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.


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.

  • $\begingroup$ 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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.