Question

A Lissajous pattern on the oscilloscope is stationery and has 6 vertical maximum values and 5 horizontal maximum values. The frequency of horizontal input is 1500 Hz. The frequency of vertical input is
1. 1800 Hz
2. 1250 Hz
3. 45000 Hz
4. None of the above

Answer 1

Correct Answer - Option 2 : 1250 Hz

Concept:

The following is a procedure to measure the frequency of a Lissajous pattern obtained from CRO.

• For a given Lissajous figure, draw both horizontal and vertical lines passing through the Lissajous figure (Never draw either horizontal line (or) vertical line via pre-existing intersection in the Lissajous figure)
• Count the number of cuts made by both lines
• The unknown to known frequency ratio can be measured as follows.

\(\frac{{{f_v}}}{{{f_h}}} = \frac{{{n_h}}}{{{n_v}}}\)

Where

fv = unknown vertical frequency

fh = known horizontal frequency

nh = no. of cuts as made by horizontal

nv = no. of cuts as made by a vertical line

Calculation:

From the given Lissajous figure,

Vertical tangencies (NV) = 6

Horizontal tangencies (NH) = 5

The frequency ratio of the vertical signal to the horizontal signal is

\(\frac{{{f_V}}}{{{f_H}}} = \frac{{{N_H}}}{{{N_V}}} = \frac{5}{6}\)

⇒ f_H = 1250 Hz