# Linked Questions

18 questions linked to/from Why is the FFT "mirrored"?
10 votes
1 answer
14k views

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4 votes
1 answer
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### Why can the FFT always be mirrored in the middle of the x axis? [duplicate]

Plotting the FFT of a signal with Matlab e.g. by the code: y_fft = abs(fft(y)); f = fs*(0:length(y)-1)/length(y); plot(f,y_fft); where ...
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0 votes
0 answers
34 views

### Why are these excess frequency components in my FFT spectrum? [duplicate]

I am experimenting with digital signal processing and I took the FFT of a sinusoid in Matlab. I got the frequency components that I expected to be present but there are two excess spikes that mirror ...
118 votes
12 answers
102k views

### What is the physical significance of negative frequencies?

This has been one of the holes in my cheddar cheese block of understanding DSP, so what is the physical interpretation of having a negative frequency? If you have a physical tone at some frequency ...
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17 votes
5 answers
14k views

### FFT of sine wave not coming as expected i.e single point

The cyan plot is a spectrum of 50 Hz, and the magenta one is a 50.1 Hz sine wave (having amplitude 0.7). Both are sampled at 1024 samples/s. I performed a 1024 point FFT to get this spectrum. Why is ...
• 1,370
8 votes
2 answers
32k views

### Amplitude of the Signal in Frequency Domain different from Time Domain

How does the Amplitude of the Signal is changed when taking the FFT of a Signal, Have a look, The amplitude had changed from 2 to 30, and Here is my code for generating the above output, ...
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3 votes
1 answer
3k views

### 2D Fourier transform of Sobel kernel

Can someone explain me the highlighted text parts regarding this image ? Here is a pseudo-code of how it was created: ...
• 113
0 votes
1 answer
2k views

### Conjugate symmetry of the DFT of real-valued sequences

I have read about Fourier transformation that real signals are "mirrored" in the real and negative halves of the Fourier transform because of the nature of the Fourier transform. For ...
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2 votes
1 answer
3k views

### FFT of a pure sine wave is not a single peak

For fun, I computed the FFT of a pure sine wave. I chose chose the sample length to be an even multiple of the signal period, so that I don't see windowing-effects. Here is the Matlab code that I used:...
• 149
1 vote
2 answers
4k views

### FFT of Square Wave and Sin Wave

When we have FFT of Square wave of a pure real signal with high frequency(close to f(max), less than half of sampling frequency) we see only 2 symmetric high peaks but when we decrease the frequency ...
• 137
0 votes
2 answers
2k views

### Discrete Fourier transform of even function

I have a cosine function - Hence it is even. Considering only the real parts of the DFT, on performing DFT, I am getting something like this. Could anybody tell me where I am going wrong: DFT: ...
0 votes
1 answer
992 views

### Complex signal low pass filter

I'm trying to implement a low pass filter in python for a complex signal but the output doesn't look right. I've created a simple example below where I've mixed a 15Hz complex sine wave and a 30Hz ...
1 vote
1 answer
685 views

### Why does the frequency window affect the inverse fourier transform oscillation frequency?

I have an oscillating pulse in the frequency domain that I would like to inverse Fourier transform. My signal looks like: Which was coded in MATLAB using the following code: ...
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0 votes
2 answers
286 views

### How Discrete sequence could have more than one Frequency

I'm trying to understand Fast Fourier Transform in Detail to find out what exactly the frequencies when dealing with discrete sequence, Here is what I've done, First I've taken a Sinusoidal wave ...
• 2,175
2 votes
1 answer
427 views

### General question about DFT

I am learning Fourier analysis and without any teacher, just trying to read books on my own. I think I have made some decent progress but they are a couple of points which are still very unclear to me ...
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1 vote
1 answer
64 views

### How to get some law parameters from its FFT figure?

Given an hidden law $$I(t)=\frac{1}{2}[A+B.cos(2\pi\nu_0t+\phi_0)]$$ I observe its signal with 256 samples and compute its FFT over this interval, and represent the FFT result from 0 to 1 (normalised ...
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0 votes
1 answer
63 views

### Sampling of sine wave :which one to choose? start with zeroth sample or 1st sample

sampling rate=200KHz signal freq =1KHz When I sample 1KHz signal I'll get 200 samples. Should I consider 0 to 199 samples? Or 1 to 200? I want only 1 cycle.
1 vote
0 answers
68 views

### Asymmetry In The Mirroring Of An FFT Of A Sinusoidal Signal

I am trying to implement a Cooley-Tukey FFT algorithm in its iterative version. Specifically, I am implementing this Wikipedia pseudocode (please look at the section Data reordering, bit reversal, and ...
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