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Homework Help: A question in communication systems and signals

  1. Feb 20, 2012 #1
    I have problem approaching the attached question and understanding how to solve it. I desperately need some help here. :confused:
    Thanks! :smile:

    Attached Files:

  2. jcsd
  3. Feb 20, 2012 #2
    I looked up the Fourier series of triangular wave, it looks like it contain only odd harmonics:

    The first 3 components are 700KHz, 2100KHz, 3500KHz. First check f1 +/- f2 will not get to anywhere close. 2f1 +/- f2 will give DC in one case and over 700KHz in other............So I don't think the is any adding or subtracting will get the base frequency down at 50KHz or 210KHz. The reason I don't think so only because you can have all the permutations of summing and subtracting of harmonics and my head spin from thinking about it. I just did f1 +/- f2 and 2f1 +/- f2 and you can do the rest if you like!!!

    The other clue is there are two band pass filter in series, one at 50KHz and the other at 210KHz. they filter out each other's pass frequency!!!!

    So my guess is nothing comes out!!!
    Last edited: Feb 20, 2012
  4. Feb 20, 2012 #3


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    The filter has a pass band from 175 KHz to 225 KHz.

    So which odd harmonic of 700 KHz will pass through this band?

    None of them would
  5. Feb 21, 2012 #4
    Yeh, you are right, there is no mixing of the frequency, so there is no 2f1 - f2 stuffs at all. I corrected my post.
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