Abstract: The paper analyzed basic principle of measuring insulation parameters of cable by additional low-frequency signal source, and detailedly introduced design of optimal SPWM driving circuit, protection circuit of input and output and RS232 communication circuit of the signal source, in which the optimal SPWM driving circuit can output -8 V to +12 V voltage to ensure reliable opening and closing of IGBT. The experiment result showed that the signal source has stable output voltage and adjustable frequency.