I determined that the shift is somehow related to the bandwidth of my
pulse. If I make the source very narrowband (170 to 190 THz), the FFT
from Matlab ends up in the correct position.
This may be related to a previous thread:
http://ab-initio.mit.edu/pipermail/meep-discuss/2006-October/000463.html
Best,
Matt
On Wed, 3 Sep 2008, Steven G. Johnson wrote:
> On Sep 3, 2008, at 1:19 PM, matt wrote:
>> I'm pretty certain that the matlab script is correct. Is there
>> something in meep which could be causing the frequency shift?
>
> No, I can't think of anything in Meep that could cause a frequency
> shift of that sort, assuming all your units are consistent and correct.
>
> However, your Matlab script is slightly off. The number of data
> points from the DC frequency to the Nyquist frequency, inclusive, is
> NFFT/2+1, not NFFT/2. So, you should have:
>
> f = (1/dt)/2*linspace(0,1,NFFT/2+1);
> y=plot(f,2*abs(A(1:NFFT/2+1)));
>
> if I'm reading your code correctly. (The DC frequency is "bin" 0 of
> the DFT, and Nyquist is the N/2-th bin, but because Matlab's indices
> are 1-based these become 1 and N/2+1, respectively.)
>
> Regards,
> Steven G. Johnson
>
>
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