本文引用自乱七八糟 《MATLAB中freqz函数的用法》  

 

FREQZ 是计算数字滤波器的频率响应的函数

   [H,W] = FREQZ(B,A,N)returns the N-point complex frequency response

    vector H and the N-point frequency vector W in radians/sample of

    the filter:

 函数的输出：a.滤波器的频率响应H（N点）  b.频率向量W(N点，且单位为弧度)

其中，滤波器形式如下：

 

 

given numerator and denominator coefficients in vectors B and A. The

    frequency response is evaluated at N points equally spaced around the

    upper half of the unit circle. If N isn't specified, it defaults to    512.

 滤波器的系数：

分子为B，分母为A  

  频率向量W，是均匀分布在滤波器的上半区，即：0：pi，这些点上的频率响应都将通过此函数计算出来。

举例:

假设滤波器的系数A=1，B=【1 1/2 1/3 1/4】;通过FREQZ函数便可以得到此滤波器的频率特性

其中，需要考察的是H和W的长度，看是否符合我们设置的N。

 

     [H,W] = FREQZ(B,A,N,'whole') uses N points around the whole unit circle.

    函数给出了滤波器的双边频率特性，是N点的

   H = FREQZ(B,A,W)returns the frequency response at frequencies

    designated in vector W, in radians/sample (normally between 0 and pi).

    函数的输出依然是滤波器的频率响应H，只不过这个H是限定了范围的，不再是全频率（0：pi）上的了，这个范围由W来指定，单位是弧度。

举例：W=【0.5pi----pi】,下图中，红线部分便是此函数的输出，可见，只描述了【0.5pi----pi】之间的频率响应。

   [H,F] = FREQZ(B,A,N,Fs) and [H,F] = FREQZ(B,A,N,'whole',Fs)return

    frequency vector F (in Hz), where Fs is the sampling frequency (in Hz).

注意下图的横坐标与程序中fs的关系

   H = FREQZ(B,A,F,Fs)returns the complex frequency response at the

    frequencies designated in vector F (in Hz), where Fs is the sampling

    frequency (in Hz).

   FREQZ(B,A,...)with no output arguments plots the magnitude and

    unwrapped phase of the filter in the current figure window.