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Fourier transform of RF signal with a prism ?

  1. Nov 7, 2013 #1
    Fourier transform of RF signal with a "prism"?

    We can use a prism to decompose visible light into components of different frequencies. This is a Fourier transform by nature. For an ideal prism, the energy is conserved in the process.

    How about RF signals? There is no fundamental difference between an RF signal and visible light -- they are both EM waves. Is there a device/circuit that can separate the RF signal into different frequencies spatially just like a prism to visible light?

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  3. Nov 7, 2013 #2
    A prism has the effect it does because the speed of light (refractive index) inside the material varies according to the frequency. This is called dispersion. All you have to do is find a material where the refractive index continues to vary into the radio range, and make a triangle out of it.

    Alternatively you could probably simulate such a material by making a metamaterial, which would probably be fairly easy for radio wavelengths since the element size would be quite large.

    Actually, far more exotic devices have been built, such as lenses for gamma rays and neutrons.

    So my answer is yes, it is definitely possible, but I have not personally heard of someone doing it.
  4. Nov 7, 2013 #3
    The ionosphere is the Nature's device for dispersing RF.

    A bit of a problem with RF is that the wavelength is large, so dispersing devices will need to be large as well.

    Hertz in what was probably the very first experiments with RF built a prism made of asphalt, which weighed half a ton, and he was able to detect refraction. He used centimeter waves, longer waves would have made that completely impractical.
  5. Nov 7, 2013 #4


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    there is a difference and something you haven't appeared to consider.....
    Visible (white) light is a collection of frequencies ( wavelengths) and can be split into its components
    A transmitted RF signal ( unmodulated carrier) is generally a single spot frequency ( with some given bandwidth) you cannot split it up ....
    that would be the same as taking the blue or red light that comes out of a prism and trying to split it further .... you cant its still either blue or red

    You can use a spectrum analyser and look at a section of spectrum say 100MHz to 1000 MHz and look at all the individual frequencies that are being transmitted in that range that are within the receive capability of the analyser.
    You can look at an individual freq and see its bandwidth, its sidebands ( due to the modulation etc) its signal strength etc

    Last edited: Nov 7, 2013
  6. Nov 18, 2013 #5
    Is it possible to use a series of transformers to extract signal components of different frequencies simultaneously?
  7. Nov 18, 2013 #6


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    what signal components ?
    did you understand what I wrote in my last post ? :smile:

  8. Nov 18, 2013 #7
    Hi Dave, I was thinking about a modulated RF signal, whose frequency is not a spot but a range. How do we separate the RF signal into different frequency components? I suspect a series of transformers may do the job, but am not sure.
  9. Nov 18, 2013 #8


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    Its not a spot freq because of the modulation, that is, its the modulation that gives the signal "x" amount of bandwidth .... so there isn't a range of RF frequencies being combined.
    for AM( Amplitude Modulation) its a spot frequency that has a changing amplitude
    ( yes sidebands are produced but they are endeavoured to be kept to a minimum)
    FM ( Frequency Modulation) is a spot frequency that is varies slightly with the modulation

    if its a modulated signal, then you put it through a demodulator to recover the modulation.
    That modulation may be voice, data etc

    in the most basic radio signal demodulator, say an AM transmission, we use a diode to detect and recover the audio from the RF signal have a look at this link
    it's just one of 100's of links in google for demodulating/recovering the audio etc

    Last edited: Nov 18, 2013
  10. Nov 19, 2013 #9
    That seems self-contradictory. Any modulation results in a range of RF frequencies; that's what "bandwidth" means.

    Which means it is a range of RF frequencies.
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