The Operation of an Oscilloscope (AQA A Level Physics): Revision Note

Oscilloscope as a DC & AC Voltmeter

• A Cathode-Ray Oscilloscope (CRO) is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits

  • It can therefore be used as an a.c and d.c voltmeter

• An a.c voltage on an oscilloscope is represented as a transverse wave

  • Therefore you can determine its frequency, time period and peak voltage

• 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)

Diagram of Cathode-Ray Oscilloscope display showing wavelength and time-base setting

• The period of the wave can be determined from the time-base

  • This is how many seconds each division represents measured commonly in s div^-1 or s cm^-1

• The frequency is then determined through:

• Where:

  • T = time period (s)

• Dividing the total time by the number of wavelengths will give a value for T 

  • Use as many wavelengths shown on the screen as possible to reduce uncertainties

C.R.O Controls for An A.C Waveform

• Time-base

  • When the time-base is switched off, only a vertical line on the voltage-gain axis is seen with its relevant amplitude

  • When the time-base is switched on, a wave will appear across the whole screen and the time period can be measured

  • This control has units of time cm^-1 or time div^-1 and has a range of 100 ms – 1 μs per cm, or division

• Voltage-gain (sensitivity)

  • This controls the vertical deflection, or amplitude, of the wave

  • The peak voltage (V₀) is the maximum vertical displacement measured from the time axis

  • The peak-to-peak voltage is the vertical displacement between the minimum and maximum values of voltage

  • When the voltage-gain is switched off, only a horizontal line on the time axis will be seen

  • This control has units of volts cm^-1 or volts div^-1

• For a d.c waveform, only a horizontal line is displayed at the relevant voltage

  • The time-base settings are irrelevant since there is no time period

  • The voltage-gain setting is relevant since this is used to read the value of the d.c voltage

Examples of an alternating and direct voltage on a CRO with and without the time base