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how to get a real signal by inverse FFT

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Old   December 28, 2005, 04:06
Default how to get a real signal by inverse FFT
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xijing
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First I have a real pulse signal of disturbance, I calculated its power spectrum by applying FFT, then I need to remove a specific band of frequency by setting those energy as zero, after that I applied an inverse FFT to get a new signal. But it is a complex series of time. How can I get a real signal which has the energy spectrum absent that specific frequency band? Thank you very much!
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Old   January 3, 2006, 01:01
Default Re: how to get a real signal by inverse FFT
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carno
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What you have done is very correct. What makes you think that it is wrong? If you modify the FFTed signal and do reverse FFT it is bound to happen.
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Old   January 4, 2006, 09:13
Default Re: how to get a real signal by inverse FFT
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xijing
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Y, thank you! I realized that idea is only for ideal filter.so I cannot get a real modified signal by inverse FFT. Now I try to use matlab to remove a narrow band of requency. But I still has problem, someboby please help me: I have a real signal in the time domain.I need to remove a band of frequency from it to get a new real signal. So first I FFT this input real signal and multiply the FFT ouput by a filter function I chose from matlab. I hope I can get a new power spectrum with that designated band of frequency zero. But I tried many ways, I can not get it successfully.Below is my matlab file:

% the original pulse input t=0.3721124:0.3721124:0.3721124*160; w=51*1034.631154/1000000; x=0.0082*cos(-t*w); plot(x(1:160)) title('singnal quarter of cosine') xlabel('timestep')

% I want to remove the low frequency band from 0.05-0.07, my sampling interval is about 0.372114

ff = [0 0.04*0.37 0.05*0.37 0.07*0.37 0.08*0.37 1]; aa = [1 1 0 0 1 1]; nn = 160; bb = firls(nn,ff,aa); [h,w] = freqz(bb); plot(ff,aa,w/pi,abs(h)) legend('Ideal','firls Design')

X=fft(x); Xh = filter(bb,1,X); % fft to get the whole frequency content f=1/0.3721124/160*(0:20); Pxx=Xh.* conj(Xh)/160 We=exp(-10*log(10)); Ub=log10((Pxx)+We); plot(f,Ub(1:21)) title('frequency content') xlabel('frequency')
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Old   May 31, 2011, 06:25
Default frequency versus position in FFT
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amir
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amirmohebbi is on a distinguished road
hi
dear all
i am new in fluent
i solve a vibration flow field jet
how i can plot frequency of jet in a line versus position?
thanks a lot
amirmohebbi is offline   Reply With Quote

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