Signal Generators & Oscilloscope (OCR A Level Physics)

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Signal Generators & Oscilloscope

Signal Generator

  •  A signal generator is an electronic test instrument used to create repeating or non-repeating waveforms
    • They can be adjusted for different shapes and amplitudes

  • These are often used for designing and repairing electronic devices, to check they are working as expected
  • Signal generators are used to create signals to then show on oscilloscopes

signal-generator, IGCSE & GCSE Physics revision notes

A signal generator can be used to create signals for a CRO

Cathode-Ray 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

Oscilloscope, downloadable AS & A Level Physics revision notes

A cathode-ray oscilloscope displays the signal generated by the signal generator

  • 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)

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

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

C.R.O Controls for a D.C waveform

  • 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

CRO Examples 1, downloadable AS & A Level Physics revision notesCRO Examples 2, downloadable AS & A Level Physics revision notesCRO Examples 3, downloadable AS & A Level Physics revision notes

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

Generating & Measuring Waves

  • Waves can be generated and measured using:
    • A microphone and loudspeaker
    • A ripple tank
    • A vibration transducer
    • A microwave / radio wave source

Microphone and Loudspeaker

  • A microphone and loudspeaker can be used to create sound waves
  • Microphones turn sound into electrical signals

moving-coil-microphone, IGCSE & GCSE Physics revision notes

Sound waves entering a microphone

  • When sound waves reach the microphone, for example, generated by a person's voice, the pressure variations cause the diaphragm to vibrate
  • This in turn causes the coil to move back and forth, through the magnetic field
  • The coil, therefore, cuts through the magnetic field lines, inducing an alternating e.m.f in the coil

 

  • Loudspeakers turn electrical signals into sound

 

loudspeaker-cross-section, IGCSE & GCSE Physics revision notes

Cross-section of a loudspeaker

  • An alternating current pass through the coils of the loudspeaker, which then creates a changing field around the coil
  • This changing field interacts with the field from the permanent magnet, exerting a force on the coil
  • This makes the coil and the speaker cone oscillate, making the air particles oscillate and therefore creating sound waves

Ripple Tank

  • Ripple tanks are used to create water waves

Ripple tank, downloadable AS & A Level Physics revision notes

Ripple tank creating wavefront seen on a screen

  • In a ripple tank, a motorised wooden straight-edged bar produces plane (straight) waves while a small dipper produces circular waves
  • When a light is shone from above, the bright bands seen on the screen below the tank show the wave crests (wavefronts)
  • The diagram below shows how the wavelengths differ with frequency in a ripple tank
    • The higher the frequency, the shorter the wavelength
    • The lower the frequency, the longer the wavelength

Ripple tank vibrations, downloadable AS & A Level Physics revision notes

Ripple tank patterns for low and high frequency vibrations

Vibration Transducer

  • Vibration transducers can detect vibrations in machines and transform them into a signal
    • These signals are then sent to a digital indicator to test machines

  • These are particularly useful in industry to see how machines behave under vibrations before any damage can occur

Vibration Transducer, downloadable AS & A Level Physics revision notes

A vibration transducer

Microwave / Radio Source

  • A microwave or radio source produces microwaves or radio waves to be used for diffraction and interference experiments
  • Radio waves can be produced by connecting an antenna to a high frequency alternating current power source

Radio waves are produced by high-frequency alternating currents and induce similar currents when they are received

  • The charge, oscillating up and down the antenna, produces radio waves that can be absorbed by similar antennae some distance away
  • When absorbed, the radio waves induce a similar alternating current in the receiving antenna, which can then be detected

 
  • A microwave source could be produced by artificial devices such as:
    • Circuits
    • Masers (microwave lasers)
    • Microwave ovens
    • Microwave signal generators

  • Or, from natural sources such as the Sun or atoms and molecules

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.