Analyzing the phase and amplitude noise of RZ, NRZ and RF DACs

Abstract

This article considers a multi-mode RF DACs phase and amplitude noise for non-return-to-zero (NRZ), return-to-zero (RZ), and mix/RF reconstruction waveforms under the assumption that two switch pairs (or two channels) in each current switch cells are alternately active in every clock cycle, Analysis shows that each channel of multimode DAC is completely characterized with RZ mode, and all other modes can be derived from this mode, in particular NRZ mode is not a complete analog of conventional DAC performance when reconstruction pulse duration and period are equal. When determining phase and amplitude noise of multi-mode RF DAC, one should keep in mind the phase shift between two switch pairs output pulses spectral components in NRZ and RF modes. This phase shift depends upon DAC mode (NRZ or RF) and output oscillation frequency. As consequence an analysis indicates that the 3 dB improvement of 1/F phase noise can be achieved in 1st Nyquist zone when using NRZ mode, the same in 2nd Nyquist zone with RF mode if amplitude noise of DAC switch pairs is negligible. The independence of 1/F noise from clock frequency has simple physical interpretation. As correlation period of 1/F noise is significantly greater than output oscillation period, the delay of this oscillation caused by phase fluctuations in DAC is the same for any mode, and analysis of noise spectral density for conventional DAC can be extended to multi-mode RF DACs.