A continuous-time sigma-delta modulator includes a VCO-based quantizer, a rotator, a truncation circuit and a digital-to-analog converter (DAC). The VCO-based quantizer is arranged to generate a thermometer code based on an input signal and a feedback signal. The rotator is coupled to the VCO-based quantizer, and is arranged to generate a phase-shifted thermometer code based on the thermometer code and a phase shift, and generate a rearranged thermometer code based on the phase-shifted thermometer code to comply with a specific pattern. The truncation circuit is coupled to the rotator, and is arranged to extract a most significant bit (MSB) part from the rearranged thermometer code. The DAC is coupled to the truncation circuit, and is arranged to generate the feedback signal according to at least the MSB part. Two alternative continuous-time sigma-delta modulators are also disclosed.