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Physics Factsheet September 2002

Number 39

Using Oscilloscopes What is it?

What can I use it for?

An oscilloscope is essentially a device for displaying a graph of an electrical signal. Usually the graph shows how signals change over time – the vertical (Y) axis represents voltage and the horizontal (X) axis represents time.

Oscilloscopes are used a great deal in electronic engineering, for everything from television repairs to circuit design, but they are not limited to the world of electronics.

This graph can tell you many things about a signal; • how much of a signal is direct current or alternating current;

A transducer is a device which produces an electrical signal in response to a physical stimulus – for example, a microphone is a transducer which converts sound waves into an electrical signal. With the correct transducer an oscilloscope can display and be used to measure all kinds of phenomena such as sound, pressure, light or temperature.

• •

how the signal varies with time – the ‘shape’ of the waveform; from measurements of voltage and time, you can calculate the frequency.

The display of an oscilloscope is like a small television – in fact it works in essentially the same way as a TV, using a cathode ray tube. It is sometimes called a cathode ray oscilloscope, or CRO. The display screen has a centimetre square grid drawn on it for making measurements, with smaller (usually 2mm) markings on the centre lines.

Where do I begin? The first step is to get a clear display on your oscilloscope screen using the display controls. If there is already a spot or a horizontal line on your screen when you have switched on, then this is a simple case of using the intensity (brightness) and focus adjustments to get a clear display. If there is nothing on screen, then the simplest method is to:

The control panel can look confusing at first and the layout varies considerably from one model to another, but it normally divides into control sections for: • horizontal (timebase)

• •

vertical (voltage)

trigger sections. There are also controls for the display (intensity, focus) and input connections. Dual-beam oscilloscopes allow two waveforms to be displayed at the same time and these will have two sets of controls for the vertical (voltage) axis – one for each display. A typical control panel is shown in Fig 1.

a)

Turn the intensity up to maximum

b)

Use the ‘Y position’ and ‘X position’ controls to move the display up and down or from side-to-side until it is on screen

c)

Now turn the intensity down and adjust the focus until you see a clear image without any ‘halo’ or ‘bloom’ around the edges.

Fig 1. A typical oscilloscope control panel Horizontal (timebase) controls

Display controls

Trigger controls

Input A

Input B Vertical (voltage) controls - two channels

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Physics Factsheet

Using Oscilloscopes

cell will appear as a horizontal line 1.5cm from the centre line – indicating a d.c. voltage of 1.5V. An a.c voltage of amplitude 1.5V will show as a waveform between 1.5 cm above the centre line and 1.5cm below the line. A similar a.c. voltage which varies between 0V and 3V (i.e. which is always positive) may look similar to the previous one, but this time the display will be between the centre line and 3cm above the centre line. The line stays in the top half of the display, showing that the voltage is always positive – always greater than zero.

The time axis The most common use of an oscilloscope is to display a voltage varying with time – with time being displayed on the horizontal (X) axis. The timebase setting controls the speed with which the display spot sweeps across the screen from left to right. The markings on the timebase control normally show the time (in seconds, or milliseconds, or microseconds) it takes for the spot to cover 1cm of the display. (Most oscilloscope screens are about 10cm wide).

If you changed the setting on the input channel to ‘AC’, then both of these last pair of signals would appear identical on-screen, because the ‘AC’ setting removes any d.c. component from the input and just displays the alternating part. It is no longer possible to say (from the screen) whether voltages are positive or negative relative to earth, because this setting will display all signals as thought they are centred on the centre line of the screen.

If you turn the timebase control off, then you should see a single spot of light, which you can place anywhere you choose using the X and Y position controls. If you move this spot to the left hand side of the screen and turn the timebase to the slowest setting, you will see this spot sweep across a full screen width from left to right, then immediately appear again at the left hand side for a repeat performance. The spot will only look like a spot on the very slowest settings.

