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Algorithm - Detect carrier

Discussion in 'Electronic Design' started by Ishira, Jul 22, 2003.

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  1. Ishira

    Ishira Guest

    Hi,

    I am trying to write a program which detects if a signal is modulated
    or not; i.e whether there is a carrier or not.

    Any suggestions would be greatly appreciated. Also, if you can direct
    me to some tutorials/books which handles algorithms of this sort, it
    would be great.

    Thanks.
    Ishira
     
  2. John Larkin

    John Larkin Guest

    Could you please be less specific?

    John
     
  3. Ken Smith

    Ken Smith Guest

    I assume that you have a series of numbers that is the digitized data from
    some signal source.

    I also assume that the sample rate is high enoungh that aliasing is not a
    problem.

    I also assume that the processing is done after the fact so that you have
    all of the data before you process it.

    Given all that, I'm still not sure I understand what you are looking for
    but I will suggest that you do an FFT on the data and look at each
    peak. Find the peak, fold the spectrum around that peak and do a
    correlation. If the abolute value of the correlation is less than 0.707
    chances are you are not dealing with a simple AM or FM modulation of that
    frequency.

    Hmmm now that I've written that, it looks like something that should lead
    to a calculation involving a Hilbert transform.
     
  4. John Fields

    John Fields Guest

     
  5. Ishira

    Ishira Guest

    I assume that you have a series of numbers that is the digitized data from
    YES


    AHEM! lOOKS LIKE I AM TOTAL DUNCE IN THIS FIELD! BUT I AM REALLY
    EXCITED ABOUT IT AND WOULD LIKE TO LEARN MORE.

    SO HERE IS THE THING

    1. I know that the signal data can only be BB, Constant Carrier
    Modulation(CCM), FSK or C_FSK.

    So basically, I need to classify the data into one of these modulation
    schemes.

    2. After that I need to demodulate and then decode.

    But one step at a time. I know that I first need to get a hang of FFT
    and basic signal concepts. Any papers/journals/books/ tutorials at all
    on the algorithms?

    Anything at all that you would like to say would be greatly
    appreciated. If I dont understand something, it will only lead to more
    learning.

    Thank you.
    Ishira
     
  6. Ken Smith

    Ken Smith Guest

    You could just try each scheme and see if you get something that looks
    right. If the number you need to select from is low and well defined
    this may work about as well as any other method.
    Try google with "FFT". You'll find lots about the FFT


    Basically the FFT bins the data according to frequency. Most modulation
    methods make side bands on both sides of the carrier that are either
    mirror images of each other or inverted mirror images. If you guess the
    right carrier, you can see if the side bands make sense for the suspected
    type of modulation.
     
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