A Sigma-Delta (ΣΔ) modulator for converting an analog input signal having a frequency bandwidth around a variable center frequency f0 to a digital output signal at a sampling frequency fs. The ΣΔ modulator comprises a quantizer (420) for generating the digital output signal and a loop filter for shaping the quantization noise. The loop filter comprises at least one subfilter (430, 410) centered around a frequency f0 and constant noise shaping coefficients (451, 452, 453). The ΣΔ modulator further comprises a tunable delay element (455), a frequency adjuster (480) for adjusting the sampling frequency fs such that the normalized center frequency f0/fs is constant, and a delay adjuster (490) for adjusting the loop delay td implemented by the quantizer and the tunable delay element (455), such that the normalized loop delay td/Ts falls in a predetermined range [tmin, tmax], where Ts=1/fs.