说明：  信号发生器由波形选择开关控制波形的输出, 分别能输出正弦波、方波和三角波三种波形, 波形的周期为2秒
(Signal generator by the waveform selector switch to control the output waveform, respectively, to the output sine wave, square and triangular waveforms in three waves, waveform cycle for 2 seconds)