利用matlab绘制傅里叶分解三维图_三维频谱图fft

目的：

1. 理解傅里叶变换分析及其他方法的分析。
2. 理解傅里叶变换频域与时域的关系。
3. 利用MATLAB绘制傅里叶变换图形三维图。

内容和原理：

本次主要使用的函数及其说明如下（此处只取采用用法的意义说明）：
1.abs：abs(X) is the absolute value of the elements of X. When
X is complex, abs(X) is the complex modulus (magnitude) of
the elements of X.
2.figure: figure, by itself, creates a new figure window, and returns
its handle.
3.square: square(T) generates a square wave with period 2*Pi for the
elements of time vector T. square(T) is like SIN(T), only
it creates a square wave with peaks of +1 to -1 instead of
a sine wave.
4. Fft: fft(X,N) is the N-point fft, padded with zeros if X has less
than N points and truncated if it has more.
5. plot3：plot3(x,y,z), where x, y and z are three vectors of the same length,
plots a line in 3-space through the points whose coordinates are t