This paper discusses an important aspect of the grid-connected power converters control for electric trains. An analysis and synthesis of the second order Synchronous Reference Frame Phase Locked Loop (SRF-PLL) is considered. The problems of synchronization, stability, and noise immunity are studied using the discrete-time theory. This approach allows one to directly study the system implemented in software, skipping the usual application of the continuous-time model using the bilinear transform and Euler's approximation. The discrete-time SRF-PLL model with the first-order loop filter is considered. This article is of interest to engineers of converter control, which should be synchronized to the grid.