21.1 What an oscilloscope displays

An oscilloscope displays voltage on the vertical axis against time on the horizontal axis. A changing voltage moves the trace up and down while the time base sweeps it across the screen. A microphone, signal generator, transformer secondary or other sensor can provide the input.

A traditional cathode-ray oscilloscope and a digital storage oscilloscope differ internally, but basic controls and measurements are similar. The display is divided into squares or divisions.

21.2 Y-gain or volts per division

The vertical sensitivity states volts per division. If a peak is 3.2 divisions above the zero line and the setting is 0.50 V/div, peak voltage is 1.6 V. Peak-to-peak voltage is the vertical distance from maximum positive to maximum negative multiplied by V/div.

First identify the zero reference. For a centred symmetrical a.c. trace, peak-to-peak is twice peak amplitude. For a shifted or non-symmetrical signal, measure directly rather than assuming.

21.3 Time base

The time-base setting gives seconds, milliseconds or microseconds per division. Measure the horizontal length of one complete cycle and multiply by time per division to find period T.

Frequency is f = 1/T. Convert time to seconds before calculating hertz. Measuring several cycles and dividing by their number reduces uncertainty when the trace is small or thick.

21.4 Trigger and stable traces

The trigger starts each sweep at the same point on a repeating waveform, making the trace appear stationary. Without suitable triggering, the pattern may drift horizontally and become difficult to measure.

Adjust trigger level and slope, then time base and vertical sensitivity to use much of the screen without clipping. A large well-positioned trace gives lower percentage reading uncertainty.

21.5 d.c. and a.c. traces

A steady d.c. voltage gives a horizontal line displaced from zero. Reversing polarity moves it to the opposite side. A sinusoidal a.c. voltage crosses the zero line repeatedly, showing alternating polarity.

D.c. coupling shows both average and changing components. A.c. coupling may remove the steady offset and show only variation. At O Level, interpret the displayed trace according to settings provided.

21.6 Comparing signals

A trace with greater vertical amplitude at the same Y-gain has greater peak voltage. A trace with shorter horizontal period at the same time base has higher frequency. Settings must be considered before comparing two displays.

Two traces that look the same size may represent different voltages or frequencies if control settings differ. Always multiply measured divisions by the scale.

21.7 Practical measurement and uncertainty

Use the greatest safe sensitivity and fastest suitable time base that keep the waveform visible. Align the trace with grid lines, measure from equivalent points such as peak to peak, and average across several periods.

Uncertainty is often about half the smallest readable division at each endpoint. Measuring many cycles reduces the fractional uncertainty in period.

21.8 Electrical safety

An oscilloscope input has voltage limits and may share an earth connection. Incorrect connection to mains can create a short circuit or expose dangerous voltage. School work must use isolated low-voltage signals and approved probes.

Never connect an ordinary oscilloscope directly to mains unless specialist equipment and procedures are used by qualified personnel. Check probe attenuation and ground connection before measurement.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. A peak spans 3 divisions at 0.5 V/div. Find peak voltage.
2. One period spans 5 divisions at 2 ms/div. Find frequency.
3. What is the purpose of the trigger?