This might not appear useful on first sight, but there are occasions where a signal has an a.c. component and a d.c. component, and it is useful to be able to separate them. For example, when an a.c. voltage has been changed into a d.c. voltage (by a full-wave rectifier), it is then often passed through further circuitry to smooth out the lumpy a.c. wave shapes and make it more like ‘true’ d.c. – like the straight line you might get from a battery. After this smoothing has taken place, there is usually just a small remnant of the a.c. signal left – so that instead of the d.c. signal being a perfectly straight horizontal line, there is a small amount of variation which is known as ‘ripple’. The size of the ripple voltage can be important, because some types of circuit (particularly fast switching circuits, computers and so on) can be sensitive to these small fluctuations on the supply voltage. If you try to measure the size of the ripple voltage on the ‘DC’ setting of the oscilloscope, then you are stuck with a problem. Suppose the supply has been rectified to provide a 12V d.c. voltage and the ripple voltage is about 0.5V. If you display the voltage on ‘DC’ at a sensitivity of 1V/cm (in order to measure the ripple) then your ripple is off screen – because it is 12V (which means 12cm) from the zero volts centre line. If, however, you try to bring it back on screen by reducing the sensitivity down to 5V/cm, then the display is back on screen but now the ripple amounts to only 1mm of fluctuation up and down – almost too small to measure with any accuracy. You need the ‘AC’ setting! This removes the 12V d.c. component and allows you to look at the ripple as clearly as you like. You can now set the sensitivity to, say, 0.2V/cm and then measure the size of the ripple with ease.

Once the timebase is set a little faster, then your eyes are not quick enough to discern that it is in fact still a moving spot, and the display appears to be a solid horizontal line. (Don’t ever forget, however, that it is still moving at a steady speed and always from left to right, or you won’t understand and be able to read the graphs it produces). Often, in addition to the outer timebase control knob which clicks from one setting to the next, there is a smaller inner control knob which allows you to speed up or slow down the timebase smoothly between settings. This is not an awful lot of use, in practice, and it can cause you problems when you try to make actual measurements of time later on. There will be one position for this variable time control marked ‘cal’ or ‘calibrated’ – sometimes the knob will ‘click’ into this position. It is important that you leave it in this position when you want to take measurements – because only when it is here do the markings on the outer timebase control actually mean what they say! For example, if the outer control is set to read 0.1s/div (division) or 0.1s/cm then you might reasonably expect the spot to be moving at a speed of 0.1 seconds per centimetre, covering the full screen width in 1 second. It IS moving at this exact speed, BUT ONLY IF the variable time control is set to the ‘calibrated’ position. It will make your life easier to leave the control knob in the calibrated position! The voltage axis The Y axis of the oscilloscope is a voltmeter. It displays voltage as movements of the display up and down the screen, and the scale of these movements can be set using the ‘volts/cm’ control. So, for example, if this control is set to 1V/cm (1 volt per centimetre) and a single 1.5V cell is connected to the input for that channel, then you should see the display move 1.5 cm up the screen (or down the screen if you connected the cell the other way round).

4V d.c. with a 1V a.c. component on the d.c. setting

There are usually a few extra toys with the voltage display. The first is a switch (or sometimes a set of push-buttons) to choose between GND / AC / DC. ‘GND’ (ground) is simple – choosing this position automatically sets the Y-input to earth voltage (i.e. zero voltage) and so the display doesn’t do anything up or down whatever is connected to the input. Its only purpose is to allow you to set the position of your display on-screen so that you can make accurate voltage measurements. So you might, for example, choose to set your input to ‘GND’ and then use the Y-position control to move the line so that is exactly over the centre-line on-screen. All future measurements of voltage can then be made above centre (for positive inputs) or below centre (for negative inputs).

The same waveform on the a.c. setting

The DC (direct current) setting measures all voltages relative to zero (earth or ground) voltage. It is the setting you will use most often even, confusingly, when you are looking at AC (alternating current) signals. So, as we said before, on the ‘DC’ setting with the volts/cm control set to 1V/cm, a 1.5V

So, to summarize, whilst the ‘AC’ setting has its uses they are fairly specialized and you are best advised to stick to the ‘DC’ setting for most everyday uses; including when you are looking at most types of a.c. signals!

