Reading a cathode-ray oscilloscope

8 minAS Level7.1.3

By the end of this lesson

By the end you can use a CRO's time-base to find the period and frequency of a signal, and its y-gain to find the amplitude.

Key to the symbols

The main symbols used in this lesson:

Symbol	Meaning
$f$	frequency

A cathode-ray oscilloscope (CRO) draws a graph of voltage against time on a screen ruled in squares.

The screen itself does not know what a square means. Two settings tell you:

the time-base sets how much time each horizontal square represents, in seconds per division ( $\text{s div}^{-1}$ );
the y-gain sets how much voltage each vertical square represents, in volts per division ( $\text{V div}^{-1}$ ).

Read the number of squares, multiply by the setting. That is the whole method.

Definition

Time-base and y-gain

Time-base: the time represented by one horizontal division on the CRO screen.

Y-gain: the voltage represented by one vertical division on the CRO screen.

For a periodic signal:

period $T$ = horizontal length of one complete cycle (in divisions) × time-base;
frequency $f = 1/T$ ;
amplitude = vertical height from the equilibrium line to a peak (in divisions) × y-gain.

Worked example: Worked example: frequency and amplitude from the screen

A CRO trace shows one complete cycle across $4.0$ horizontal divisions. The time-base is $2.0\ \text{ms div}^{-1}$ .

The peak of the trace is $3.0$ divisions above the centre line, and the y-gain is $0.50\ \text{V div}^{-1}$ .

Find the period, frequency and amplitude of the signal.

Have a go first?

Quick check

CalculateQuestion 1 · Apply

CalculateQuestion 2 · Recall

Answer the 3 questions above to continue.