Abstract: Today, most spectrum analyzers(SA) are designed with digital IF technology to increase frequency resolution and decrease the manufacture difficulty of analog IF and video circuits. SA converts analog signal to digital by ADC(analog to digital converter),and processes the digital signal by digital down converter, digital filter, digital detector and FFT. So that the SA can get the amplitude and frequency information of input radio signal, and finally get a clear spectrum trace by video filter. The methods introduced in this article have been proved in our products. Using this methods, the SA we developed has excellent, steady and reliable performance.