我正在尝试一个相当简单的东西,即获取具有固定长度的信号频谱中的正确值。我不知道包含的频率,因此无法相应地选择fft节点的数量。这使我面临泄漏问题。我尝试通过使用窗口来淡去我的时间信号的开头和结尾来避免这种情况。但是,我在获取正确振幅方面遇到了问题。到目前为止,我所做的如下:
我建立了一个小例子,计算了三个频谱:
- 通过乘以一个因子来补偿窗口处理
- 将双边频谱转换为单边频谱 -> 将频谱幅度乘以2
- 通过将频率节点数`N`除以规范化频谱振幅
我建立了一个小例子,计算了三个频谱:
- 使用带窗函数的fft()
- 使用不带窗函数的fft()
- 使用spectrogram()
SampleTime=0.001;
Fs=1/SampleTime;
t=0:SampleTime:10;
y=2*sin(2*pi*10*t)+3*cos(2*pi*30*t); % Time Signal
fn = Fs/2; % Nyquistfrequency
N = 1001; % FFT-length (use N=2^x to use DFT-Algorithm)
df = Fs/N; % frequency resolution
%% Generating spectrum using fft()
win = hann(length(y))'; % generate Hanning window for Time Signal
ywin = y.*win*length(win)/sum(win); % amplitude correction to get correct values with window
Hwin = fft(ywin, N); % spectrum of windowed signal
H = fft(y,N); % complex spectrum of signal without window
amplHwin = abs(Hwin); % absolute values of spectrum of windowed signal
amplH = abs(H); % absolute values of spectrum without window
% Double sided spectrum -> single sided spectrum of windowed signal
amplitudengang = fftshift(amplH/N);
amplitudengang = amplitudengang(ceil(length(amplitudengang)/2+1):end);
amplitudengang(2:end) = amplitudengang(2:end)*2;
% Double sided spectrum -> single sided spectrum of unwindowed signal
amplitudengangwin = fftshift(amplHwin/N);
amplitudengangwin = amplitudengangwin(ceil(length(amplitudengangwin)/2+1):end);
amplitudengangwin(2:end) = amplitudengangwin(2:end)*2;
%% Generating spectrum using spectrogram()
noverlap = floor(0.5*N);
window = hann(N);
[B,F,T] = spectrogram(y,window,noverlap,N,Fs);
% Scaling
FFTScalingFactor = 2 * 1 / N; % 2 for Hanning window, 2^(-1/2) for RMS, 1 for peak
A = FFTScalingFactor * B;
B = FFTScalingFactor * abs(B);
B(2:end-1,:) = 2.*B(2:end-1,:); % single side spectrum with even block length
%% Plots
x_fn = 0 : df : fn-df; % Frequency vector
figure
subplot(2,2,1)
plot(x_fn,amplitudengangwin,'r')
hold on
plot(x_fn,amplitudengang)
legend('Hanning window','no window')
title('FFT()')
subplot(2,2,3)
plot(t,y)
title('Time signal')
subplot(2,2,2)
plot(F,B(:,1))
title('Spectrogram()')