3.7.5.5
Reading AC values from an oscilloscope trace
Alternating Currents — AQA A-Level Physics
- An oscilloscope displays voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. on the y-axis and time on the x-axis. The trace shows the waveform in real time.
- Two settings control what you see: the Y-gain (volts per division on the vertical axis) and the time base (seconds per division on the horizontal axis).
Reading peak voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference. from the trace
- Count the number of vertical divisions from the centre line (zero) to the top of the wave. This gives the peak amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m). in divisions.
- Multiply by the Y-gain setting to get V₀ in volts.
- Alternatively, count peak-to-peak (top of wave to bottom of wave), multiply by Y-gain, then divide by 2.
- To get V_rms: divide V₀ by √2.
Reading periodThe time taken for one complete oscillation or wave cycle. Measured in seconds (s). and frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). from the trace
- One complete cycle goes from a point on the wave back to the same point moving in the same direction. Easiest to measure between two adjacent zero crossings going the same way (e.g. both going up).
- Count the number of horizontal divisions for one complete cycle.
- Multiply by the time base setting to get the periodThe time taken for one complete oscillation or wave cycle. Measured in seconds (s). T in seconds.
- Calculate frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz).: $f = 1/T$.
Worked Example
An oscilloscope displays an AC signal. The Y-gain is set to 5.0 V/div. The time base is 2.0 ms/div. The trace shows a peak height of 3.2 divisions above the centre line, and one complete cycle spans 4.0 divisions. Calculate the frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). and the RMS voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference..
Show Solution
1
Find the peak voltage
- Peak height: 3.2 divisions
- Y-gain: 5.0 V/div
2
Convert to RMS voltage
$$V_{\text{rms}} = \frac{V_0}{\sqrt{2}} = \frac{16.0}{\sqrt{2}}$$
$$= 11.3 \text{ V}$$3
Find the periodThe time taken for one complete oscillation or wave cycle. Measured in seconds (s).
- One cycle: 4.0 divisions
- Time base: 2.0 ms/div
4
Calculate the frequency
$$f = \frac{1}{T} = \frac{1}{8.0 \times 10^{-3}}$$
$$= 125 \text{ Hz}$$Answer
Frequency: $f = 125$ Hz. RMS voltage: $V_{\text{rms}} = 11.3$ V.
Examiner Tips and Tricks
- When reading an oscilloscope trace, be precise about where the waveform crosses the grid lines.
- A half-division error in peak height can cost you the mark.
- If the peak falls between grid lines, estimate to the nearest 0.1 division.
Common Mistake
MEDIUM
Students often: Measuring the period as half a cycle (peak to trough) instead of a full cycle.
Instead: A full cycle goes from one peak to the next peak, or from one upward zero-crossing to the next upward zero-crossing. Peak to trough is only half a cycle. If you measure half a cycle, your period is halved and your frequency is doubled.
Instead: A full cycle goes from one peak to the next peak, or from one upward zero-crossing to the next upward zero-crossing. Peak to trough is only half a cycle. If you measure half a cycle, your period is halved and your frequency is doubled.