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

From: jeff millar
Subject: Re: [Discuss-gnuradio] Why is fft output complex?
Date: Sat, 11 Oct 2014 17:58:31 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.1.2

On 10/11/2014 05:38 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.

The output of an FFT is complex because it contains magnitude _and_ phase.  If the output is I,Q (cartesian coordinates) then the magnitude is the length of the vector from 0,0 to I,Q or sqrt(i^2 + q^2).  The phase of the signal is atan2(Q,I).

Note that the FFT transforms Voltage samples into Voltages per frequency bin.  When you say "energy" you may mean power which is the magnitude squared or (i^2 + q^2).

jeff, wa1hco


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