Uses of an oscilloscope

O Level / IGCSE Physics · Electricity and magnetism. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

An oscilloscope draws a graph of p.d. against time on its screen. The p.d. being measured moves the spot up and down, and the timebase moves the spot steadily from left to right.

The screen is marked with a grid of divisions. The Y-gain setting tells you how many volts each vertical division stands for. The timebase setting tells you how much time each horizontal division stands for.

So to measure a p.d., count vertical divisions and multiply by the Y-gain. To measure a time, count horizontal divisions and multiply by the timebase. Changing a setting changes the size of the picture on the screen, not the signal itself.

Rules to remember

Common mistake

Using the whole height of the wave, from trough to peak, as the peak p.d. Measure from the centre line, or halve the trough-to-peak height.

Worked example

A wave on an oscilloscope reaches 2.5 divisions above the centre line, and one complete wave is 8.0 divisions long. The Y-gain is 4 V per division and the timebase is 0.5 ms per division. What are the peak p.d. and the frequency?

  1. Peak p.d. = 2.5 × 4 = 10 V.
  2. Period = 8.0 × 0.5 ms = 4.0 ms = 0.0040 s.
  3. Frequency = 1 ÷ period = 1 ÷ 0.0040 = 250 Hz.

Answer: Peak p.d. 10 V, frequency 250 Hz

Practice questions

Try each one, then open the answer.

1. A 250 Hz signal is displayed with the timebase set to 1.0 ms per division. How many complete waves appear across the 10 horizontal divisions?

  1. A
    0.25
  2. B
    2.5
  3. C
    4
  4. D
    25
Show answer

Answer: B. Period = 1 ÷ 250 = 0.004 s = 4.0 ms, so one wave takes 4 divisions. 10 divisions show 10 ÷ 4 = 2.5 waves. 4 is the number of divisions per wave, not the number of waves.

2. An oscilloscope trace shows exactly 2 complete cycles across 10 horizontal divisions. The timebase is set to 0.1 ms per division. What is the frequency of the signal?

  1. A
    200 Hz
  2. B
    1000 Hz
  3. C
    2000 Hz
  4. D
    5000 Hz
Show answer

Answer: C. The 10 divisions last 10 × 0.1 ms = 1.0 ms and hold 2 cycles, so the period is 0.5 ms = 0.0005 s. Frequency = 1 ÷ 0.0005 = 2000 Hz. Taking 1.0 ms as one period gives 1000 Hz.

3. An alternating p.d. is connected to the Y-input of an oscilloscope and the timebase is switched off. What is seen on the screen?

  1. A
    a sine wave
  2. B
    a horizontal line
  3. C
    a vertical line
  4. D
    a single dot at the centre
Show answer

Answer: C. With no timebase the spot does not move sideways, but the alternating p.d. moves it rapidly up and down, which the eye sees as a vertical line. Its length shows the peak-to-peak p.d.

More questions on this topic

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