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Square wave(Simple Question)

  1. Jul 10, 2008 #1
    Friends,

    While studying Fourier Transforms, i saw that a square wave with amplitude +A and -A can be replaced with a sqaure wave with amplitude 2A.

    Just wondering how it was done? I'm new to Signals and Systems .

    thanks.
     
  2. jcsd
  3. Jul 10, 2008 #2

    Integral

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    It is just a coordinate system shift. Translate by -A.
     
  4. Jul 10, 2008 #3
    If you look at a square wave with amplitude of +/- A on a graph, you will see that it covers a range of 2A (A- -A=2A). This wave has a amplitude that fluctuates between these two values. However, an alternate wave with amplitude 2A has the same range (2A), yet it fluctuates between 0 and 2A. The two are identical in their form, they only differ how values are assigned.
     
  5. Jul 10, 2008 #4
    So that means that an X shift can be done in the same way. For e.g -5 to +5(on the x-axis) can be from 0 to 10 on the positive side.
     
  6. Jul 10, 2008 #5

    NoTime

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    I think you meant y-axis (the normal up/down designation), but yes.
     
  7. Jul 11, 2008 #6
    NO, i meant X-axis . I assumed that when the coordinate shift can be done on Y-axis, the same can be done on X-axis.

    Also, when x(t) has amplitude of (-A to +A), the book says that we can write it as x1(t) - A, where x1(t) has an amplitude of 2A. Any explainations.

    thanks.
     
  8. Jul 11, 2008 #7

    NoTime

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    Your statement explicitly says x(t) is an amplitude.
    So t is the location on the x-axis and x(t) is a sample point value on the y-axis.
    Adding a constant to x(t) just shifts the waveform up or down on the y-axis.

    If you want to shift the wave on the x-axis then you would write something like x(t-n).
    This has no overall effect on the y-axis amplitude excursions.
     
  9. Jul 11, 2008 #8

    Redbelly98

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    I don't see that anywhere.
     
  10. Jul 11, 2008 #9

    NoTime

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    Do you think the following says something else?
     
  11. Jul 12, 2008 #10

    Redbelly98

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    Missed that, sorry.

    At any rate, if this is just a math exercise in shifting coordinates, a signal can be shifted horizontally by one's choice of zero time. Looking back at message #4, we can say either the signal goes from -5 sec to +5 sec, or 0 to 10 sec.

    This assumes I'm interpreting alextsipkis's statements correctly.
     
  12. Jul 12, 2008 #11

    NoTime

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    I agree with you about the coordinate shifting part, you can do both.
    Seems to me that the OP defined x to be a square wave generator of amplitude (+A, -A).
    That gives A the definition of amplitude.
    Unless A got redefined somewhere, I don't see how you could get a time shift by adding a voltage value to the square wave.
    Did I miss a redefinition somewhere?
    If so, I'm not seeing it, but I've been known to be blind sometimes.
     
  13. Jul 12, 2008 #12

    Redbelly98

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    No, adding a voltage can't produce a time shift. It's simply a matter of defining where "zero" is. Whether it's voltage or time, 0V or 0 sec is an arbitrary reference.

    If I have a graph that covers a 10 sec range, from 0 to 10 sec, you might ask "when does t=0". And I might answer that t is zero at 2:00 p.m. today. But we could just as easily say t is zero at 5 seconds past 2:00 p.m., in which case the graph would run from -5 to +5 sec.
     
  14. Jul 12, 2008 #13
    Offsetting the signal on the amplitude axis introduces a 0 Hz component in the frequency domain.

    Shifting the signal on the time axis will introduce a phase change of frequency amplidudes. If you are transforming into sines and cosines the relative amplitudes for at any given frequency will vary inversily. (actually as a^2+b^2=constant, but that may be too much detail)
     
  15. Jul 13, 2008 #14

    NoTime

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    I admit to being a little rusty on this stuff.
    It has after all been a good 20 years since I've been paid to do any of this.

    Out of curiosity is a 0 Hz component just a fancy way of saying DC offset or is there a deeper implication.

    Rereading this thread it looks like I read "same way" in the following statement to be -5v to +5v, but now notice there is no units specification at all.
    I don't know if the OP is confused or not.
     
  16. Jul 14, 2008 #15

    Redbelly98

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    Yes.
     
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