High School Physics

Measuring frequency & voltage of an Alternating Current (AC) with CRO

An oscilloscope can be used to measure the frequency and voltage of an alternating current. There are two types of oscilloscopes. The traditional cathode-ray oscilloscope (CRO) uses an electron beam.

The alternative is a digital oscilloscope, which is likely to be much more compact and which can store data and display the traces later. Practical Activity with CRO explains how to measure the frequency and voltage of an alternating current.

How to use the controls of a CRO?

The X-shift and the Y-shift controls move the whole trace in the x-direction and the y-direction, respectively.

The two controls that are very important are the time-base and the Y-gain, or Y-sensitivity.

Figure 1: The controls of a typical CRO

It is important to remember that on the CRO screen, the x-axis represents time, and the y-axis represents voltage.

Time-base control

You can see in Figure 1 that the time-base control has units marked alongside.

Let us suppose that this time-base control reads 5 ms/cm, although it might be 5 ms/division. This shows that 1 cm (or 1 division) on the x-axis represents 5 ms.

Varying the time-based control alters the speed with which the spot moves across the screen.

If the time-base is changed to 1 ms/cm, then the spot moves faster and each centimetre represents a smaller time.

Y-gain control

The Y-gain control has a unit marked in volts/cm, or sometimes volts/division.

If the actual marking is 5 V/cm, then each centimetre on the y-axis represents 5 V in the applied signal.

Determining frequency and amplitude (peak value of voltage) from CRO trace

If you look at the CRO trace shown in Figure 2, you can see that the amplitude of the waveform, or the peak value of the voltage, is equivalent to 2 cm and the period of the trace is equivalent to 4 cm.

If the Y-gain or Y-sensitivity setting is 2 V/cm, then the peak voltage is 2 × 2 = 4 V. This is the amplitude of the voltage.

If the time-base setting is 5 ms/cm, then the period is 4 × 5 = 20 ms = 0.02 sec

In the example: frequency = 1/Time-period = 1/0.02 = 50 Hz

figure 2: A typical trace on the screen of a CRO

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