# Thread Subject: frequency vs. wave# plot; fft2 of E field

 Subject: frequency vs. wave# plot; fft2 of E field From: Woehrle Date: 29 Jun, 2012 03:57:17 Message: 1 of 4 Greetings all, I'm having some difficulty producing a frequency vs wavenumber plot from some E field data. (I'm trying to produce something comparable to this: http://i.imgur.com/0pGO2.png) Anyways, I know what dt (time step) and dz (distance increment) for each E value. So, an fft2 of Ey(z,t) should produce w,k? I might have this wrong... Here is what I've been working on.... %load data file load woctrlpts.dat %Time & sample stuff Nz = 44; %Number of samples collected along first dimension Nt = 4097; %Number samples collected along second dimension dz = .0017; %Distance increment dt = 5.770885e-13; %time increment %Nyqusit Nyq_k = 1/(2*dz); Nyq_f = 1/(2*dt); dk = 1/(Nz*dz); df = 1/(Nt*dt); k = 0 : dk : Nyq_k - dk; f = 0 : df : Nyq_f - df; %Data array E = woctrlpts(:,5); %create E(z,t) matrix space_time = reshape(E,44,[]); %fft2 of space_time, might be right... fft2result = fftshift(fft2(space_time))*dz*dt; plot(k,f,abs(fft2result)) What are the necessary modifications I need to make to my code to produce a plot similar to the one mentioned above? Thanks!
 Subject: frequency vs. wave# plot; fft2 of E field From: TideMan Date: 30 Jun, 2012 03:56:47 Message: 2 of 4 On Friday, June 29, 2012 3:57:17 PM UTC+12, Woehrle wrote: > Greetings all, > > I'm having some difficulty producing a frequency vs wavenumber plot from some E field data. (I'm trying to produce something comparable to this: http://i.imgur.com/0pGO2.png) > > Anyways, I know what dt (time step) and dz (distance increment) for each E value. > So, an fft2 of Ey(z,t) should produce w,k? I might have this wrong... > > Here is what I've been working on.... > > > %load data file > load woctrlpts.dat > > %Time & sample stuff > Nz = 44; %Number of samples collected along first dimension > Nt = 4097; %Number samples collected along second dimension > dz = .0017; %Distance increment > dt = 5.770885e-13; %time increment > > %Nyqusit > Nyq_k = 1/(2*dz); > Nyq_f = 1/(2*dt); > > dk = 1/(Nz*dz); > df = 1/(Nt*dt); > > k = 0 : dk : Nyq_k - dk; > f = 0 : df : Nyq_f - df; > %Data array > > E = woctrlpts(:,5); > > %create E(z,t) matrix > space_time = reshape(E,44,[]); > > %fft2 of space_time, might be right... > fft2result = fftshift(fft2(space_time))*dz*dt; > > plot(k,f,abs(fft2result)) > > What are the necessary modifications I need to make to my code to produce a plot similar to the one mentioned above? > > Thanks! That's a contour plot. Try: contour(k,f,abs(fft2result))
 Subject: frequency vs. wave# plot; fft2 of E field From: Yash Date: 30 Jun, 2012 12:48:09 Message: 3 of 4 i would say try a mesh plot or a surface plot for this will work best makeyourassignment@yahoo.com
 Subject: frequency vs. wave# plot; fft2 of E field From: Woehrle Date: 1 Jul, 2012 04:27:09 Message: 4 of 4 No luck so far. I'm not getting a useful graph. I might doing something wrong. I'll update tomorrow.

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