A student is investigating an electrical signal using a cathode-ray oscilloscope (c.r.o).

Question 15

A student is investigating an electrical signal using a cathode-ray oscilloscope (c.r.o).

The frequency of the signal is 50 kHz.

Which time-base setting on the oscilloscope should be used?

A 50 ms cm^-1                   B 1 ms cm^-1                     C 10 μs cm^-1                 D 0.5 μs cm^-1

Reference: Past Exam Paper – March 2017 Paper 12 Q4

Solution:

Answer: C.

Frequency = 50 kHz = 50 × 10³ Hz

On the screen of the c.r.o, the period of the signal can be found.

Frequency f = 1 / T

Period T = 1 / f = 1 / 50×10³

Period T = 2.0×10-5 s = 20 × 10^-6 s

Period T = 20 μs

The time-base setting should be such that a complete wave may be seen.

A and B are obviously incorrect.

If the time-base setting is 10 μs cm^-1, one complete wave may be observed within 2 cm.

 
If the time-base setting is 0.5 μs cm^-1, one complete wave may be observed within 20 cm. The screen of the c.r.o is smaller than 20 cm. So, a complete wave cannot be seen on the screen. [D is incorrect]