7.10.2 The Operation of an Oscilloscope

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
• 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

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.

What is the frequency of the sound wave?

A    2.4 Hz          B     24 Hz             2.4 kHz             24 kHz

Exam Tip

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

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