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DJV3-Newcomer
3-Newcomer
November 11, 2023
Question

Need help with FFT function

  • November 11, 2023
  • 2 replies
  • 1862 views

I am using MathCAD for the first time today, and have a problem that requires your assistance.  I have to create a waveshape, and I do so, but the points are separated by 1 unit, not the 1 microsecond units that  are the reality.  For this reason, I have no idea what the output of the FFT X axis is.  I've given it no frequency reference, so is it using 1 Hz as the basis for the difference between each point I've entered (not the truth).  I'd like to know how to get this software to accept 1 microsecond as the difference between each point that I enter.  I also need to know how to interpret the output of the FFT. 

 

The spreadsheet is here:

 

Re: How do I change the graph in the attached file... - PTC Community

 

There are various example mathCAD sheets attached.  I've tried various things, but can't get them to work.

 

Thanks.

 

2 replies

ttokoro
21-Topaz I
ttokoro21-Topaz I
21-Topaz I
November 12, 2023

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All show same dft results. Their fundamental spectrum is same. But the frequency is f.

t.t.
F
Fred_Kohlhepp23-Emerald I
23-Emerald I
November 12, 2023

An FFT of a vector of observations has no frequency information; it only uses the number of values in the vector.  To assign a frequency you must define the "sampling frequency", or the time interval between the values in the vector..

 

Attached is a sample sheet done as I transitioned from real Mathcad to Prime Express.  It explains the concept of sampling frequency, then moves on to a whole lot of things you may not care about.

 

Don't forget--the FFT can only tell the truth at frequencies below half of the sampling frequency (Nyquist frequency), so you need to be sampling pretty fast!  

File is Prime 4.0 Express.

D
DJV3-NewcomerAuthor
3-Newcomer
November 12, 2023

Okay, I knew the Nyquist frequency existed..  So, if I sample at a 1 microsecond interval, my sampling frequency is 1 megaHertz, and the x axis of the MathCAD plot needs to be read in terms of megaHertz.  In other words, the 1 on the output plot is 1 Megahertz?  It plots up to 500 Mhz due to Nyquist limit?

 

Thank you, sir.  

 

D
DJV3-NewcomerAuthor
3-Newcomer
November 12, 2023

Sorry that should be up to half a Megahertz as the max frequency.  500 kHz.

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