Noise leakage suppression in VCO-based ∑Δ-modulators excited by modulated signals

Oversampling data converters, such as the ΣΔ-modulator, are widely used for analog-to-digital conversion. Proper evaluation of the in-band noise densities and the corresponding effective number of bits requires a post-processing step to adequately suppress the noise leakage. This suppression is traditionally achieved by windowing the digital output signal. Although windowing is effective for sinusoidal excitations, it is less optimal for the analysis of modulated signals. We propose a technique that not only accurately derives the in-band noise densities for these modulated signals, but also provides a measure for the in-band nonlinear distortions generated by the modulator. The performance of the technique is demonstrated on a ΣΔ- modulator employing a voltage controlled oscillator for the internal quantiser.