How to take FFT of I and Q Data

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

Author

1-Visitor

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?

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terryhendicott

21-Topaz II

Hi,

further information makes this simple.

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

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terryhendicott

21-Topaz II

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

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21-Topaz II

Hi!

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

(view in My Videos)

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F

Fred_Kohlhepp

23-Emerald I

See if this example can help

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1_QPSK-demodulation.zip

21-Topaz II

Hi,

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

Greetings

FM

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F

Fred_Kohlhepp

23-Emerald I

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.

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21-Topaz II

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.

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21-Topaz II

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Author

1-Visitor

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?

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Author

1-Visitor

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?

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21-Topaz II

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1_IQ16.zip

Author

1-Visitor

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.

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