3B Scientific Training Oscilloscope - Manual (Page 4)

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• With the fine adjustment, search for a frequency
at which a standing image appears, which shows
a clear period of oscillation.
At which other saw-tooth frequencies does a standing
image also appear?
5.4 Lissajous figures
Additional equipment required:
1 Function generator (50
Ω, with amplifier if possi-
ble), and 1 AC power supply or 2 function generators.
• Set up the experiment as in fig. 5.
• Attach a coil to the metal ring pointing inward,
with the axis aligned horizontally.
• Connect the inputs (green, yellow) to the AC
power supply or to the second function generator
(set to a 50 Hz sinusoidal voltage). Select the am-
plitude so that the line appearing on the screen is
approximately half the length of the screen di-
ameter.
• With the circular magnet, adjust the line horizon-
tally to centre it.
• Mount another coil to the metal ring pointing
inward, with the axis aligned vertically.
• Connect the inputs (green, yellow) to the first
function generator (set to a 50 Hz sinusoidal volt-
age).
An ellipse appears that changes shape at faster or
slower speed, depending on how well the frequencies
of the input signals match. This takes the shape of a
sloped straight line twice per cycle.
• Adjust the amplitude of the first function genera-
tor so that the slope of the straight line is 45° and
that a circle emerges during transition.
The simplest Lissajous figures can be observed al-
ready. The shapes depend on the frequency ratios and
on the phase shift. Due to a small deviation from the
exact target frequency on either of the two function
generators (usually, the inaccuracy of the devices is
already sufficient), the phase shift cycles automati-
cally, and all figures for a specific frequency ratio can
be observed in succession.
• Set the frequency of the first function generator
to a multiple of the horizontal frequency (50 Hz).
Observe Lissajous figures for the frequency ratios 2:1,
3:1 and 4:1.
• Further Lissajous figures are created by fractional
multiples of the horizontal frequency (e.g., 3:2 (75
Hz), 4:3 (66.7 Hz).
Fig.2 Electrical deflection of the electron beam (left: with power supply U8521371, right: with power supply U33000)