Hi all I have a question on FFT scaling difference between periodic and a periodic signals. if I have a periodic signal x1 (sinewave), the FFT of this signal is normalized by the number of points N to get the correct amplitude spectrum scale < abs(fft(x1))/N >. However if there is aperiodic signal e.g. x2 (square pulse), the normalization by the number of points representing pulse amplitude (k) gives amplitude spectrum with a maximum of 1. what I know is that the spectrum of a square pulse is a sinc waveform of amplitude (A*w). where A is the pulse amplitude and w is the pulse width. does this mean the normalization is different for periodic and aperiodic signals? and if so, how does spectrum analyzers know how to normalize different signals? Any hints? many thanks N = 4096; % number of FFT points ts = 1e-3; % Sampling time t = [0:N-1]*ts; % Time Vector x1 = sin(2*pi*1*t); % Sinewave X1 = abs(fft(x1))/N; % Amplitude spectrum of the sinewave k = 2000; % number of points representing pulse width x2 = [zeros(1,1000) ones(1,k) zeros(1,1096)]; % square pulse of width 2 seconds X2 = abs(fft(x2))/k; % Amplitude Spectrum of the square pulse subplot(2,1,1),plot(X1) subplot(2,1,2),plot(X2)

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10/30/2010 6:14:04 PM

On Oct 30, 11:14=A0am, "abees Tero" <abee...@hotmail.com> wrote: > Hi all >... > > does this mean the normalization is different for periodic and aperiodic = signals? Yes > and if so, how does spectrum analyzers know how to normalize different si= gnals? Spectrum analyzers don't know. In fact, the signal being analyzed may have components that should be scaled in different ways. > > Any hints? > many thanks > ... The three cases are signals that have a power spectrum (tone like), signals that have a power spectral density (PSD) (noise like) and signals that have an energy spectral density (ESD)(transients). As I have posted here before: -begin quote- The PSD, ESD and power spectrum can be calculated via fft based methods in Matlab. Manufacturers of dynamic signal analyzers have provided these functions, properly scaled, for years. Some have been nice enough to accurately document their functions and make and keep the documentation available. Take a look at "Choose your Units!" from B&K: http://www.bksv.com/doc/bo0438.pdf and for more detail, "Signals and Units" on page 29 of: http://www.bksv.com/doc/bv0031.pdf For an discussion of the signal processing, consider pages 5-21 of: http://www.rssd.esa.int/SP/LISAPATHFINDER/docs/Data_Analysis/GH_FFT.pdf Particularly "3 Introduction" on page 5 and "9 Scaling the results" on page 15. I consider "13 Testing the Algorithm" on page 21 and on as a more practically oriented discussion of pwelch than the Matlab docs. -end quote- Dale B. Dalrymple

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10/30/2010 6:37:25 PM

dbd <dbd@ieee.org> wrote in message <1bded1a9-6241-4f2c-85be-d9020e5a2b78@t13g2000yqm.googlegroups.com>... > On Oct 30, 11:14 am, "abees Tero" <abee...@hotmail.com> wrote: > > Hi all > >... > > > > does this mean the normalization is different for periodic and aperiodic signals? > Yes > > and if so, how does spectrum analyzers know how to normalize different signals? > Spectrum analyzers don't know. In fact, the signal being analyzed may > have components that should be scaled in different ways. > > > > Any hints? > > many thanks > > ... > > The three cases are signals that have a power spectrum (tone like), > signals that have a power spectral density (PSD) (noise like) and > signals that have an energy spectral density (ESD)(transients). > > As I have posted here before: > -begin quote- > The PSD, ESD and power spectrum can be calculated via fft based > methods in Matlab. > Manufacturers of dynamic signal analyzers have provided these > functions, properly scaled, for years. Some have been nice enough to > accurately document their functions and make and keep the > documentation available. > > Take a look at "Choose your Units!" from B&K: > > http://www.bksv.com/doc/bo0438.pdf > > and for more detail, "Signals and Units" on page 29 of: > > http://www.bksv.com/doc/bv0031.pdf > > For an discussion of the signal processing, > consider pages 5-21 of: > > http://www.rssd.esa.int/SP/LISAPATHFINDER/docs/Data_Analysis/GH_FFT.pdf > > Particularly "3 Introduction" on page 5 and "9 Scaling the results" on > page 15. > I consider "13 Testing the Algorithm" on page 21 and on as a more > practically oriented discussion of pwelch than the Matlab docs. > -end quote- > > Dale B. Dalrymple Dear Dale,,, I would like to thank you for your kind, fast, really useful response. you cleared all my doubts. thanks again. regards,,,,

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10/30/2010 7:25:05 PM

I am trying to calculate the FFT of some sampled data, and have had some trouble determining the correct way to scale the resulting FFT data. Technical note 1703 states to properly scale the FFT (Mx) of a signal (x), the FFT should be divided by N, where N=length(x) However, I have determined that this only applies for the case where the signal x is <periodic>. What I want to know is how to scale the FFT is the signal is NOT periodic. Is there a way to accomplish this? I need to determine the proper magnitude of the FFT for data resulting from an input sine-sweep, or from a series of...

I need help in matlab code for applying fft and perform fft waveform of a given signal....who can help me?? Chai wrote: > > > I need help in matlab code for applying fft and perform fft > waveform > of a given signal....who can help me?? perhaps My_Spectrum = fft(My_Signal); My_Spectrum_Conventional = fftshift(My_Spectrum); plot(real(My_Spectrum_Conventional),'r');hold on; plot(imag(My_Spectrum_Conventional),'r');hold off; maybe Dave Robinson Thanks for your kindness. I still working on my project.more question will be coming if you don mind helping me. s...

