Discussion in 'General Electronics Chat' started by Thomas.R, Mar 30, 2008.

  1. Thomas.R

    Thread Starter Member

    Aug 16, 2007

    I've made a signal generator with a square and triangle wave. The circuit is made out of a Schmitt-trigger and a integrator. So when there is a low voltage on the Schmitt.. the integrator will rise and when there is a high voltage it will fall. Now i hooked up both the signals to a scope and put the scope in XY position, to see a square on the screen. I sort of get what the function does but i don't understand the phase link of the answer (the square)

  2. SgtWookie


    Jul 17, 2007
    When the X-Y button is pressed IN, the oscilloscope does not display a V/t (Voltage vs time) graph. Instead, one axis is controlled by the input to CH I/A, and the other axis is controlled by the input signal to CH II/B. This allows the oscilloscope to be used to display a V/V (voltage vs voltage) graph.

    The X-Y control is used when you want to display component characteristic curves, or Lissajous figures.

    When I put my 'scope display to X-Y, connect a square wave to channel A and a triangle wave to channel B, I get a box with bright vertical sides and very dim horizontal sides. This is because the square wave is controlling the X-axis, and the triangle wave the Y-axis.

    If you hooked up the probes to either ends of the secondary windings of a high-quality step-down transformer with the primary plugged into mains power, you should see a diagonal line on the display. This is because the phases are 180° from each other.

    You can exploit this phenomenon as a very handy troubleshooting tool. A Navy technician came up with "The Octopus" back in the 1930's. Here is a page that describes some of the things you can do with an Octopus:

    Here is a 33-year-old archive describing the Octopus; scroll about 3/4 of the way down:

    Here's a link to a BK Precision article about using X-Y mode:
  3. scubasteve_911

    Senior Member

    Dec 27, 2007
    Excellent post Wook! I really enjoyed reading through that stuff, since I had forgotton the purpose of X-Y mode since I used it in tech. college.

  4. Thomas.R

    Thread Starter Member

    Aug 16, 2007
    so, if i see a square as a result that would mean that the two signals are 90 degrees out of phase?
    i see exactly what you see on the scope. sgtwookie.
    (i don't know the word for it in English so i will try to explain)
    the phase of one signal is 90 degrees before or after in phase in comparison to the other signal?
  5. relicmarks

    Active Member

    Oct 13, 2006
    The X-Y control is used when you want to display component characteristic curves

    Do you have any more charts or info about component characteristic curves using in XY mode please?
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    You need 90 degrees phase shift to get a circle. Zero and 180 yield diagonal lines of opposite slopes.
  7. SgtWookie


    Jul 17, 2007
    OK, with a square wave on Ch A/I and a triangle wave on Ch B/II:

    The signal level on channel A (square wave) is rapidly switching between two DC levels.
    This causes the "dot" (electron beam) to alternate between two different X-axis (horizontal) positions. It isn't an instantaneous transition, as the square wave takes a bit of time to go from one DC level to the other.

    Meanwhile, channel B has a triangle wave that causes the "dot" to move in the Y-axis (vertically) relatively smoothly.

    If you slow your signal generator way down (to perhaps 1Hz) you'll be able to see the dot move up one side of the box, then down the other side - but the transition of the square wave from low to high and high to low will be so rapid that you won't be able to see the top and bottom of the box being "drawn."

    The square and triangle waves are "sort of" out of phase. They aren't the same waveform; more like the inverse of one another. One has no stable DC level, the other has two stable DC levels. One has very brief rise and fall times, the other is always rising or falling.

    Try some experiments using 4000 series CMOS IC's. 4093 quad NAND Schmitt inputs and 40106 hex Schmitt triggers are very useful for simple square and (rough) triangle wave generators. You can also connect three inverters in series (using resistors and caps) to create a phase-shift oscillator, where the output waveforms are roughly 120° apart.

    Just make certain that unused inputs are tied to either Vdd(+V) or Vss (ground) so that they don't oscillate.