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I'm so confused, and it would be nice if someone could clear my head, or at least direct me to suitable references.

Assume that I have two gaussian pulses with the same bandwidth in time, but for 2 different wavelengths. I'm going to handle them separatly- 2 separate graphs of one gaussian in time.

Both graphs are the same because they have the same bandwidth. Then, I use fft on each graph. Of course I'm going to have the same graphs after the fft- because I had the same graphs before.

Now, I want to creat the frequencies axis for each signal. In order to do that, I use the exact direction from matlab. df=1/T, T=N·dt. So the frequencies axis are also the same!

But how can it be if f=c/lambda? I'm talking about two different wave length, how is it possible that the both have the same spectrum of frequencies?

Further more, I don't understand what is the different between F and df and why is F=N·df. I do understand that T is the time in which I sample, dt=T/N. but that's it.

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You forget that what you did was describing the passband signals (i.e. some waveform on a carrier frequency == on a wavelength) in baseband.

So, of course, after calculating the baseband spectrum, you need to shift it back where it came from - to the carrier frequency. It's as easy as that.

If this isn't clear to you, you might want to revisit what (complex equivalent) baseband and passband are.

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  • $\begingroup$ But why did I describe my passband in baseband? where in the process? Thank you. $\endgroup$ Jan 15 at 0:23
  • $\begingroup$ in the end of your question you're trying to consider the spectrum of the signal on the carriers. That's passband. Really, read up on baseband and passband signals – this doesn't seem to be clear to you, and it's literally in every textbook on communications engineering, in many on signals and systems, … $\endgroup$ Jan 15 at 7:16
  • $\begingroup$ thank you so much for your help! $\endgroup$ Jan 15 at 10:25

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