Measure frequency and amplitude on a CRO
Use oscilloscope time-base and vertical gain scales to read waveform period, frequency and amplitude.
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 7. These original wave examples and diagrams are conceptual study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use frequency, period, wavelength, amplitude, speed and trigonometric ratios with consistent units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which CRO control or scale gives time per horizontal division?
What you will learn
- Explain what the CRO time-base and y-gain scales represent
- Determine frequency from the displayed period
- Determine signal amplitude from the vertical scale
Read the horizontal time scale to find period
A cathode-ray oscilloscope (CRO) displays a changing signal against time. The time-base states the time represented by each horizontal division. Measure the horizontal divisions for one complete cycle to find period T.
Frequency is f = 1/T. Use a cycle-to-cycle interval rather than measuring from a peak to an arbitrary point. If several cycles fit clearly on the display, measure across several and divide by the number of cycles to reduce reading uncertainty.
Read vertical deflection with the y-gain
The y-gain gives the voltage represented by each vertical division. Multiply the centre-to-peak deflection by the y-gain to determine amplitude. Peak-to-peak deflection is twice amplitude for a centred symmetric signal.
Record the scale settings and units before reading divisions. A trace shifted above or below the centre line may have a non-zero mean; amplitude is the distance from that mean level to a peak, not from the screen edge.
Pause and explain
If a signal has a 4.0 ms period, what is its frequency?
Worked example
One cycle spans 2.5 horizontal divisions at 2.0 ms per division. The waveform peak is 2.0 divisions above its centre line at 0.50 V per division. Find frequency and amplitude.
Show the worked solution
- T = 2.5 × 2.0 ms = 5.0 ms = 0.0050 s.
- f = 1/T = 200 Hz.
- Amplitude = 2.0 × 0.50 V = 1.0 V.
Answer Frequency 200 Hz; amplitude 1.0 V.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A trace shows 3.0 cycles across 12 horizontal divisions. The time-base is 1.0 ms per division. Find the period and frequency.
Give me a hint
Find the divisions per cycle, then multiply by time per division.
Compare my reasoning
- Divisions per cycle = 12/3.0 = 4.0.
- T = 4.0 × 1.0 ms = 4.0 ms = 0.0040 s.
- f = 1/T.
T = 4.0 ms; f = 250 Hz.
Look for these in your work
- Cycles used to average period
- Milliseconds converted
- Frequency reciprocal correct
A CRO trace reaches 3.0 vertical divisions above its centre line. The y-gain is 0.20 V per division. Find amplitude and peak-to-peak voltage for a centred symmetric wave.
Give me a hint
Amplitude is centre-to-peak; peak-to-peak is twice that value.
Compare my reasoning
- Amplitude = 3.0 × 0.20 V.
- Amplitude = 0.60 V.
- Peak-to-peak = 2 × 0.60 V.
Amplitude 0.60 V; peak-to-peak voltage 1.2 V.
Look for these in your work
- Vertical scale applied
- Amplitude distinguished from peak-to-peak
- Units stated
A learner counts 2.0 cycles across 8.0 divisions at 0.50 ms per division, but forgets the scale setting and reports 4.0 kHz. Explain the error and give the correct frequency.
Give me a hint
The divisions per cycle must be converted to a real time using the time-base.
Compare my reasoning
- There are 4.0 divisions per cycle.
- T = 4.0 × 0.50 ms = 2.0 ms = 0.0020 s.
- f = 1/0.0020 = 500 Hz; the incorrect result treated divisions as a time unit.
Correct frequency is 500 Hz. The learner omitted the time-base setting, which converts four divisions into a 2.0 ms period.
Look for these in your work
- Scale setting applied
- Period in seconds found
- Error explained
Common mistakes
- Using divisions without multiplying by the scale setting
- Taking peak-to-peak deflection as amplitude
- Forgetting to convert milliseconds to seconds before calculating hertz
What can you explain now?
Which CRO control or scale gives time per horizontal division?
Compare with the explanation
Time-base
The time-base maps horizontal screen divisions to time.
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Make it stick
Why can measuring several full cycles improve a CRO frequency estimate?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.