Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Using a CRO to show different waveforms

Demonstration

The CRO (cathode ray oscilloscope) can show sinusoidal, square and triangular waveforms.

Apparatus and materials

Oscilloscope

Signal generator, with selectable waveform output

Loudspeaker

Leads, 4 mm, 4

Health & Safety and Technical notes


A cathode-ray tube requires voltages classified as 'hazardous live'. The casing nearly always has ventilation holes, some of which may give access to these voltages. Classes should be warned not to poke anything through the holes.

A video showing the use of an oscilloscope is freely available at the National STEM Centre eLibrary. There is also a video showing the use of a signal generator.


Procedure


Apparatus set-up

a Set the oscilloscope with the volt/cm control at 1, the time base control at 1 ms/cm and the AC-DC switch to AC.

b Set the signal generator to 2 V, 50 Hz, sinusoidal output. Connect the high-impedance output terminals to the input of the CRO.

c Show the effects of varying the output voltage of the signal generator (amplitude changes) and the frequency (period changes).

d Switch between the different output waveforms (sinusoidal, square and triangular) to show these.

e Connect a loudspeaker to the low-impedance output of the signal generator. Repeat steps b to d above so that students can hear the effects and relate these to the traces on the screen.

f If your signal generator can provide a low frequency output (less than 1 Hz), show this on the CRO also. Set the time-base to its slowest setting, and watch the spot as it moves slowly up and down.

 

Teaching notes


1 It is useful if students can appreciate that sinusoidal waveforms are not the only shape which an alternating voltage can take. Square waveforms are at the heart of modern digital electronics. 

2 You could also show the 'Sweep' output of one CRO on the screen of another. This is a saw-tooth waveform, used to move the spot at a steady rate left to right across the screen, returning very rapidly to the left at the end of each trace.

This experiment was safety-checked in January 2007