Using an Oscilloscope
Introduction


The practical work supporting Signals introduces you to using an oscilloscope to display voltage–time (Vt) graphs. This is a very powerful technique for investigating and understanding circuit behaviour and for correcting circuit faults.

What does an oscilloscope do?
An oscilloscope is easily the most useful instrument available for testing circuits because it allows you to see the signals at different points in the
circuit
A circuit is a closed conducting path.
circuit
.

The best way of investigating an electronic system is to monitor signals at the input and output of each system block, checking that each block is operating as expected and is correctly linked to the next. With a little practice, you will be able to find and correct faults quickly and accurately.

An oscilloscope is an impressive piece of kit:

Figure 1.   Oscilloscope controls.
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Fig.1 shows a Hameg HM 203-6 oscilloscope, a popular instrument in schools and colleges. Your oscilloscope may look different but will have similar controls. Zoom in to see things in more detail.

Faced with an instrument like this, students typically respond either by twiddling every knob and pressing every button in sight, or by adopting a glazed expression. Neither approach is very helpful. Following the systematic description below will give you a clear idea of what an oscilloscope is and what it can do.

The function of an oscilloscope is extremely simple: it draws a Vt graph, a graph of
voltage
Potential difference, or voltage V is a measure of the difference in energy between two points in a circuit. Charges gain energy in the battery and lose energy as they flow round the rest of the circuit.
voltage
against time, voltage on the vertical or Y-axis, and time on the horizontal or X-axis.

Figure 2.   Oscilloscope screen.
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As you can see in Fig.2, the screen of this oscilloscope has 8 squares or divisions on the vertical axis, and 10 squares or divisions on the horizontal axis. Usually, these squares are 1 cm in each direction.

Many of the controls of the oscilloscope allow you to change the vertical or horizontal scales of the Vt graph, so that you can display a clear picture of the signal you want to investigate.

'Dual trace' oscilloscopes display two Vt graphs at the same time, so that simultaneous signals from different parts of an electronic system can be compared.

Setting up
1. Someone else may have been twiddling knobs and pressing buttons before you. Before you switch the oscilloscope on, check that all the controls are in their 'normal' positions. For the Hameg HM 203-6, this means that:

  • All push button switches are in the 'out' position.

  • All slide switches are in the 'up' position.

  • All rotating controls are centred.

  • The central TIME/DIV and VOLTS/DIV and the HOLD OFF controls are in the calibrated, or CAL position.



Check through all the controls and put them in the positions shown in Fig.3.

Figure 3.   'Normal' control positions.
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Some Hameg oscilloscopes have calibration controls which should be rotated fully clockwise rather than anticlockwise. Look for a small CAL symbol near the control which shows the correct position.

2. Set both VOLTS/DIV controls to 1 V/DIV and the TIME/DIV control to 0.2 s/DIV, its slowest setting:

Figure 4.   VOLTS/DIV and TIME/DIV controls.
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3. Switch on, using the red POWER button at the top centre of the oscilloscope:

Figure 5.   POWER on/off.
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The green
LED
An LED, or light-emitting diode, is illuminated when current passes through it in the forward bias direction.
LED
illuminates and, after a few moments, you should see a small bright spot, or trace, moving fairly slowly across the screen.

4. Find the Y-POS. I control:

Figure 6.   Y-POS. I control.
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What happens when you turn the Y-POS I control?
  • Click here to mark the question


For the present, adjust the trace so that it runs horizontally across the centre of the screen.

5. Now investigate the INTENSITY and FOCUS controls:

Figure 7.   INTENSITY and FOCUS controls.
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When these are correctly set, the spot will be reasonably bright but not glaring, and as sharply focused as possible. (The TR control shown in Fig.7 is screwdriver adjusted. It is only needed if the spot moves at an angle rather than horizontally across the screen with no signal connected.)

