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Homework Help: How to draw a Amplitude and Phase spectrum

  1. Sep 18, 2012 #1
    The problem statement

    Sketch the Amp spectrum of the following....
    Additionally, for x3, sketch the phase spectrum...

    ( j is the imaginary number)

    x1(t) = cos(10pi*t) + cos(3pi*t)

    x2(t) = cos(10pi*t) + cos(5pi*t) - j*sin(10pi*t)

    x3(t) = cos(10pi*t + (pi/6)) + j*sin(10pi*t)

    The attempt at a solution

    For x1 I am fairly certain that the plot should appear as two lines at f = 3/2 and f = 5 with A = 1 for both. I am only confident in this answer as the signal is real and not complex.

    For x2 I have drawn two lines once more for f = 1/5 and f = 2/5 with respective A values of sqrt(2) and 1

    For x3 I have f = 1/5 and a value of sqrt(2) for it. I have no idea how to interpret the phase spectrum.

    There was considerably more to the problem, but I have completed everything but these plots, which I simply cannot find how to draw online. I found one source, but was weary as it did not explain what to do in the event of a phase shift. Does a phase shift effect the Amp spectrum?
  2. jcsd
  3. Sep 18, 2012 #2

    rude man

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    Don't know what an "Amp spectrum" is, nor a "phase spectrum". Is this in relation to Fourier transform?
  4. Sep 18, 2012 #3
    We have yet to be formally taught the Fourier transform, but an Amplitude spectrum is a plot of amplitude vs frequency and a phase spectrum is a plot of phase shift vs frequency.
  5. Sep 18, 2012 #4

    rude man

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    That's right. Amplitude spectra convey no information as to phase,only amplitude.
    Better check your f's on that one! For the rest, see below.
    Again, what is the frequency in Hz? :uhh:

    Your amplitude is (probably) not correct. But I don't know how to find it!

    The thing is, the problem's terminology is not only unconventional but downright misleading. It mixes phasors and time functions. Complex representations of time functions are phasors and are not time-dependent. Neither are they frequency-dependent. A general phasor is Ae where A is the phasor amplitude and θ is its phase angle.

    So the expression "jsin(10πt)" is basically nonsense, and I don't know what to do with it really.

    You could maybe interpret "cos(10πt + π/6)" as as phasor with amplitude 1/√2 and phase angle π/2 + π/6. Why? Because sin(10πt) can arbitrarily be defined to have zero phase. That transforms sin(10πt) to a phasor of 1/√2, jsin(10πt) to a phasor of j(1/√2),
    10cos(10πt) to a phasor of (1/√2)ejπ/2 and cos(10πt + π/6) to (1/√2)ej(π/2 + π/6). So the total phasor of x3 would be X3 = (1/√2){ej(π/2 + π/6) + j}. But that's just a WAG until they supply you with a legitimate time or phasor representation of x3.

    NOTE: The 1/√2 is just a definition of phasor amplitude. It's there for a good reason of course. You should know what it is, or find out.
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