The signal generator is introduced and is used to produce a square wave, which is similar as ladder and could be set beforehand. The signal's scope and width could be easily adjusted through the keyboard in real-time. The system utilizes small keyboard microprocessor AT89C51 and complex logic device (CPLD).The software program is written by C language and VHDL language,respectively. The system construction is compact and the volume is sma1l, and the keyboard operation is flexible and convenient.