6. The TIME/DIV control determines the horizontal scale of the graph which appears on the oscilloscope screen.

With 10 squares across the screen and the spot moving at 0.2 s/DIV, how long does it take for the spot to cross the screen?
  • Click here to mark the question


Estimate the time taken for the spot to cross the screen with TIME/DIV set to 0.2 s. Is your answer correct?

Now rotate the TIME/DIV control clockwise:

Figure 8.   TIME/DIV control.
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With the spot moving at 0.1 s/DIV, it will take 1 second to cross the screen.

Continue to rotate TIME/DIV clockwise.
With each new setting, the spot moves faster. At around 10 ms/DIV, the spot is no longer separately visible. Instead, there is a bright line across the screen.
This happens because the screen remains bright for a short time after the spot has passed, an effect which is known as the persistence of the screen.
     It is useful to think of the spot as still there, just moving too fast to be seen.



Keep rotating TIME/DIV. At faster settings, the line becomes fainter because the spot is moving very quickly indeed.

7. The VOLTS/DIV controls determine the vertical scale of the graph drawn on the oscilloscope screen.

Check that VOLTS/DIV I is set at 1 V/DIV and that the adjacent controls are set correctly as shown in Fig.9.

Figure 9.   VOLTS/DIV and adjacent controls.
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The Hameg HM 203-6 has a built-in source of signals which allows you to check that the oscilloscope is working properly. A connection to the input of channel 1 (CH I) of the oscilloscope can be made using a special connector called a BNC plug, as shown In Fig.10.

Figure 10.   Connecting 2 V square wave output to channel 1 input.
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Fig. shows a lead with a BNC plug at one end and crocodile clips at the other. When the crocodile clip from the red wire is clipped to the lower metal terminal, a 2 V square wave is connected to the input of CH I.

Adjust VOLTS/DIV and TIME/DIV until you obtain a clear picture of the 2 V signal, which should look like this:

Figure 11.   Oscilloscope trace of 2 V square wave.
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Check on the effect of Y-POS I and X-POS:

Figure 12.   Y-POS I and X-POS controls.
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Y-POS I moves the whole trace vertically up and down on the screen, while X-POS moves the whole trace horizontally on the screen. These controls are useful because the trace can be moved so that more of the picture appears on the screen, or to make measurements easier using the grid which covers the screen.

You have now learned about and used the most important controls on the oscilloscope.
You know that the function of an oscilloscope is to draw a Vt graph.
You know how to put all the controls into their 'normal' positions, so that a trace should appear when the oscilloscope is switched on.
     You know how the change the horizontal scale of the Vt graph, how to change the vertical scale, and how to connect and display a signal.


Figure 13.   Using an oscilloscope.
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What is needed now is practice so that all of these controls become familiar.

Connecting a function generator
Fig.14 shows the appearance of a Black Star Jupiter 2010 function generator – one of many types used in schools.

Figure 14.   Function generator.
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Again, your function generator, or signal generator, may look different but is likely to have similar controls.

The Jupiter 2010 has an on/off switch next to the socket for the mains connecting lead on the rear panel, with push button and rotating controls on the front panel.

Start by clicking in the 1k FREQUENCY RANGE button, select a sine wave signal and check that the ATTENUATOR switch is set to 0 dB:

Figure 15.   Frequency range, function, and attenuator controls.
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Most often the 600 Ω output is used. This can be connected to the CH I input of the oscilloscope using a BNC–BNC lead, as follows:

Figure 16.   Connecting function generator to oscilloscope.
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Switch on the function generator and adjust the output level to produce a visible signal on the oscilloscope screen. Adjust the TIME/DIV and VOLTS/DIV to obtain a clear display and investigate the effects of pressing the waveform shape buttons.

The FREQUENCY RANGE switch is used together with the rotating FREQUENCY control to determine the frequency of the output signal. With the settings shown in Fig.15, the output frequency will be close to 1 kHz. The
7-segment
A 7-segment display consists of individual LEDs, or other devices, which can be illuminated to represent the numbers from 0–9.
7-segment
displays show the exact frequency produced. The small LEDs next to the
digital
In a digital circuit, information is represented by discrete voltage levels. A high voltage is called logic 1, or 1, while a low voltage is called logic 0, or 0.
digital
display light up to indicate the measurement units in use.

