Principles of Physics II (PHY112)
Lab
Experiment no: 3
Determination of the line frequency of Alternating Voltage signal generated by the electric
power plant with the help of an alternating voltage signal of known frequency by forming
Lissajous curves in an oscilloscope and the determination of the time base of the
oscilloscope
Name: Sowmik Das Gupta
Id: 22301107
Section: 16
Data
Table 1: Data for working out the unknown frequency of an alternating voltage signal
Known
frequency
(frequency of the
alternating
voltage signal
generated by
function
generator) ‘a’
(Hz)
Number of
Number of
complete
complete cycles
cycles along along the y axis
the x axis
for one
for one
complete cycle
complete
around the
cycle around Lissajous curve
the
Lissajous
ny
curve
𝑏 = 𝑎 . 𝑛𝑥 / 𝑛𝑦
(line frequency)
‘b’ (Hz)
Mean value
of
‘b’
(Hz)
nx
4
5
1
20
5
4
1
20
10
2
1
20
20
● Find the slope from ‘a’ vs. 1/nx graph (Calculate m from best-fit line. It should be a
straight line passing through the origin):
Best fit line:
Solution:
From graph we find that, m = 20
Frequency to be determined (line frequency), ‘b’ (Hz) = 20 Hz
Table 2: Data for determining time base of the oscilloscope
Frequency of the
alternating voltage
signal generated by
function generator
f (Hz)
250
Number of horizontal
divisions between two
consecutive peaks
300
17
0.000168
350
14
0.000204
400
12
0.000208
450
11
0.000202
D
20
Time base
(time/division)
TB (s/Div)
Average value of the
Time base
(time/division)
TB (s/Div)
0.0002
0.000196
● Find the slope from 1/D vs. f graph (Calculate m from best-fit line. It should be a straight
line passing through the origin):
Best-fit line:
From Graph we find that, m = 0.0002
Time base of the oscilloscope, TB = 0.0002
Answer the following questions:
1) See the Figure:
For this Lissajous curve what is Tx / Ty ?
Ans: 0.2
2) What is the specific phase condition required to achieve a
perfect circular Lissajous curve when both input signals 'a' and
'b' share identical frequencies?
Ans: 0.51 π radians
3) Provide an explanation for the similarity in the Lissajous figures when inputting values a = 1
and b = 3 compared to a = 5 and b = 15.
Ans: Both has a T x / Ty ratio of 1/3 as ratio of 5/15 is also 1/3
4) Define the “Time Base” of an Oscilloscope.
Ans: The amount of time represented by a single division of the horizontal side of a square
drawn on the display box of the oscilloscope.
5) Go to Virtual Oscilloscope | Academo.org - Free, interactive, education. If we were to keep the
input frequency in our virtual oscilloscope fixed and change other parameters in the following
ways, what would happen in each case? (i) doubling gain (ii) halving the time base & (iii)
doubling the volts/div. Discuss any change in values as well as in the output in our oscilloscope
screen.
Ans: (i) The vertical division which is the amplitude doubles. For 500Hz, previously the
amplitude was 5 divisions, after doubling gain, it is 10 divisions.
(ii) The horizontal division of each sine wave which is the period doubles. For 500Hz,
previously the period was 10 divisions, after halving the time base, it is 20 divisions.
(iii) The vertical divisions become half. For 500Hz, previously, the amplitude was 5 divisions,
after doubling, it is 2.5 divisions.