Signal Generators & Oscilloscope (OCR A Level Physics)
Revision Note
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
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
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
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