and if the quantizer output is averaged over a sufficiently large number of samples, the quantizer noise is eliminated. Reducing the number of quantization levels increases the magnitude of the quantization noise and consequently increases the number of samples over which the quantizer output must be averaged; and hence the sampling rate of the convertor (and its oversampling ratio).
However, the quantizer noise increases as the frequency of the input signal rises, leading to errors at nigher frequencies in the output of the convertor. To reduce this effect, a filter may be placed in the path so as to filter the quantizer noise before it is subtracted from the next sample. The filter has a prediction characteristic so that when the path is added the spectral distribution of the quantizer noise is "shaped", to reduce the noise level at frequencies below the Nyquist frequency of the signal to be converted, and consequently increase the noise at higher frequencies outside the signal band.
Alternatively, a similar effect may be achieved by placing a different low pass filter in the signal path at a point following that at which the quantizer noise has been introduced by an overall feedback path from the quantizer output.
The quantizer may, as described in GB2183115, have only two levels (in other words, it may be a one bit quantizer). In this case the output of the quantizer may (after suitable buffering or amplification) be integrated or otherwise low pass filtered using an analogue filter to provide an analogue signal corresponding to the digital input signal. Alternative types of convertor employ a quantizer having a greater, but still small, number of levels and consequently producing output signals comprising a small number of bits (for example, 3) . However, since the output of such quantizers remains a multi-bit digital signal some form of conversion to an analogue signal is skill required. It has therefore been proposed to employ, following the quantizer, a pulse width modulator controlled by the digital output of the quantizer to produce a pulse of constant height but of length determined by the quantizer output. This is advantageous because, whilst control of the precise height of a pulse requires high precision analogue circuits, control of the pulse lengths (with

REFERENCES:
patent: 4947171 (1990-08-01), Pfeifer et al.
patent: 5339054 (1994-08-01), Taguchi
Cataltepe et al.; IEEE ISCAS 89; May-1989; "Digitally Corrected Multi-Bit . . ." pp. 647-650.

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