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when i compute the fft of sine wave, the output doesnt appear to peak signal/ delta signal. in fact it appears as a cluster of random signal at specific frequency

09-14-2020 09:13 AM

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09-14-2020 12:24 PM

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First. Please use capitalization in you your sentences.

Second, you describe something here, but the words aren't adequate to understand what you are talking about. Why don't you attach some pictures. Perhaps even a VI with the results saved as default and any values in controls saved as default.

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** LabVIEW Champion. It all comes together in GCentral **

**What does "Engineering Redefined" mean??**

09-14-2020 12:25 PM - edited 09-14-2020 12:48 PM

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You need to show us your VI.

- What is the input (1D array, waveform, dynamic data)?
- There are many ways to do a FFT. Which function are you using?
- A FFT is complex. Do you look at the real and imaginary parts? (A pure sine wave has everything in the imaginary part!)
- What is the meaning of "Peak signal/delta signal". Very ambiguous!
- If the data does not have timing information, do you correctly transform t0,dt into f0,df?

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09-14-2020 03:11 PM

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Hi Amandeep,

Please post picture or upload VIs to enable community member to help and to get the solution at the earliest.

By the way, please find the attached VI of FFT of sine wave, might be helpful.

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09-15-2020 05:16 AM

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It looks exactly as I would expect. You have a peak at the frequency of sine wave, then low level noise; which is expected because the signal is time sliced an quantized.

"If you weren't supposed to push it, it wouldn't be a button."

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09-15-2020 08:43 AM

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If you want to measure properties of a pure sine signal (frequency, amplitude,phase), you should use sine approximation methods (LSME-FIT) or try the tone detection vi.

DFT (or FFT)* can *give correct results under very certain conditions... rarely met without good knowledge of digital signal processing. The tone detection vi provided use FFT but care for those conditions (or correct side effects)

Greetings from Germany

Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

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** LabVIEW Champion. It all comes together in GCentral **

**What does "Engineering Redefined" mean??**

09-15-2020 09:12 AM

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@amandeepkaur wrote:

A day later and you have not followed up on our questions. Did you solve your problem?

(Also, the term "cluster" has a specific meaning in LabVIEW, but you seem to use it to describe something else).

An FFT of a sine wave is a trivial problem for non-pathalogical data. You get distortions if your input consists of a non-integer number of periods. You might get alias frequencies if your sampling rate is insufficient.

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09-15-2020 12:52 PM

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How to calculate the fft of sine wave and plot its real and imaginary parts.

I tried using two methods. The screenshots of both VIs are attached below.

I am not able to obtain a peak value at specific frequency of sine wave.

Download All

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09-15-2020 12:55 PM

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I have posted the screenshot of VI in my current question in this discussion

forum

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09-15-2020 12:58 PM

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hey!

yes, i have solved my previous problem.

This platform has been of great help.

I think I was not very clear in describing my current problem. I have attached the screenshot of VI in my current ques. Kindly look into it