Re: [Discuss-gnuradio] Why is fft output complex?
Brad: 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. Bob On Sat, Oct 11, 2014 at 5:38 PM, k1...@comcast.net 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. Thanks Brad. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio -- 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) ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Discuss-gnuradio] Why is fft output complex?
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. ThanksBrad.___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] Why is fft output complex?
On 10/11/2014 05:38 PM, k1...@comcast.net 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 Thanks Brad. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] Why is fft output complex?
On Oct 11, 2014, at 5:38 PM, k1...@comcast.net k1...@comcast.net 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. Thanks Brad. _ Dear Brad, What you write is mostly correct. But you are thinking of the power spectrum, which is the modulus of the FFT and thus entirely real. An FFT contains more information. For example, a Fourier Transform (the continuous version of an FFT) can be inverted using an Inverse Fourier Transform to exactly reproduce the original time-domain waveform.That would not be possible using the power spectrum alone, since information is lost in going from the complex Fourier Transform to the power spectrum. --Dan Marlow ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Re: [Discuss-gnuradio] Why is fft output complex?
Thank you this helps a lot. I'll place my complex to mag block directly at the fft output. Then I can send that to a vector sink and review my bin data. [Brad Hein]-Original Message-From: wa1...@wa1hco.netTo: discuss-gnuradio@gnu.orgCc: Sent: 2014-10-11 17:59:24 GMTSubject: Re: [Discuss-gnuradio] Why is fft output complex? On 10/11/2014 05:38 PM, k1...@comcast.net wrote: I cant 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? Im trying to write a python script to analyze the energy across frequency bins but I dont 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 Thanks Brad. ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio ___ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
