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Lissajous figure and oscilloscopes problem

  1. Feb 23, 2005 #1
    i'm learning about oscilloscopes, and i dont understand
    why this Lissajous figure can be go from a straight line to
    a circle. Also, this sweep time/div control setting, i dont
    understand why the wave cycles vary with this control setting.
     
  2. jcsd
  3. Feb 23, 2005 #2
    You questions are not very clear and are therefore had to answer.
    Here is a java applet for lissajous lab. It did not work with my browser but maybe it will for yours.
    http://www.math.com/students/wonders/lissajous/lissajous.html
    here is one that worked on my browser.
    http://abc.net.au/science/holo/liss.htm

    The oscilloscope takes some getting use to. Play with it a little and don’t be afraid of breaking it, most are rather resilient. I have only seen people mess up oscilloscopes when they mess with the output impendence of the oscilloscope.
     
  4. Feb 23, 2005 #3

    Integral

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    The horizontal time setting (time/div) controls the time it take the sweep to cross the screen. It is specified as the time required to travel 1 major division on the screen. If you are displaying periodic signal the number of cycles on the screen will be determined by the horizontal time setting. So if you are looking at a 60hz it has a period of ~17ms. So a setting of 10ms/div will mean that 1 cycle of the signal will occur in about 1.7 divisions. If there are 10 divisions across the screen you will see about 5 cycles. Now if you change the time base to 5ms/division each cycle will take about 3 divisions and only ~3 cycles will be displayed.

    To create Lissajous patterns you must provide a signal to drive the horizontal sweep. This means that your horizontal speed controls are disabled. The normal signal applied to the horizontal deflection is a sawtooth, this causes the sweep to cross the screen at the uniform rate specified by the time base. When you apply a sinusoidal signal to the horizontal plates the rate the sweep travels is no longer linear, it now varies with the phase of the input signal. When you apply equal frequencies to both horizontal and vertical plates the result is the circle. The appearance of the pattern is determined by the ration of the horizontal to vertical frequency. A figure 8 means a 2:1 ratio, a 1:2 yields a [itex] \infty [/itex] Higher ratios yield more interesting results.

    The key to understanding the Lissajous pattern is realize that the horizontal sweep rate is not linear.
     
    Last edited: Feb 23, 2005
  5. Feb 23, 2005 #4

    Claude Bile

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    If your two inputs are in-phase, they will produce a straight line, if they are 90 degrees out of phase, they will produce a circle (provided the two inputs are the same frequency of course).

    Claude.
     
  6. Feb 24, 2005 #5

    Integral

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    Yes, reflecting on my post, I realized that I had neglected to mention the importance of the phase relationship between the signals. Thank you Claude.
     
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