在使用buttord()设计butterworth滤波器时，我得到了N的零值或负值

Question

这些是我的低通巴特沃斯滤波器所需的规格。

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delta_l = 0.2929;
delta_h = sqrt(0.1);

passband_min = 1-delta_l;
stopband_max = delta_h;

% the passband and stopband edge frequencies, normalized from 0 to 1 where
% 1 corresponds to pi rad/sample
Wp = 0.25; %*pi
Ws = 0.4; %*pi

%in the input to buttord(), Rp is the passband ripple
%"no more than Rp dB of passband ripple"
Rp = mag2db(delta_l)

%in the input to buttord(), Rs is the stopband attenuation
Rs = mag2db(1-delta_h)

[n, Wn] = buttord(Wp, Ws, Rp, Rs)

输出为

Rp =

  -10.6656


Rs =

   -3.3018


n =

    -1


Wn =

    0.3805

这显然是错误的。

使用数学方法，我得到n= 3的截止频率为0.7854。一旦我获得这些值，我计划将它们放入butter()函数中。

谢谢你的帮助。

Answer 1

我使用了错误的衰减公式：

衰减级别( dB )= 20 * log10(vout/vin)