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Physics Factsheet

Using Oscilloscopes A second complication arises when your oscilloscope offers dual beam display. This is an invaluable function allowing you a lot more scope for comparing waveforms and making other measurements – but we will deal with that a little later on. For the moment, you need to know that two beam display means two identical inputs (usually called channel A and channel B) and two sets of input controls. There are also ‘mode’ options, allowing you to choose to display either channel individually, or both channels together. Whilst using your oscilloscope as a single channel device, choose the channel A mode and leave channel B controls alone.

(i.e. ‘+’ = ‘going up’ or ‘–‘ = ‘going down’). Additional controls in this section may allow you synchronise this trigger with additional external signals or in particular ways for particular applications – such as analysing TV or HF (high frequency) radio signals. They are specialized uses for specific branches of electronic engineering and I would advise you to avoid them until you are up and running. If the trigger controls include an ‘auto’ option, it is a good place to start – and will usually give you a clear display starting at the left from zero volts, positive slope. + slope

Just like the timebase control described above, the voltage control usually has a multi-position switch to choose voltage ranges (marked in V/cm or V/div, down to mV/cm: remember that 1V = 1000mV). There is often also a central variable control to allow you to increase or decrease the sensitivity (or gain) between the marked values. It is just as important to remember to place this control in the ‘calibrated’ position when you are using the oscilloscope to measure voltages.

− slope

zero volts Input signal

Trigger controls The trigger controls let you stabilize the display of repeating waves. It makes repeating waveforms appear to stand still on the oscilloscope screen. If you display a simple sinusoidal voltage wave (like the output of a laboratory power supply) without any triggering then what happens is this. The spot moves across the screen from left to right at whatever speed you have chosen with the timebase control. As it moves, it also moves up and down to indicate the changing input voltage at each instant. Suppose your input waveform is alternating at a frequency of 50Hz. This means that one complete cycle occurs in one-fiftieth of a second, or 0.02s. Setting your timebase to 0.01s/div (or 10ms/div, which is the same thing) would give you one wave every 2cm, or approximately five complete waves across a 10cm-wide screen.

Triggering on the Positive Slope with the level set to 3V

3V

When it gets to the extreme right of the screen, it jumps back to the beginning and starts displaying the waveform again. Unless you are extremely lucky, it will start this second journey at a different point in the wave from the first time. What you will see on screen is not one waveform but several, all flickering and seemingly moving to left or right – most unpleasant and useless for most practical purposes.

Triggering on the Negative Slope with the level set to 3V

3V

Taking measurements Now you are able to produce a clear, stable display on your oscilloscope screen, you can take accurate measurements of time or voltage and use these to calculate other values. Remember to put all variable time and voltage controls to the calibrated position. The best position to make measurements is along the centre lines (vertical and horizontal) of the display screen, because there you have additional marks at 2mm intervals along the axes. If the part of the wave you want to measure isn’t on those axes, use the Y position and X position controls to move it there – these controls don’t affect any other aspect of the display. If you want to measure actual voltages relative to zero (earth) voltage, then use the ‘GND’ setting first to set your zero volts line on screen; then take all subsequent measurements above or below that line.

The only way to get a stable display of a repeating waveform on screen is for each traverse of the screen to start at exactly the same point in a wave cycle as the last traverse – so each display falls exactly on top of the previous one and our eyes detect a single sharp image. This is what the triggering controls are designed to achieve.

To make time measurements you need to use the central horizontal scale line. For example, if you want to measure the period of a repeating waveform, move the waveform using X and Y position controls so that the same position in two successive cycles can be seen along this central axis. You can either speed up the timebase so that one cycle almost covers the whole screen width, or you can measure the width of several successive waves on screen and divide by the number of cycles to find the period. It is more accurate to choose a point in the wave where it crosses the centre line (at zero volts) than to choose the peak of successive waves – because it is harder to identify exactly where the centre of the peak is. Once you have measured the period, then use 1 frequency = period to calculate frequency.

