Tutorial 1 B - The Cathode Ray Oscilloscope

Learning Objective

Recall that the CRO is used to display AC waveforms.

Recognise that it plots a voltage-time graph.

Measure peak to peak voltage and the time period from the CRO

Derive quantities such as Vrms and frequency.

Key Questions

How do we display AC waveforms?

How do we use the CRO?

 

The CRO is the electrical and electronic engineer's best friend.  So it's worth getting to know the instrument.

The CRO is connected in exactly the same way as a voltmeter, i.e. in parallel with a component. 

This is a typical display of a sinusoidal waveform on the CRO screen:

  • We measure the voltage on the vertical  axis. 

  • We can adjust the sensitivity by turning the knob marked y-gain.

  • The horizontal direction is determined by the time base setting.  It controls the number of times the beam is swept across the screen every second.

The screen has a grid of 1 cm squares and the y-gain is marked in volt cm-1.  The time-base is marked in ms cm-1 or ms cm-1.

This CRO screen shows the idea:

The horizontal scale is the time period, measured in seconds, or, more usually, in milliseconds (ms).

1 ms = 1 10-3 s

We count how many squares there are on the horizontal axis, then calculate the time by multiplying the number of squares by the time per square.

T = length (cm) time-base (s cm-1)

This time is the period.  To get the frequency, we need to take the reciprocal of the period.

 

Question 1

Show how the CRO can be used as:

(a) A voltmeter;

(b) an ammeter

Answer

Question 2

The CRO screen below shows a sinusoidal waveform:

The time base is set at 2 ms/cm and the y gain at 0.5 V/cm

(a) What is the peak to peak voltage?                                                                               

(b) What is the peak voltage?                                                                                           

(c) What is the rms voltage?                                                                                              

(d)  What is the period?                                                                                                    

(e)  What is the frequency?                                                                                              

Answer

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