How should you change these settings to obtain an output frequency of 50 Hz?
This is done by clicking in the RANGE switch marked '100' and rotating the FREQUENCY control until the digital display shows 50.00.
     The frequency meter adjusts the measurement units automatically.


Figure 17.   Frequency controls.
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Experiment with the controls shown in Fig.17 to produce other frequencies of output signal, such as 10 Hz, or 15 kHz. Whatever frequency and amplitude of signal you select, you should be able to change the oscilloscope settings to give a clear Vt graph of the signal on the oscilloscope screen.

Figure 18.   Displaying a sine wave.
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Find out what happens when you click the triangle and square wave FUNCTION buttons.

The remaining features of the function generator are used less often.
For example, it is possible to change the output signals by connecting suitable signals to the SWEEP IN input.
The D.C. OFFSET control is normally kept in its central position. By rotating it, you can add a d.c. component to the output signal, producing a complex wave, as described in Signals.
The ATTENUATOR switch is normally set to 0 dB:
Figure 19.   ATTENUATOR switch in 'normal' position.
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This gives an output signal with a peak amplitude which can be easily adjusted up to several volts. In the −20 dB and −40 dB positions, the amplitude of the output signal is reduced to a few millivolts. Such small signals are used for testing amplifier circuits.
The TTL output produces pulses between 0 V and 5 V at the selected frequency and is used for testing logic circuits.
The 50 Ω output allows you to deliver more current to the circuit under test. There are occasions when this is useful.
     The SYM button is used together with the SYMMETRY control to vary the shape of the output waveform. It is interesting to try this to see how it works, but you are not at all likely to need this feature.

Microphones, audio signals, and amplifiers
This section is an investigation of microphones, audio signals, and amplifiers, intended to develop your
prototype board
Prototype board is used for building temporary circuits. Connections are made by pushing components and wire links into the holes in the prototype board.
prototype board
skills and giving you experience of using the oscilloscope to monitor signals in a simple circuit.

Fig.20 shows an easily available type of
microphone
A microphone is an input transducer which converts sound energy to electrical signals.
microphone
, called an electret microphone.

Figure 20.   Electret microphone.
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The microphone has separate + and 0 V connections. Can you see that the 0 V connection is connected to the metal case? Check these connections on the real component.

To get the microphone to work, you need to provide a voltage across it using a
voltage divider
A voltage divider consists of two resistors, Rtop and Rbottom, connected in series. Vin is connected across both resistors. Vout is the voltage across Rbottom. Vout is calculated from:

voltage divider
circuit such as that shown in Fig.21.

Figure 21.   Microphone circuit.
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From the voltage divider formula, the voltage expected across the microphone is:

     Substituting:




Build the voltage divider part of the circuit on prototype board as follows:

Figure 22.   Voltage divider circuit.
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Measure the voltage between the resistors. Write down or enter your meter reading.


Expected voltage / V Measured voltage / V
1.58



Small differences can arise if you have not adjusted the power supply voltage to exactly 9 V and also because the resistors may not have precisely their marked values. Remember, resistors are manufactured to a
tolerance
Tolerance describes manufacturing accuracy. If a component is manufactured to a tolerance of ±5%, this means that its actual value is guaranteed to be within 5% of its marked, or nominal value.
tolerance
, usually ±5%, so that their values are not exact.

Now add the microphone to the circuit as shown in Fig.23, taking care to get its + and 0 V connections the right way round.