The effect of the trigger is to make the spot ‘wait’ at the left hand side of the screen until exactly the same point in the wave cycle before it starts its sweep across the screen. You will appreciate that this means detecting not just the voltage at that point in the wave, but also the direction the voltage is changing in – otherwise, the display might start on one sweep at the ‘start’ of a cycle (where it passes through zero volts on the way up) and on the next sweep halfway through the cycle (where it passes through zero volts on the way down). The trigger controls allow you to choose both the level (i.e. the voltage at which the trigger fires to start the next sweep) and the slope

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Physics Factsheet

Using Oscilloscopes Exam Workshop

(b) A second voltage waveform of the same frequency is input on channel 2. This has a peak voltage of 12V and lags the first wave by a phase difference of π . Sketch this on the diagram above. 2 Show all your working. (6)

This is a typical student’s answer to an exam question. The comments explain what is wrong with the answers and how they can be improved. The examiner’s answer is given below. Both channels of the CRO display shown below are set to a time base of 5ms /div. and a vertical sensitivity of 4V / div.

A phase difference of π/2 means a quarter of a complete wave, so the second line should peak where the first woveform reaches zero.

(a) A 16V peak-to-peak alternating signal of frequency 40Hz is input on channel 1. Sketch this on the screen above. Show any calculations you make below. (6)

The amplitude is correct this time since the question says ‘peak voltage’ not ‘peak-to-peak’. The quality of the line is poor although the student has shown the correct phase difference. (S)he has still made a mistake with the phase difference since the second curve leads the solid one by π/2

Time period = 1 / frequency = 1 / 40 = 0.025s (or 25 ms). On a timebase scale of 5ms/div, this means that one cycle = 5 divisions.

Examiner’s answer (a) 16V peak to peak, amplitude=8V= 4 divisions on CRO (2V / div); ! T = 1/f = 1/40 s = 1000/40ms = 25ms; ! 25ms is 5 divisions on CRO (5ms / div); ! Correct amplitude on sketch; ! Correct period on sketch; ! Quality of curve; ! (b) 12V peak, amplitude = 12V = 6 divisions on CRO; ! Phase difference π/2 , is ¼ of period=5/4=1.25 divisions on CRO;! Phase lag, so voltage reaches peak later in time than first voltage;! Amplitude and period correct on sketch; ! Phase relationship correct on sketch; ! Quality of curve; !

Correct for the time axis – and one of the marks is for calculating this correctly, so it is important to show your working. Unfortunately, this student has made the classic error of confusing peak-to-peak value with peak amplitude. Peak-to-peak is equal to twice the amplitude – so a wave of peak-to-peak value of 16V has an amplitude (from the centre line) of 8V. It is important to be fairly precise with the position of ‘key points’ on hand-sketched graphs, such as the position of the peaks and the points where they cross the zero line. This example goes a little way below the –16V line and sloppiness like this might lose marks.

This example question shows one of the ways in which a dual-beam oscilloscope can be useful – for comparing two waves at the same time and in particular for examining the phase relationship between them.

Question The equipment shown in the diagram is set up for an experiment. loudspeaker

Answer (a) (i) Two traces drawn, out of phase eg Labels shown

microphone

signal generator

(ii) Phase difference increases Amplitude of microphone signal decreases

CRO

(b) Place microphone so traces are in phase Gradually move microphone away Each time traces are back in phase, microphone has moved one wavelength Count the number of wavelengths moved Measure distance moved and calculate wavelength

(a) The CRO displays two traces: one directly from the signal fed to the loudspeaker, the other from the microphone. (i) Draw a labelled diagram of a typical display you would expect to see on the CRO. (ii) State two aspects of this display that would change as the distance between the loudspeaker and microphone is increased.

Acknowledgements: This Physics Factsheet was researched and written by Keith Penn. The Curriculum Press,Unit 305B, The Big Peg,120 Vyse Street, Birmingham, B18 6NF. Physics Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136

(b) Explain how the CRO measurements can be used to find the wavelength of the sound. Exam Hint:- Knowledge of oscilloscopes is often required for practical examinations and coursework. Written exam questions may refer to the use of an oscilloscope in a number of topic areas, in particular those covering a.c. electrical circuits and waves.

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