Electricity Tutorial 10 - The Cathode Ray Oscilloscope

 Contents

The CRO

The cathode ray oscilloscope is the electronics engineer's best friend.  It is an instrument that displays alternating wave-forms.  These can be simple, such as the sine-wave in alternating current.  Or the waveforms can be complex, such as the pattern made by someone's voice.  Cathode ray oscilloscope is a bit of a mouthful, so it is shortened to CRO.

You are not expected to describe how it operates, but you are expected to be able to set it up and interpret displays on the screen.

The CRO is connected in exactly the same way as a voltmeter, i.e. in parallel with a component.  The input resistance is very high indeed and the electron beam acts as a pointer of negligible inertia.  It is also robust and not easily damaged by overloading.

The CRO can be used as a DC voltmeter.  We get a horizontal line or a dot, depending whether the time base is on.   If it is used as an AC voltmeter, it will show the sinusoidal waveform.  It will, of course show more complex waveforms of audio signals, but we won't worry about these. The CRO can also be used to measure the voltage across a resistance of known value.  Therefore it can be used as an ammeter. The CRO is shown in the picture below: The most important controls that we use are:

• The vertical sensitivity, voltage gain, or y-gain setting, calibrated in volts per centimetre (V cm-1).

• The time base, in seconds per centimetre (s cm-1).

The CRO is a perfect voltmeter as its input resistance is very high indeed. The horizontal display represents the period.  It is NOT the wavelength.

Remember:

• We measure the voltage on the vertical  axis.  We can adjust the sensitivity by turning the knob marked y-gain or voltage gain.

• The horizontal direction is determined by the time base setting.   We can change this by using the time base knob.

As well as analysing the waveform, there are two measurements we can make with the CRO:

• We can determine the peak voltage of the AC waveform shown below.

• We can also read the period, which in turn allows us to work out its frequency. Notice that:

• The peak to peak voltage is 12.8 V.  Often engineers read the peak to peak voltage off the CRO as the determination of the 0 level is not always easy.  The peak to peak voltage is half of the peak to peak voltage.

• The root mean square voltage, which we use in electrical calculations, is the peak voltage divided by Ö(2)

• Therefore the Vrms = 6.4 ¸ Ö2 = 4.5 V

 Look at the CRO display: The time base is set at 2 ms/cm and the y gain at 0.5 V/cm (a) What is the peak to peak voltage? (b) What is the peak voltage? (c) What is the rms voltage? (d)  What is the period? (e)  What is the frequency? Different Kinds of Waves on the CRO

Not every waveform displayed on a CRO is sinusoidal.  The picture below shows a square waveform: With a square wave, there is no RMS voltage.

 The time base is set at 2 ms cm-1 and the voltage gain is 2.0 V cm-1. (a) What is the period? (b) What is the maximum voltage? (c) What is the minimum voltage? (d) Is this an alternating waveform?

Some waveforms are not alternating, but unidirectional.  Now look at this picture: The settings on the CRO are the same as they are in Question 2. (a) What is the maximum voltage? (b) What is the minimum voltage? (c) Is this an alternating waveform?

There are more waveforms that are not sinusoidal.  This is a triangular alternating waveform: This waveform is a saw-tooth waveform.  It is unidirectional. Note that the first set of waves has a positive ramp.  The second set has a negative ramp.  You have to have one or the other.

The picture below shows a complex alternating waveform. To study a complex waveform, you really need to have a storage oscilloscope, which is one that has a memory.  A normal CRO will display a waveform for as long as it is generated.  As soon as the source of waves is turned off, the waveform will no longer be displayed.

Nowadays it's possible to buy a CRO sensor that connects to a computer in the same way as a data logger. Before you read this, you need to understand how electrons behave in magnetic fields (Magnetic Fields Tutorial 3) and electric fields (Fields Tutorial 4). Diagram from Wikimedia Commons

Electrons are boiled off the filament of the electron gun (2) and are accelerated by the anode to form a beam (3).  The beam is focussed by a magnetic coil (4) to give a sharp image on the phosphor screen (5).   When you adjust the focusing knob on the control panel, you are adjusting the current through this coil.  Another pair of coils allows you to adjust the position of the beam on the screen.  (This can be quite temper-trying!)

The beam passes through the deflection plates (1) which produce a uniform electric field. The plates are arranged like this: The x-plates respond to the time base input.  If there is no input voltage, we see: If there are ten 1 cm squares across the screen, and the time base is set to 10 ms cm-1, the electron beam will take 100 ms to cross the screen.  Therefore the frequency is 1 ÷ 0.100 s = 10 Hz.  This is a slow sweep, and you will see the beam (a spot) crossing the screen.

If we turn the time base off and apply an alternating voltage, we see a vertical line, as the the y-plates respond to the voltage gain input.  They are deflecting the electron beam vertically, so we see: The beam is vertical

If neither set of plates is switched on, we see a spot in the middle of the screen. If we apply a positive DC voltage, the spot moves upwards: The spot moves downwards if the voltage is negative.

If the voltage is a positive DC voltage and the time base is turned on, we see a straight line above the centre like this: If the DC voltage is negative, the line will be below the centre line.

If the time base is turned on, and an alternating voltage is applied, we get a sine wave: The y-plates are connected to the output of amplifier that amplifies the voltage from 10-6 V to several hundred volts to make the field between the plates.  The x-plate drivers sweep the beam across the screen at a frequency as high as 106 Hz.

If you want to find a more detailed account of the CRO, click on this link: