Fft Definition and 181 Threads

I've been trying to code an algorithm to compute the convolution of two probability distributions. using the FFT. This relies on the "convolution theorem": (p*q)[z] = FFT^{-1}(FFT(p) \cdot FFT(q)) However, when I test it using the distributions p={0.1, 0.2, 0.3, 0.4} q={0.4, 0.3, 0.2, 0.1}...

Hi all. I have been recording the thermal noise produced by a number of resistors. I have created a excel sheet that takes the readings from the 8 1/2 DVM and performs a FFT on them, in order to give me the frequency domain plot. However I am not 100% certain of what I am seeing because of my...

Hi, I was told that in order to analyze cycles, wave patterns etc in empirical data, the time frequency analysis using the discrete Fourier transform (or the fast Fourier transform) are most appropriate (instead of say the autocorrelation spectrum). Using the Python scipy.fftpack as...

Is it possible to calculate the phase and signal amplitude from data gained from FFT? For instance, if I have a samples from a signal B+A*cos(ψ), is it possible to obtain A and ψ? Extra challenge: is it possible to do so without division? (I am looking to put this on a DSP and division is...

Hello all, I have a vector, let's say A, which has measured values of the Earth's magnetic field (and if it is relevant, two vectors X and Y with the coordinates of the point each value was measured). I need to calculate the amplitude, phase and energy spectrum using FFT in MATLAB X...

Hey all! I have to ask for help. I’m in too deep and can’t find my way out. Does anyone know where to find a simple code for missing samples recovery? All I can find are math equations that I can’t interpret. I’m not into signal processing here, I’m using FFT for numeric data only, so I’m...

Hi all! First off: I don't know which forum to post this. I'm no grad student or PHD so academic forum seemed wrong, and it's not really just math, since it was the result of a measurment and the signals can come from environment, etc. Ok. I measured the magnetic field using a sensor...

How do you accurately identify the fundamental harmonic if you get a peak of similar amplitudes at frequencies that are really close to each other. And furthermore, once you identify the fundamental harmonic, are the second, third fourth harmonics just multiples of the fundamental frequency...

Hey all, I'm new here. Currently I'm struggling with the phase of FFT. I read that the phase of FFT is relative to the start of the time domian signal. In my measurement I recorded a signal which is a sinus sweeping from 100hz to 3000hz. after FFT i obtained a phase spectrum within the...

Hello, I have a simple cosine f = 50Hz. When I generate Matlab code to produce 1/2 second of this signal and take the FFT, the response correctly shows a spike at 50 Hz. However, when I bracket the signal with 1/2 seconds of zeros on either side, the frequency response is showing a spike at...

Hello! My partners and I are building an NSOM microscope and it is mostly finished. Now we need to find the Point Spread Function. My questions is how do I find the PSF in labview and how do I use it to deblur in labview? What follows is some background on what I have attempted so far...

I am new to Fortran. Is there an ultimate best library for FFT for Fortran (95)? I found this one FFTPACK. I am not sure if this is the best one. Is there anywhere a simple example how to use it? I have huge time files (260 MB each) that I like to read in Fortran and perform FFT and...

Hi, I have a complex time signal x(t)=X1*exp(i*∅)+X2*exp(i*2*∅). On converting to frequency domain, i expect frequency components at ω and twice of ω (when ∅=ω*t). FFT gives the desired frequencies, but the amplitudes don't correspond to X1 and X2. I understand that this is due to the phase...

I'm trying to find out how to "properly" fourier-transform a velocity-auto-correlation function (VAC), for calculation of phonon density of states (DOS), from molecular dynamics simulation. The problem I'm running into is that my calculated DOS: a) doesn't fall of to zero as the frequency...

I am trying to compute the Fourier transform of a square pulse with MATLAB's FFT. Code: Fs=1000; %Sampling rate (Hz) T=1/Fs; %Sampling time interval P=10; %Period of pulse t=0:1/Fs:P/2; %Time axis N=length(t); x=rectpuls(t,P); %Pulse amplitude n=pow2(nextpow2(N)); %Number of frequency...

Spatial Frequency Transform by doing FFT twice ?? This is for my own knowledge in relations to acoustics. I am trying to determine the location of sound using an array of sensors. Very similar to what they are doing in the webpage given. Im looking at the material at...

Homework Statement Hello people For a few days I'm having trouble with periodic boundary 1D poisson solver code on matlab. I'm trying to figure out discritized Fourier transform (fft), and ifft commands. To make things simpler I want to type a simple code. Aim on the code is integrating a...

Hi, Let's say I have an input signal f(t,0)= \sum_n A_n cos(w_nt) And we know that f(t,x)= \sum_n A_n cos(k_nx-w_nt) and we also have the relationship between k and w. We can find the coefficients A_n by taking a Fourier Transform of the relationship for f(0,t). Here's my...

Hi, I have the following problem that I would like to solve by using the 'fft' function in matlab. Some background on the problem: The evolution of sea surface height is given, to first order, by \eta_o(x,t)=A\cos\left(kx-\omega t\right) From this, we see that there are three parameters...

I'm attempting to use Matlab fft functionality to reconstruct Fourier transform tables in my textbook (brigham), but to little success. Here is code to take the Fourier transform of cos(2*\pi*x*f_0), which should be \frac{\delta (f + f_0) + \delta (f - f_0)}{2} I can *almost* get it, but...

it seems like a pretty commonly used computational/mathematical method in analyzing experimental data, such as voltage signals

Matching orientations of 2-d arrays of values -- using fft? I was discussing the following problem (a subproblem of a personal project I'm working on) with a professor: we're given two 2-d arrays of values. We know them to be identical, but they might not be oriented corrected -- i.e., 0 0...

I am using an FFT package (GFT) to analyze a signal from a pressure sensor. I am unsure of the results I am getting, so I decided to try to verify the FFT package using a known input. I am using a signal generator to produce a sine wave of +/- 5V @ 5 and 100 Hz. When I run the FFT on the...

Hello. I've made a lab experiment of interference and with a CCD camera photographed fringe pattern which one can understand as a periodic signal. My question is: When I apply an FFT to the image is there any limit to the amplitude information I may get from lower frequencies? I read in the...

I was browsing another topic, and I suddenly remembered that I've never quite figured out what units matlab's fft returns. I've used the fft before and found where I had signal noise by taking the power density of the Fourier coefficients, which is all I really wanted to use the fft for in the...

Suppose you have the FFT of a sinusoidal signal sampled at 100 kHz and it has energy peaks at k = 100 and k = 412. How many points is the FFT? What is the frequency of the signal?

Fourier Transform -- Completely Flustered About Recursive FFT Hi all. I have been banging my head about this problem for the last week and a half-- Fourier Transform. Some background about me: I am a rising Junior at an accredited university majoring in Computer Engineering & Computer...

Confusion with 2D DFT Hello everyone, I'm trying to figure out why the third transform in the picture I attach here is different to the other ones, why it's 'distorted'. I know that it's related with the fact that if you copy the image in a grid the resulting image has discontinuous lines...

Does a PNA show FFT If I'm looking at gain across a band on a PNA could one say that I'm looking at an FFT or is that complete nonsense?

Homework Statement The following is the output of a 16 point computation of the discrete Fourier Transform for data taken at 10 kHz. Complete the table below by determining the frequencies associated with each of the FFT values. I attached an Excel workbook with the table. Homework...

Homework Statement Find the Fourier Transform of a unity amplitude rectangular pulse of width 2T, where T=[ims]. Use MATLAB to plot the magnitude response of this signal's FT. Homework Equations The Attempt at a Solution I got something that LOOKS right, but the first...

I understand that for the FFT the resolution or bin size is a function of the number of samples in your signal and that while padding the signal with zeros will make the graph look more precise it will not enable you to resolve between two frequencies if they are contained in the same bin. What...

Homework Statement I want to solve FFT and IFFT in the same problem. My function is Exp[2*(1 + I*c)*T^2]. For FFT, I used the following code: ClearAll["Global'*"] c = 0; f[t_] = Exp[-(1 + I*c)*2*t^2]; sampleTime = .01; data1 = Table[f[t], {t, -5, 5, sampleTime}]; freq1 =...

I am wondering what the minimum sampling duration must be in order to accurately resolve a 60Hz wave. I know the FFT works with periodic waves about the number N but I am not sure how this relates to say a sampling duration that is 1 half the sampling period of a 60 Hz wave. I'm worried that the...

I took the FFT of a signal that was taken at non constant time period (T), but at a sampling frequency(Fs) of 6.5 hz. Can I take the FFT of a signal with non constant T, and does it mean anything? Please see attachment. Ps. I used Matlabs, FFT command, which i think uses the Cooley–Tukey FFT...

I have about 40 tabs open on this right now and something important is slipping my grasp. I know this has been covered a million and a half times, but for some reason I cannot seem to find a straight answer (or more probably realize and understand it when I see it). When I take the Continuous...

Hi, everyone: As I am sure will be clear from my post, I am not--nor have I ever been-- an EE :). (For the sake of giving some context to help taylor the answer, I am a mathematician-in-training. I only know the most basic ideas of signal-processing but I do know...

I have data in excel file and want to convert that to frequency domain. I wrote the following code but its giving me the wrong results. If my data is 2,2,2,5,5,6 Shouldn't Amplitude =2 and frequency =3 Amplitude =5 and frequency =2 Amplitude =6 and frequency =1 %Read in the...

Does anyone know a good free library to do Fourier Transforms (FFT or DFT). I know FFTW but I'm having some problems with it. I want an alternative that do FFT in two dimensions with complex numbers. The libraries I have found doesn't fulfill this requirements. Thank you

Hello, I have a very basic FFT question. Correct me if I am wrong, but the FFT is a fast implementation of the DFT (which is essentially the same as Discrete Time Fourier series). The DFT is periodic in time and frequency. Thus if you input a time domain made of 4 times samples for example to...

Im writing a program in python to simulate the propagation of a gaussian beam through a thick lens and to the focussing point using Fourier optics. Due to the strength of the focussing I need a lot of data points so that I have a decent resolution at the focus. To speed things up and to...

Hi, I have to solve a FFT problem by mathematica.I have attached my problem(exercise 2) here. I do not understand fft algorithm well.I also try to solve it by mathematica but mathematica can not solve this by forier transform directly.It will be great if anyone help me to solve this problem.

I have collected time-domain acceleration signals from the test rig through an accelerometer. Voltage signals are scaled to acceleration signals based on voltage sensitivity of accelerometer. Please refer the link given at the last to get the link for PDF attachment to see the time domain...

Hello, Please help me with the following problem. Given a measured signal which has to be smoothed. A lowpass filter has to be applied. The signal is available as measured data (a data file on the hard disk, numerical, not real-time) ( it has to be applied in C or C++). The filter...

Hi There, Im having a little trouble with scalings required for a fft performed in Matlab. So here is the story: I construct a position space gaussian, plot this guy out to 6 stdDev. My frontfactor is such that my probability density is normalized. I know what the actual Fourier transform...

Hi All, Its my first post here. I want to know how the digital output which comes out of an ADC can be used to calculate FFT. Because FFT algorithms usually needs real and imaginary parts of a sampled signal? Thanks.

Hi, I'm analyzing a laser diffraction experiment. Does Matlab has a reverse FFT function? Thank you Naftali

I'm trying to do FFT convolution via the FFTW3 library for image processing purposes. Basically I have a kernel and I am convolving it with an image. A problem I encountered has to do with the dimensions of the image. When the width is not equal to the height, the pixel values I get, seem to...

Hi, I am a bit confused regarding using FFT to calculate the 1D single slit diffraction pattern.. Let's take the simple setup shown here: http://electron9.phys.utk.edu/optics421/modules/m6/images/sslit.gif" If I wanted to do a quick calculation using Matlab, I would create a 1xN matrix...

Homework Statement hello! i'm given a signal h(t) = v(t)*g(t) where g(t) is a distortion/noise that got added and has a very low frequency compared to v(t) i need to devise a method to clean up g(t) The Attempt at a Solution i'm thinking of to do the fft on the signal h(t)...