Signals and systems must be learned first. Signals and systems include analog and digital signals and system analysis methods. Digital signal processing is to study the analysis of digital domain signals and the design of digital filters.

Before studying these two courses, it is best to learn Fourier series, differential, integral and complex variable functions in advanced mathematics well. Circuit analysis is just ... In fact, it is an incomprehensible frequency domain transformation between the signal and the system. As long as we can understand Fourier series, transform time function into frequency function and master Paswa's law, it is the key to design filtering and understand bandwidth.