Cathode-Ray Oscilloscope

A cathode-ray oscilloscope is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits
An a.c. current on an oscilloscope is represented as a transverse wave
- The oscilloscope can be used to determine its frequency and amplitude
A d.c voltage on an oscilloscope is represented as a horizontal line at the relevant voltage
The x-axis is the time and the y-axis is the voltage (or y-gain)

Cathode-ray oscilloscope display

CRO diagram, downloadable AS & A Level Physics revision notes

Diagram of cathode-ray oscilloscope display showing wavelength and time-base setting

The period of the wave can be determined from the time-base
- Time-base is how many seconds each division represents measured commonly in s div^-1 or s cm^-1
Count the number of wavelengths shown on the screen
Divide the total time by the number of wavelengths to obtain the time period T
Calculate the frequency of the wave using $f = \frac{1}{T}$

Worked example

A cathode-ray oscilloscope (c.r.o.) is used to display the trace from a sound wave. The time-base is set at 7 µs mm^-1. WE - CRO question image, downloadable AS & A Level Physics revision notes What is the frequency of the sound wave?

A 2.4 Hz B 24 Hz C 2.4 kHz D 24 kHz

Answer: C

Step 1: Determine the period of the wave:

One full wave covers three horizontal squares, which is a distance of 6 cm
There are 60 mm in 6 cm
- Each mm represents a time of 7 μs
- Time period, T = 60 × (7 × 10⁻⁶) = 4.2 × 10⁻⁴s

Step 2: Use the frequency equation to determine frequency:

Recall the frequency equation

$f = \frac{1}{T} = \frac{1}{4.2 \times 10^{- 4}} = 2380 Hz$

Recall that 1000 Hz = 1 kHz and the answer must be given to 2 significant figures

$f = 2.4 kHz$

Examiner Tip

The time-base setting varies with units for seconds (commonly ms) and the unit length (commonly mm). Unit conversions are very important for the calculation of the time period and frequency

Cathode-Ray Oscilloscope (CIE A Level Physics)

Revision Note