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Re: [Discuss-gnuradio] Why is fft output complex?

From: Robert McGwier
Subject: Re: [Discuss-gnuradio] Why is fft output complex?
Date: Sun, 12 Oct 2014 10:46:50 -0400


You are treating an FFT as if it were a spectrum analyzer which produces a magnitude or energy profile of how much signal is at a particular frequency.  The FFT does much more than that.  It tells not only what magnitude is at a frequency but what phase angle the signal has there.

Let's take an example:

You would not want the fft of   sin(t)+cos(2t)  to be the same as sin(t)+sin(2t).   You would want the result to show that the stuff at 2t is 90 degrees out of phase depending on what signal you input.


On Sat, Oct 11, 2014 at 5:38 PM, <address@hidden> wrote:
I can't wrap my head around why fft transform of complex signal produces a complex output. After all the output reflects the amount of energy per frequency bin and frequency bins and energy are both real numbers, no?

I'm trying to write a python script to analyze the energy across frequency bins but I don't know where to insert a complex to mag block. I think if I can understand the fft I will know to put the complex to mag.


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Bob McGwier
Co-Owner and Technical Director, Federated Wireless, LLC
Professor Virginia Tech
Senior Member IEEE, Facebook: N4HYBob, ARS: N4HY
Faculty Advisor Virginia Tech Amateur Radio Assn. (K4KDJ)

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