Hi, I want to generate periodic signals of ones and zeros by matlab. It's a periodic signal with active and passive periods, the active period has a value of one and the passive period has zero value. It consists of 7 passive and 6 active periods, starting with passive. Each period is 42 seconds. Could you guide me through this, thanks. On Apr 2, 10:43=A0am, "BrnoM " <kentman...@seznam.cz> wrote: > Hi, > > I want to generate periodic signals of ones and zeros by matlab. It's a p= eriodic signal with active and passive periods, the active period has a v...

Hellow! anyone can tell me how to scale the FFT? suppposed i have a 10 seconds data x(t),after FFT(x(t)),i will plot Freq-Amplitude,then how to scale the frequency? thanks! dt = 1/Fs t = dt*(0:N-1) T = N*dt df = 1/T f = df*(0:N-1) Note that max(t) = T - dt max (f) = Fs - df Hope this helps. Greg ...

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Fs=1024;% sampling frequency Ts=1/Fs;%sampling interval N=256;% number of samples t=(0:Ts:(N-1)*Ts); x = load('-ascii', 'signal.txt'); %FFT koko taajuusalueelta f=(0:Fs/N:(N-1)*Fs/N); Xm=(1/N)*abs(fft(x)); figure, plot(f, Xm);AXIS([0 Fs/2 0 1]) Hello Is there anything wrong with the fft of the signal, the plot looks more like a time domain signal, whereas i was expecting at least some Uspikes representing the signal frequencies! Any one who knows about this? Thanks ! Umutesi Faith skrev: > Fs=1024;% sampling frequency > Ts=1/Fs;%sampling interval > N=256;% number o...

i'm doing my project in matlab for ecg noise cancellation i would like to know how to find fft in matlab for ecg signal On 2/2/2011 1:47 AM, sugasini vaithiyanathan wrote: > i'm doing my project in matlab for ecg noise cancellation i would like to know how to find fft > in matlab for ecg signal How about using fft? --Nasser ...

hi, This is a student's problem. ;) I fight with prediction in Matlab with NNT ToolBox. There is some example about how to use enclosed function but....i spent more than 16 hours on it and I'm not sure if I'm right becouse it isn't work correct! I have 1000 samples of signal "W" - this is not period. I want to learn my net and use it to predict next 20 samples from: 1001 to 1020. I tryed : net = newlin([min max],1,0,0.1) % this create a new linear layer with min/max value, 1 output, 0 delay,0.1 learning rate %!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!...

I have read 'a guide to the FFT' in the 'File exchange' , but confusing about the FFT The author recommend the following commands when we'd like to see the spectrum in the frequency domain. 1. fftshift(fft(fftshift(�)))/Fs for (non-periodic signal) Here, Fs: sampling frequency 2. fftshift(fft(fftshift(�)))/Np for (periodic signal) Here, Np:number of samples But what's the difference? and Should i have to divide it by Np for periodic signal? beom lee wrote: > > > I have read 'a guide to the FFT' in the 'File exchange' > > , but co...

hi iam doing my assainment in biomedical engg.im unable to write a program to find period of a signal using matlab. could any body help me. "rami reddy" <reddy435@yahoo.co.in> wrote in message news:ef2fe59.-1@webx.raydaftYaTP... > hi > iam doing my assainment in biomedical engg.im unable to write a > program to find period of a signal using matlab. > could any body help me. help pwelch may get you started. BTW, the wide keys at the left and right ends of the second row of your keyboard will allow you to use upper case letters. ...

DSP Experts, I am analyzing a physiologic signal that has a varying fundamental frequency. To create a more accurate reconstruction from the FFT, I wish to include the bins immediately and symmetrically on either side of the fundamental bins. I refer to this as the "distribution method." I am confident I have been able to do a reconstruction using the fundamental bins (a standard reconstruction). I also performed a reconstruction for the distribution method. The mean squared error (comparing both reconstructions to data in the time domain) for the distribution method is ...

I've browsed a bit, so hopefully I've got all my information in order. I haven't been able to find an answer for this specific problem. It's for image processing. I'm simplifying 2D regions by applying low-pass filters to their contours. Specifically, I take an 8-connected clockwise contour of (arbitrary) n points, and make two new functions, x(i) and y(i), out of the points' coordinates. Then I do a DFT on each, apply a Gaussian or low-pass cutoff filter, inverse DFT, and replot. It works like a charm if I use a naive O(n^2) DFT implementation - but of course, it'...

Hi all, I am trying to convolve a sine wave with a random signal. Instead of using conv function, I am multiplying them in the frequency domain and then taking the ifft. I have taken care of the zero padding issue in the time domain. The problem is instead of getting a dual-dirac histogram, I always end up with the histogram of random signal. Can anyone suggest a proper convolution code between a sine wave (or any periodic waveform) and random signal? Thanking everyone in advance..... Nitin Check the fft of the sine wave alone - do you get two peaks and zeros everywhere else? (To get this...

I have been collecting emg signal and would like to look at the frequency of the signal. In Matlab I know there is an FFT function which I have been using, but ran into a problem when using it. The example given in the help says it is using a 512-point FFT. I tried using a 512-point FFT and a 2056-point FFT and noticed that they look very different. Is the only difference that the 512 gives less points to plot than the 2056? I noticed when I plotted signal of a specific length (1:1000) then plotted more of the same signal (1:2000), the frequency spectrum overlapped completely if I used th...

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