Phase noise vs. jitter analysis in continuous-time LP and BP ΣΔ modulators with interferers

Abstract

Continuous-time (CT) ΣΔ modulators are well-known for their sensitivity to jitter. This paper intends to compare and clarify the effect of both colored phase noise and white jitter as measures for non-ideal clocks on the performance of CT ΣΔ modulators, which becomes of utmost importance under the influence of strong out-of-band (OOB) interferers. The jitter-induced noise is shown to be a convolution in the frequency domain between the clock phase noise and the CT derivative of the modulator output. Simulations on lowpass and bandpass modulators show that depending on the shape of the phase noise, the SNR of the modulator is limited by the input signal or the quantization noise. In the scenario where the SNR is limited by the input signal, techniques regarding feedback DAC waveforms cannot reduce the impact of jitter, and the SNR is further reduced in the presence of OOB interferers. Consequently, a realistic clock phase noise should be specified to allow frequency-domain analysis and thus an accurate estimation on the output spectrum and performance of CT ΣΔ modulators. Moreover, interferer attenuation techniques should be applied to reduce the clock jitter influence.