AQA A Level Physics复习笔记7.10.2 The Operation of an Oscilloscope

• 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)

• 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

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          C     2.4 kHz          D     24 kHz

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