Matlab FFT 使用方法

clc;clear;close all;
Fs = 1000;            % Sampling frequency                    
T = 1/Fs;             % Sampling period       
L = 1500;             % Length of signal
t = (0:L-1)*T;        % Time vector
 
S = 0.7*sin(2*pi*50*t) + sin(2*pi*90*t);
 
f = Fs*(0:(L/2))/L;
 
Y = fft(S);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
  
figure,plot(f,P1) 

创建一个20Hz和50Hz的叠加波，使用并使用高通滤波器进行滤波后，在使用fft查看频谱，查看低频波是否被正常过滤，代码如下

clc;clear;close all;
Fs=1000;
T=1/Fs;
L=1500;
t=(0:L-1)*T;
 
X= sin(2*pi*t*20)+sin(2*pi*t*50);
 
subplot(4,1,1);
plot(t,X);
 
f=(0:L/2)*Fs/L;
 
Y=fft(X);
 
P1=abs(Y/L);
P2=P1(1:L/2+1);
subplot(4,1,2);
plot(f,P2);
title('傅里叶结果');
xlabel('频谱 Hz');
ylabel('振幅');
 
cutoff_freq=30;
order=9;
[b,a]=butter(order,cutoff_freq/(Fs/2),'high');
filtered_signal=filter(b,a,X);
subplot(4,1,3);
 
 
plot(t,filtered_signal);
 
% 计算滤波后的信号的傅里叶变换
Y_filtered = fft(filtered_signal);
 
% 计算振幅谱
P1_filtered = abs(Y_filtered/L);
P2_filtered = P1_filtered(1:L/2+1);
 
% 计算频率向量
f_filtered = (0:L/2)*Fs/L;
 
% 绘制滤波后信号的频谱图
 subplot(4,1,4);
 
plot(f_filtered, P2_filtered);

效果如图所示：