Oscilloscope

If you have ever looked at a graph showing alternating voltage against time, you may wonder how this sinusoidal wave-like shape was created using a typical analogue voltmeter. In truth, you can't record the data needed to make this graph unless you use an apparatus called an oscilloscope. We can analyse most types of waves, including sound waves, with an oscilloscope. We simply need to convert the signals into a format that can be understood by the oscilloscope.

Definition of oscilloscope

We need to begin to understand what an oscilloscope is and how it operates. The best place to start is with the definition.

Types of oscilloscopes

There are two main types of oscilloscopes; a digital oscilloscope or digital storage oscilloscope (DSO) and a cathode-ray oscilloscope (CRO). They both have the same function but carry it out in different ways.

Cathode Ray Oscilloscope (CRO)

The cathode ray oscilloscope makes use of a cathode ray tube that fires electrons onto the screen of the oscilloscope. The screen is coated with phosphorus that is excited when electrons are incident upon it and releases energy in the form of light which we observe as a bright dot on the screen. If a signal is passed through the CRO (AC voltage, for example), then the electron beam from the cathode moves relative to imposed electric and magnetic fields.

The electrons are then deflected by the electric and magnetic fields in a region between two deflecting plates. The moving electrons strike the screen at different points and hence a pattern representing the voltage will be created (it would be sinusoidal in the case of AC). Any adjustments to the input voltage or frequency will change the strength of the applied fields, creating a different waveform on the screen. The waveform can be used to make direct measurements of voltage ($\mathrm{mV}$), frequency ($\mathrm{MHz}$), period ($\mathrm{ms}$) and other electrical quantities. The figure below is an example of a cathode ray oscilloscope.

An image of a typical cathode ray oscilloscope (CRO) with a signal represented on the phosphor-coated screen on the left. On the right-hand side is the controls that allow adjustment of the scale of the image.

Digital Storage Oscilloscope (DSO)

The digital storage oscilloscope is the more modern of the two types of oscilloscopes. It takes an analogue signal as input and uses sophisticated signal-processing software to convert this into a digital signal. It can store the waveform pattern in memory and convert it into a digital image that can be viewed on an LCD screen. Deflecting plates, electric fields and magnetic fields are hence not required. The DSO can also be connected to a printer to obtain a print-out of any signal stored in memory, or the image could even be stored on a USB drive. As with the CRO, the waveform in the DSO can be used to make measurements of voltage$\left(\mathrm{mV}\right)$, frequency$\left(\mathrm{MHz}\right)$etc. The image below shows a typical DSO.

An image of a modern digital storage oscilloscope with a signal displayed on an LCD screen. These oscilloscopes are slowly replacing the typical CROs due to their ability to store and transfer data.

Uses of the oscilloscope

There are many uses of the oscilloscope but we will mention only three below; testing circuits, electrocardiograms and analysing sound waves.

Uses of the oscilloscope: Testing Circuits

The oscilloscope is a device that is quite often found in the laboratories of schools and is used to demonstrate the sinusoidal behaviour of alternating currents (AC). There is, however, a much more practical use for an oscilloscope; to test an electric circuit. Oscilloscopes can be used to determine the location of a fault or simply to test currents in and out of different points of a circuit. This is done by using the oscilloscope to measure the peak voltage, period, frequency and other electric quantities of an AC power source.

The illustration below shows what the display on an oscilloscope looks like.

An illustration of the output on an oscilloscope. The divisions on the screen serve as equal intervals on a graph, allowing the voltage to be measured, Public Domain Vectors

The line shown on the screen represents a typical sinusoidal/alternating voltage and the divisions on the screen can be used to measure that voltage at different times.

Uses of the oscilloscope: Electrocardiograms (ECGs)

Electric circuits are not the only sources of electric currents; our bodies produce tiny electric currents that can be measured and monitored using oscilloscopes. In a hospital, electrodes of an electrocardiogram (ECG) are connected to patients whose frequency of heartbeats is to be monitored. Each heartbeat of a patient produces a small, but measurable current which can be detected by a digital oscilloscope.

The waveform is created by measuring the tiny voltages across the heart during each heartbeat and so the doctor is reading a real-time graph of voltage vs time. The separation between the voltage peaks is a measurement of the frequency of heartbeats. ECGs can also be used as a tool to predict possible future problems with the heart. ECGs do not produce sinusoidal graphs since heartbeats have a relaxation time in between beats and a complex beat signal, as is evident in the figure below.

An ECG waveform on an oscilloscope screen. The pulses on the screen are periodic due to the natural rhythm of a typical heartbeat.

Uses of the oscilloscope: Sound Waves

As we already know, sound waves are caused by vibrating air molecules and do not require an electric current to form. We can, however, measure sound wave properties using an oscilloscope. An audio signal can be converted via a transducer to an electrical signal (a series of voltages) which the oscilloscope can decode and represent on the screen.

CROs and digital oscilloscopes can both be used to decode and represent sound waves. The oscilloscope can be used to measure the amplitude, frequency, and period of a sound wave. The diagram below shows a setup consisting of a sound-wave generator that creates sound waves transmitted via a speaker and detected by an oscilloscope which gives a visual representation of the sound waves.

A setup that includes a sound-wave generator creating sound waves emitted by a speaker and represented on-screen by an oscilloscope. This simple setup can be used to determine the properties of the sound waves produced, Wikimedia Commons CC BY-SA 3.0

Oscilloscope example and graph