Reading an Oscilloscope | CAIE 9702 Paper 2 Notes

Paper 2

Physics9702

Reading an Oscilloscope

An oscilloscope (c.r.o.) draws a voltage against time, so a sound picked up by a microphone shows as a repeating trace.

The time-base sets the time per horizontal division; the y-gain sets the voltage per vertical division.

Reading frequency off the screen

Count how many divisions one full cycle takes across the screen.
Multiply by the time-base to get the period $T$ , then use $f = 1/T$ .

A sound wave on an oscilloscope. One full cycle spans 3 divisions here; with the time-base at 1 ms per division that is a period of 3 ms, so f = 1/T ≈ 333 Hz.

A tuning fork, microphone and cathode-ray oscilloscope connected on a laboratory bench. — The microphone converts pressure variations into voltage. The c.r.o. displays that voltage against time.

Worked example

Frequency from a trace

One complete wave spans 3.0 divisions on the screen. The time-base is set to 1.0 ms per division. Find the frequency.

Period: $T = 3.0 \times 1.0\text{ ms} = 3.0\text{ ms} = 3.0\times10^{-3}\text{ s}$ .
$f = \dfrac{1}{T} = \dfrac{1}{3.0\times10^{-3}} \approx 333\text{ Hz}$ .

Answer

f \approx 330\text{ Hz}

Reading voltage and amplitude

The y-gain turns vertical divisions into volts. Count the divisions from the middle line up to a peak and multiply by the y-gain to get the peak voltage.

Common mistake

One division is not one cycle. Read where the trace repeats to find the period, and check the time-base units: a setting in microseconds per division changes the answer by a factor of a thousand.

Your turnCRO skill check

One cycle covers 2.5 horizontal divisions and the trace has an amplitude of 3.0 vertical divisions. The time-base is 300 μs per division and the y-gain is 40 V per division. Calculate the frequency and voltage amplitude.

Show worked answer

$T=2.5(300\times10^{-6})=7.5\times10^{-4}\text{ s}$ .
$f=1/T=1.33\times10^3\text{ Hz}$ .
Voltage amplitude $=3.0(40)=120\text{ V}$ .

Answer

f=1.3\text{ kHz},\quad V_0=120\text{ V}

Reading an Oscilloscope

An oscilloscope (c.r.o.) draws a voltage against time, so a sound picked up by a microphone shows as a repeating trace.

The time-base sets the time per horizontal division; the y-gain sets the voltage per vertical division.

Reading frequency off the screen

Count how many divisions one full cycle takes across the screen.
Multiply by the time-base to get the period $T$ , then use $f = 1/T$ .

A sound wave on an oscilloscope. One full cycle spans 3 divisions here; with the time-base at 1 ms per division that is a period of 3 ms, so f = 1/T ≈ 333 Hz.

Worked example

Frequency from a trace

One complete wave spans 3.0 divisions on the screen. The time-base is set to 1.0 ms per division. Find the frequency.

Period: $T = 3.0 \times 1.0\text{ ms} = 3.0\text{ ms} = 3.0\times10^{-3}\text{ s}$ .
$f = \dfrac{1}{T} = \dfrac{1}{3.0\times10^{-3}} \approx 333\text{ Hz}$ .

Answer

f \approx 330\text{ Hz}

Reading voltage and amplitude

The y-gain turns vertical divisions into volts. Count the divisions from the middle line up to a peak and multiply by the y-gain to get the peak voltage.

Common mistake

One division is not one cycle. Read where the trace repeats to find the period, and check the time-base units: a setting in microseconds per division changes the answer by a factor of a thousand.

Your turnCRO skill check

Show worked answer

$T=2.5(300\times10^{-6})=7.5\times10^{-4}\text{ s}$ .
$f=1/T=1.33\times10^3\text{ Hz}$ .
Voltage amplitude $=3.0(40)=120\text{ V}$ .

Answer

f=1.3\text{ kHz},\quad V_0=120\text{ V}