Figure 23.   Microphone in voltage divider circuit.
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Usually, this results in a small decrease in the voltage divider voltage because the microphone is now in
parallel
Components are connected in parallel when they are joined side by side in a circuit, so that they provide alternative pathways for current flow.
parallel
with the 1 kΩ
resistor
A resistor is an electronic component with a particular resistance values. Resistors limit current.
resistor
. In other words, Rbottom is reduced. Another way of explaining this is to say that some of the
current
Current I is a flow of charged particles, usually electrons.
current
flows through the microphone, leaving a little less flowing through the 1 kΩ.

Check the polarity of a 4.7 μF capacitor (longer leg positive, stripe negative) and connect this as indicated in Fig.24 below.

Figure 24.   Adding capacitor to microphone circuit.
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The arrangement outlined in Fig.25 is a very convenient way of setting up an oscilloscope to make measurements from the prototype board.

Figure 25.   Connecting a lead to make measurements
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If you are using a battery power supply, the black lead must be connected to the 0 V rail on the prototype board.
You can do this using a crocodile clip and a wire link.
If you are using a laboratory supply, the 0 V terminal may already be connected to 'earth' through the mains.
     Often, you can leave the black lead unconnected. Connect it to a spare 0 V socket on the power supply if you prefer.

Connect the red lead to a 4 mm lead with crocodile clips at each end, or to a lead with a test probe.
Which method you use is a matter of personal preference.
     Test probes can be moved quickly from one part of the circuit to another. On the other hand, using a 4 mm lead and connecting the crocodile clip to a wire link leaves your hands free to adjust the controls of the oscilloscope.



Make a connection to the prototype board as indicated in Fig.24. Increase the sensitivity of the VOLTS/DIV control by rotating it clockwise until you can see changes on the oscilloscope screen when you talk into the microphone. Adjust TIME/DIV until the shape of the signals is clear.

Estimate the peak-to-peak amplitude of the signal and write down or enter this value.


Estimated amplitude / mV



Find out about the sort of signal which is produced when you clap your hands within range of the microphone.

When you talk into the microphone, the signals you get are small. To make them bigger, you need an amplifier. A possible device is the 741, one of a large family of integrated circuits called operational amplifiers, or op-amps:

Figure 26.   Pin connections of a 741 op-amp.
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The internal circuit of a 741 is quite complicated but it is easy to use the device simply as an amplifying subsystem. It is cheap and easily available.

As you can see, the 741 is manufactured in a small plastic package, with 8 connecting pins. These are in a dual in line, or dil arrangement.

With the index mark at the top, pin 1 is on the left and pins are numbered down the left-hand side and back up on the right. Often, there is an additional circular mark next to pin 1. This numbering convention is followed with other integrated circuits, whether there are 8, 14, 16, or more pins.

Place the 741 across the central gap in the prototype board. Check that pin 1 is correctly located. Now complete the circuit, as follows:

Figure 27.   Sound sensor with amplifier.
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If your power supply does not have dual power supply outputs, the +9 V, 0 V, and −9 V required can easily be made using two PP3 batteries.
These are connected to the prototype board like this:
Figure 28.   Connecting PP3 batteries to make a dual power supply
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     If you are unfamiliar with this type of power supply, use a multimeter as a voltmeter, with its black lead connected to 0 V, and touch the positive and negative supply points in turn with the red lead. In one case, the meter will read approximately +9 V, and in the other, approximately −9 V.



Check back with your prototype board and make sure that you have linked the sound sensor subsystem to the amplifier with a wire link. Monitor the final output of the system using the oscilloscope.

Figure 29.   Prototype board circuit.
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Estimate the peak-to-peak amplitude of the final signal and write down or enter this value:


Final amplitude / V


The voltage gain of the amplifier is given by:

The way in which this particular op-amp circuit works allows you to choose the voltage gain according to:

The minus sign appears because this is an inverting amplifier circuit, that is, the output waveform has the same shape as the input waveform, but is turned upside down, or inverted, compared with the input waveform. What matters here is that the amplitude of the waveform is increased.
The voltage gain of the circuit is calculated from:

Vout is inverted and the amplitude of the signal is increased by 47 times. Vout after the amplifier should be 47 times larger than the signal from the microphone subsystem.
     Do your observations using the oscilloscope confirm these changes?



