寫論文的時候由於需要畫出系統的零極點圖。但是之前不知道怎么用matlab畫，今天研究了一下，拿出來和大家共享。所用到的matlab函數為zplane，matlab給出的解釋如下：
ZPLANE Z-plane zero-pole plot.
ZPLANE(Z,P) plots the zeros Z and poles P (in column vectors) with the
unit circle for reference. Each zero is represented with a 'o' and
each pole with a 'x' on the plot. Multiple zeros and poles are
indicated by the multiplicity number shown to the upper right of the
zero or pole. ZPLANE(Z,P) where Z and/or P is a matrix, plots the zeros
or poles in different columns using the colors specified by the axes
ColorOrder property.
ZPLANE(B,A) where B and A are row vectors containing transfer function
polynomial coefficients plots the poles and zeros of B(z)/A(z). Note
that if B and A are both scalars they will be interpreted as Z and P.
[HZ,HP,Hl] = ZPLANE(Z,P) returns vectors of handles to the lines and
text objects generated. HZ is a vector of handles to the zeros lines,
HP is a vector of handles to the poles lines, and Hl is a vector of
handles to the axes / unit circle line and to text objects which are
present when there are multiple zeros or poles. In case there are no
zeros or no poles, HZ or HP is set to the empty matrix [].
ZPLANE(Z,P,AX) puts the plot into the axes specified by the handle AX.
假設為一個零點，多個極點系統且b=[0.2 0.8 0.64],a=1,用zplane（b,a）就可以畫出如下的系統零極點圖。
從圖中可以看出。該系統有一個極點，兩個零點，且都在單位圓以外，其中一個在單位圓附近，另一個距離單位圓較遠。由於在單位圓外附近有零點存在，故該系統在某個頻帶有比較大的幅度衰落，相位特性也會成非線性。
使用freqz函數可以畫出系統的幅度特性和相位特性如下圖所示
可見在數字高頻出，系統的幅度確實呈現大的衰減，相位變化也已經是非線性的了。
轉：http://www.52rd.com/Blog/Detail_RD.Blog_Olive_2282.html?WebShieldDRSessionVerify=1TH4xqXg8etlQJb3r3Rw