N=15, 。用Hanning窗设计一线性相位带通滤波器，观察它的实际3dB和20dB带宽。N=45，重复这一设计，观察幅频和相位特性的变化，注意长度N变化的影响。-N = 15,. Hanning window design with a linear-phase band-pass filter, observe its actual 3dB and 20dB bandwidth. N = 45, repeat the design, observe the amplitude-freque