FFT加window？

Because FFT can only deal with limited database,so actually the default window is square window which has very low roll-off speed -20dB/dec,resulting great windowing smears and spectrum leakage.
In order to get a relatively accurate spectrum analysis,different windows need to be used to get the accurate SNR or SNDR.
Take ADC for example,since we usually use a sine wave for simulation which is just one bin of the FFT result,so a high roll-off needs to be chosen,such as Han.

thanx,ilikethisplace！