From the expressions by Adrian and Earley (Proceeding of Minessota Symposium on Laser Anemometry, University of Minessota, Dept. of Conference, Minneapolis, 1976), we have calculated the signal-to-noise ratio of a Laser Doppler Anemometer system by means of the differencial Doppler Technique with backscattering configuration, for the detection and estimation of the velocity of a fluid, as a function of the size of the receiving aperture and that of the seeding particles. We present the distribution of the local reduced SNR in each point of the aperture and the total reduced SNR of the photodetector, for the case of using Polyamida-12 particles suspended in water and 2D-LDA system designed by the autors. All the results have been obtained from development of different programs implemented in C-language and Matlab.