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M
Maxplus121-Visitor
1-Visitor
February 20, 2018
Question

How to take FFT of I and Q Data

  • February 20, 2018
  • 7 replies
  • 30109 views

Hello,

I have captured I-phase and Q-phase (quadrature) data of a radio signal with QPSK modulation.  The two data are in two separate text files with an array of data in a list.

How do I do FFT using Mathcad please?

Thank you

7 replies

W
Werner_E25-Diamond I
25-Diamond I
February 20, 2018

Use the cfft or the CFFT (note the capitals) command (depends on your needs and your data)

Guess that no more could be said without more information from your side.

M
Maxplus121-VisitorAuthor
1-Visitor
February 20, 2018
Hi
Thank you.

I have seen that post and used a waveform (I believe it was a cosine) ad an example.

But I have I an Q data in text files containing 2000 data pair values to represent the complex signal. So effectively I and Q are complex.
How do I do FFT?
T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
February 21, 2018

Hi,

 

further information makes this simple.  

https://dsp.stackexchange.com/questions/19615/converting-raw-i-q-to-db

Converting raw I/Q to dB

The amplitude of an IQ signal is just the vector magnitude, sqrt(I^2+Q^2)

Then do the fft on this amplitude. The complex numbers will turn to real in the squaring of the values.

 

Cheers

 

T
21-Topaz II
terryhendicott21-Topaz II
21-Topaz II
February 21, 2018

Hi,

 

http://iitg.vlab.co.in/?sub=59&brch=163&sim=1065&cnt=1

You must demodulate the I and Q data back to the original signal data before frequency examination.

 

Cheers

-
-MFra-21-Topaz II
21-Topaz II
February 22, 2018

Hi!

Am I allowed a digression on the subject? Do you like this video?

(view in My Videos)

F
Fred_Kohlhepp23-Emerald I
23-Emerald I
February 22, 2018

See if this example can help

-
-MFra-21-Topaz II
21-Topaz II
February 22, 2018

Hi,

Regardless of the signal, in general to calculate its spectrum you should follow the procedure of the example here below, do you agree?

IQ.jpgIQ1.jpgIQ2.jpgIQ3.jpg

Greetings

FM

    F
    Fred_Kohlhepp23-Emerald I
    23-Emerald I
    February 22, 2018

    This is a very challenging analysis to follow.  I think it's good!  But I don't have either the time or inclination to verify it completely. 

     

    This looks like a set of "tools"  that has been developed, used, and trusted for a considerable time.

    -
    -MFra-21-Topaz II
    21-Topaz II
    February 22, 2018

    Yes, of course, it is one of my work done some years ago, the set up required a lot of time. For the case in particular, the time was irrelevant, I just had to put together the various topics.

    -
    -MFra-21-Topaz II
    21-Topaz II
    February 23, 2018

    IQ8.jpgIQ9.jpgIQ10.jpgIQ11.jpg

    M
    Maxplus121-VisitorAuthor
    1-Visitor
    February 24, 2018

    Hello,

    I have downloaded the IQ spectra.zip file.

    I am struggling with copying the 45001 data points into Mathcad.  I am using copy and paste command and Mathcad crashes.

    Initially, I stated IQ:= and then try to insert a table (Insert>Data Control>Table).  But the table option was greyed out

    How did you imported the data into Mathcad please?  

    M
    Maxplus121-VisitorAuthor
    1-Visitor
    February 25, 2018
    It's work in win 7 pc 32bitsbut not in win 10 64 bits.
    Anyway back to the IQ. I got my sampling time wrong - it should be 2.222E-08.

    The IQ.data was captured for 1ms with 45001 points. Therefore sampling freq is 45MHz.
    Signal frequency - it's IQ data measured from the analyser in 25MHz BW.

    1. I am unsure what FFT bin frequency have been used in your file.

    2. Here is the plot using Matlab based upon the IQ data used for mathcad and sample time as before. The plot doesn't resemble the mathcad results. What's going on?

    3. What.command was used for FFT shift?
    -
    -MFra-21-Topaz II
    21-Topaz II
    February 27, 2018

    IQ16.jpgIQ17.jpg

    M
    Maxplus121-VisitorAuthor
    1-Visitor
    February 27, 2018

    Hi MF,

    Thank you for the revised version.

    I seem to be struggling with the cursors.  They are not active for some reason even by sliding the rule.

    The Cu parameter is undefined. 

     

    Also, the import Excel is not working anymore by following your guide lines. The data doesn't get imported at all. I did the following: 1) create table, 2) Right click in table, import Data wizard and selected the file.  It says the file could not be imported for reading. 

    The way to overcome is copy and paste from my old Mathcad file.

    -
    -MFra-21-Topaz II
    21-Topaz II
    February 28, 2018

    IQ18.jpgSpectrum Analyzer.jpg

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