怎么用matlab画出信号频谱分析图

在命令窗口输入doc fft回车后，可看到例子。

%构造出信号（如已有信号，此步可省略）
Fs = 1000; % Sampling frequency
T = 1/Fs; % Sample time
L = 1000; % Length of signal
t = (0:L-1)*T; % Time vector
% Sum of a 50 Hz sinusoid and a 120 Hz sinusoid
x = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t);
y = x + 2*randn(size(t)); % Sinusoids plus noise
plot(Fs*t(1:50)，y(1:50))
title('Signal Corrupted with Zero-Mean Random Noise')
xlabel('time (milliseconds)')

NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y，NFFT)/L;
f = Fs/2*linspace(0，1，NFFT/2+1);

%FFT分析
% Plot single-sided amplitude spectrum.
plot(f，2*abs(Y(1:NFFT/2+1)))
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')

较完整的频谱图：
设抽样频率为Fs（Hz），信号点数为N，信号序列为x。
f = fftshift(fft(x));
w = linspace(-Fs/2， Fs/2， N);%频率坐标，单位Hz
plot(w，abs(f));
title('信号的频谱');
xlabel('频率(Hz)');