The authors present an analysis of effects of plasma turbulence in the solar wind on Doppler radio measurements from interplanetary spacecraft. In particular, with respect to previous work, the authors introduce physically different components of the fluctuations, e.g. discontinuities and waves in the study of variations of radio fluctuations with elongation angle. They show, correspondingly, that there is a critical angle, near maximum elongation, where the phase fluctuations of the radio waves are dominated by waves and not discontinuities. The analysis leads also to a better determination of the minimum noise level coming from plasma fluctuations and is therefore important from the point of view of gravity wave detection using Doppler techniques.