Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)A simple heterodyne scheme with digital data acquisition and offline signal processing is introduced to obtain phase noise spectra and linewidths of monomode lasers. The data are analyzed using time-domain memorized signals, enabling the evaluation of parameters such as coherence time, phase noise, laser linewidth as a function of bias current, and the identification of points where loss of coherence occurs. Results are presented for lasers with linewidths from 20 kHz up to 2 MHz.342149334940Brazilian agency FAPESP [2014/18791-7, 2007/56024-4, 2015/50063-4]Brazilian agency CNPq [150504/2015-2, 574017/2008-9, 301087/2013]Brazilian agency CNPq (INCT-Fotonicom)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)