Lissajous figure and oscilloscopes problem

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In summary, the Lissajous figure on an oscilloscope can go from a straight line to a circle depending on the phase relationship between the two input signals. This can be observed by changing the time/div control setting, which affects the number of cycles displayed on the screen. Lissajous patterns are created by applying a sinusoidal signal to the horizontal plates, which varies the sweep rate of the oscilloscope. Higher ratios of frequency between the two signals result in more complex patterns. The key to understanding these patterns is to recognize that the horizontal sweep rate is not linear.
  • #1
figs
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i'm learning about oscilloscopes, and i don't 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.
 
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  • #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.
 
  • #3
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.
 
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  • #4
figs said:
i'm learning about oscilloscopes, and i don't understand
why this Lissajous figure can be go from a straight line to
a circle.

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.
 
  • #5
Claude Bile said:
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.
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.
 

1. What is a Lissajous figure and how is it related to oscilloscopes?

A Lissajous figure is a complex curve that is created by the intersection of two simple harmonic motions, such as two waves with different frequencies. It is often used in oscilloscopes to visualize the relationship between two electrical signals, which can help identify phase differences and frequency ratios between the signals.

2. How can Lissajous figures be used in troubleshooting electrical circuits?

Lissajous figures can be used to identify problems in electrical circuits by comparing the expected pattern of the figure to the actual pattern displayed on the oscilloscope. Any discrepancies can indicate faulty components or incorrect wiring.

3. What factors can affect the shape of a Lissajous figure?

The shape of a Lissajous figure can be affected by the amplitude, frequency, and phase difference of the two signals used to create it. Additionally, any noise or interference in the signals can also impact the shape of the figure.

4. Can Lissajous figures be used to measure the frequency of a signal?

Yes, Lissajous figures can be used to measure the frequency of a signal by comparing the frequency of the horizontal and vertical components of the figure. The ratio of these frequencies can be used to calculate the frequency of the original signal.

5. Are Lissajous figures only used in the field of electrical engineering?

No, Lissajous figures can also be found in other fields such as astronomy and physics. In astronomy, they can be used to study the orbits of celestial bodies, and in physics, they are used to demonstrate complex harmonic motions and wave interactions.

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