The aim of this study was to determine students’ understanding of fundamental AC signal concepts, such as frequency, amplitude, and phase difference. The participants in the study were 179 students (44 female and 135 male) attending Izmir Vocational School who were enrolled in the Analog Electronics Course. The data of the study were collected by means of three open-ended questions and semi-structured interviews. In the questions, students were given a graph of an AC signal and asked to draw a signal with equal frequency but higher amplitude, a signal with equal amplitude but higher frequency, and a signal with a certain phase difference. The main aim of the interviews was to provide a deeper insight into the difficulties detected in the analysis of the questions. The data analysis showed that the students confused the concepts of frequency and amplitude and had difficulties in interpreting the signals that exhibited a phase difference.