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Homework Help: Discrete Fourier Transform (DFT) Matching

  1. Jun 14, 2017 #1
    1. The problem statement, all variables and given/known data
    Match each discrete-time signal with its DFT:


    2. Relevant equations

    3. The attempt at a solution

    I am mainly confused about Signal 7 and Signal 8.

    Signal 1 is the discrete equivalent to a constant function, therefore its DFT is an impulse (Dirac ##\delta##), so it corresponds to DFT 3.

    DFT of an impulse is a constant. Therefore Signal 6 corresponds to DFT 5.

    Signal 2 is a sampled version of a full period of a cosine. So we expect ##X(1)## and ##X(-1)## to be nonzero (##X(-1)## really is ##X(N-1)=X(25)##). Therefore Signal 2 corresponds to DFT8.

    By similar arguments, Signal 4 has exactly 2 cycles of a cosine and corresponds to DFT2. And Signal 3 has one and a half periods of a cosine, as we do not have complete periods we should expect spectral leakage, so signal 3 corresponds to DFT4 (the main peaks are around 1 & 2 plus negative frequencies). Likewise, Signal 5 corresponds to DFT7.

    Here is a summary of the results so far:


    Only Signals 7 & 8 and DFT 6 & 1 are left:


    What do signals 7 and 8 represent? Is Signal 7 an undersampled cosine? How do we go about matching them with their corresponding DFTs?

    Any explanation would be greatly appreciated.
  2. jcsd
  3. Jun 14, 2017 #2


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    Staff: Mentor

    Could you please clarify what the scale of the DFT represents? Is it the the same order as the output of an FFT (from 0 to highest positive frequency, followed by lowest negative frequency back to 0)?

    Both can be see as undersampled cosines, with Signal 7 being a cosine with a varying amplitude envelope.
  4. Jun 15, 2017 #3
    It is similar to the spectrum of an FFT which is not fftshifted. The zero-frequency component (DC) is the first element (##r=0##). Then it is the positive frequencies, but I think it is lowest to largest, followed by negative frequencies.

    Yes, this is right. Any ideas how to identify the DFT for each signal?
  5. Jun 15, 2017 #4


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    Staff: Mentor

    What case would correspond to a single frequency?
  6. Jun 16, 2017 #5
    Is it Signal 8?

    So, the spectrum of Signal 8 is DFT6? What would be a good explanation? Signal 7 looks like at least two cosine waves being heterodyned (i.e. a cosine wave contained in a lower frequency cosine envelope).
  7. Jun 16, 2017 #6


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    Staff: Mentor

    That's basically it. Signal 8 corresponds to a pure cosine sampled twice per oscillation period, and therefore has a single frequency. The effect of changing the envelop (modulating the amplitude) is a spreading out of the frequency, as given by DFT1 (you can see it as an "uncertainty" in the frequency due to the limited time, or pulse nature, of the signal, or due to the fact that the signal is similar to a beat coming from overlapping signals of similar frequencies).
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