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Lissajou patterns

Discussion in 'Electronic Basics' started by [email protected], Jul 20, 2005.

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  1. Guest

    Hi!

    I am having problems with displaying Lissajou patterns (for educational
    purpose) with a Tektronix 2246 and I need some help.
    I have one and only one function generator set to display sinusoidal
    signal. I put a T BNC connector on the output plug of the generator on
    plug to 50 ohms BNC to it and I have connected the 2 ends to the scope
    on CH1 and Ch2.
    I have inverted the CH2 to have the exact same signal and frequency
    inverted. I have changed the mode to XY and I am getting 2 diagonals
    lines (like this <).
    different mode, different type of current and different everything but
    nothing works.
    I would really appreciate a little help from confirmed technician which
    I am obviously not...
    Thank you.
     
  2. Chris

    Chris Guest

    Apparently your output is exactly right. The whole point of Lissajous
    figures (cue "The Outer Limits" music -- "Do not adjust your screen.
    We control the horizontal. We control the vertical") is comparing
    frequencies and phases. If they're both identical, you aren't going to
    get any more than what you have.

    You might want to try getting a second function generator, or cobbling
    up a PLL circuit or something else to get two different frequencies to
    compare.

    Actually, Lissajous figures are still vitally important in electronics.
    If I have to read a manual or data sheet or do something important on
    their problem that doesn't require wrench twiddling, and they want some
    wrench twiddling on their problem RIGHT NOW, I have been known to set
    up a Lissajous figure on my bench scope with one frequency drifting
    slightly to cause slow but constant motion on the display. I then put
    the scope on my bench next to the protoboard, with a large sign stating
    "CAUTION -- TEST IN PROGRESS!" That has a tendency to keep
    non-technical people out of my hair so I can think. For some reason,
    they stare at the display for twenty seconds or so, then walk away with
    either a blank or frustrated glance. I think there's something about,
    say, a slowly drifting 4:3 or 3:2 display that causes great inner
    confusion in non-technical persons. ;-)

    Good luck with the homework. Try googling with a different spelling --
    there are a lot of results for "Lissajous figure setup" (no quotes).

    Chris
     
  3. Don Taylor

    Don Taylor Guest

    Well I used to design these things for Tek but that was LONG ago.

    With only one signal generator you are getting exactly what I would
    expect.

    Forget scope for a moment. Just think algebra for a moment, you
    are going to plot a point (x,y) and the value for x is exactly the
    same as the value for y. So you get a point somewhere on a diagonal
    line y=x. You change to a different x, but the new y will be the
    same as the new x, so you get another point, again on that diagonal
    line.

    How about this instead. Find a little "wall wart" plug in power
    supply that outputs AC, maybe 6 volts or so. Hook one of your
    inputs to that. Probably better use your scope probe to attach to
    that, instead of connecting it directly, don't want to toast your
    input channel.

    Then set up your signal generator to be say 120 hz. Hook that to
    your other input. See if that gives you something other than a
    line. Then gently rock the generator frequency up and down a bit.
     
  4. Bob Monsen

    Bob Monsen Guest


    The point of lissajous figures is that the frequencies of the x and y
    components are related by some ratio. Thus, two signal generators are
    required.

    You can easily build a sine wave generator, using an opamp and a few
    other components. Google for 'wein bridge'. There are also signal
    generator chips that can be had for a few bucks that will generate very
    nice slow sine waves, which is what you want. The waveform needs to have
    a stable frequency, or it's hard to match.

    I saw an exhibit at the SF exploratorium, in which thin vertical rods
    were fixed at the bottom, and thinned along their axis in particular
    directions so they would oscillate at different frequencies in different
    directions when 'plucked'. That isn't a very good description, but the
    result was a lissajous figure traced out by the end of the rod when
    plucked. The amount of thinning along the rod determined what figure
    would emerge, figure 8, three loops, etc. It was very cool.

    --
    Regards,
    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
     
  5. Impmon

    Impmon Guest

    It helps if the input frequency on ch1 and ch2 is not the same or
    near. For example, put one channel on 60hz and another on 65hz and
    you get a rotating circle. If the second frequency about double (but
    not exactly) than the first, you'd get shifting figure 8.

    It would help if you have 2 different frequency generator (both on
    sine wave) and you vary the setting of one.
     
  6. Not only do you want two signal generators, but you should get two
    Function generators, which are able to generate other types of
    waveforms apart from sinusoidal. You can get some real funky lissajou
    displays with waveforms like triangle waves, ramps, square etc

    Dave :)
     
  7. Fred Abse

    Fred Abse Guest

    "Wien bridge" would be better.
     
  8. Bob Monsen

    Bob Monsen Guest

    yes, it would. thanks.

    --
    Regards,
    Bob Monsen

    If a little knowledge is dangerous, where is the man who has
    so much as to be out of danger?
    Thomas Henry Huxley, 1877
     
  9. As others have pointed out, with just one function generator all you will
    get is a diagonal line. What you see with a Lissajous setup is the
    graphical relationship between two signals. If both signals are the same,
    then what you see is what you get.

    If you can get your hands on another function generator, then you're all
    set. Or set up a circuit to turn your sine wave into a cosine wave
    (shifting it by 90 degrees) and you'll get a nice circle. But it'll be
    easier just to get another generator.

    Plus, you can experiment with wave shapes -- triangle, square, etc.

    If you really want to ham it up you can get a third generator and play with
    the insensity of the beam -- the Z axis. You can turn the beam on and off
    and get some really interesting patterns. This is what laser shows did
    before they got computers to control everything.
     
  10. colin

    colin Guest

    wein bridge = 634 hits
    wien bridge = 5500 hits

    surprisingly many people confused about how its spelt.

    Colin =^.^=
     
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