1.3.7 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

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

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)

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

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

• 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

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

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 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 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

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