Work through your circuit again using the oscilloscope to monitor the audio signal at different points in the circuit.

You are learning something important here. Developing a circuit is a progressive process. You start with simple subsystems on prototype board and investigate the performance of each subsystem before building the next. There is no point in connecting an amplifier subsystem to a sound sensor which does not work. You need to know that the sound sensor is working correctly before building the next stage.

The remaining sections of this unit describe the functions of the oscilloscope in more detail. These sections do not involve specific practical work and can be studied separately.

Trigger controls
This group of controls allows the oscilloscope display to be synchronized with the signal you want to investigate:
Figure 30.   Trigger controls.
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When the AT/NORM button is in the 'out' position, triggering is automatic. This works for most signals.

The green TRIG LED illuminates whenever a trigger point is detected.

If you change the AT/NORM button to its 'in' position, the most likely result is that the signal will disappear and the oscilloscope screen will be blank.
However, if you now adjust the LEVEL control, the display will be reinstated.
     As you adjust the LEVEL control, the display starts from a different point on the signal waveform. This makes it possible for you to look in detail at any particular point of the waveform.



The EXT button should normally be in the 'out' position. When it is pushed in, triggering occurs from a signal connected to the trigger input, TRIG INP, socket.

The slide switch to the left of TIME/DIV gives additional triggering options.
AC is the normal position and is suitable for most waveforms.
In the DC position, you use the LEVEL control to select a particular d.c. voltage on the signal waveform where triggering will occur.
The +/− button gives triggering on the upward slope of the signal waveform in the 'out' position, and triggering on the downward slope in the 'in' position.
     HF gives triggering in response to high frequency parts of the signal, LF gives triggering for low frequency components and ~ indicates that triggering will occur at 50 Hz, corresponding to UK mains frequency. You are not likely to need any of these slide switch positions.



The HOLD OFF control allows you to introduce a delay relative to the trigger point so that a different part of the signal can be seen. Normally, you will want to leave the HOLD OFF control in its minimum position.

With more experience of using the oscilloscope, you will develop a clear understanding of the functions of the important trigger controls and be able to use them effectively.

Other oscilloscope controls
In the diagram below, move the mouse over any of the oscilloscope controls to discover its function.

Figure 31.   Oscilloscope controls.
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You have learned about the most important controls already. Check the functions of the controls you used during the practical.

PC-based oscilloscopes
As an alternative to a separate oscilloscope, you may be able to use a device which plugs into the parallel port of your computer. The computer takes over many of the functions of the oscilloscope. Signals are displayed on the computer monitor. Results can be stored and transferred into other programs including word processors and spreadsheets.

Fig.32 shows the front panel layout of a PicoScope ADC200 oscilloscope.

Figure 32.   PicoScope ADC200 front panel.
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Making connections to this type of oscilloscope is very easy. The only front panel controls are the DC/AC switches. All the remaining adjustments are done within the PicoScope computer program.

Figure 33.   Using the PicoScope ADC200.
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An oscilloscope like this works by taking repeated voltage readings using a fast analogue-to-digital converter (or ADC). Sample rates of 20 million or more readings per second are possible. The readings are transferred and stored in the computer's memory.

Most experimental results included in Absorb Electronics were recorded in this way.

How does an oscilloscope work?
An outline explanation of how an oscilloscope works can be given using the block diagram shown below:

Figure 34.   Block diagram of an oscilloscope.
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Like a television screen, the screen of an oscilloscope consists of a cathode ray tube.
Although the size and shape are different, the operating principle is the same.
     Inside the tube is a vacuum. The electron beam emitted by the heated cathode at the rear end of the tube is accelerated and focused by one or more anodes, and strikes the front of the tube, producing a bright spot on the phosphorescent screen.



The electron beam is bent, or deflected, by voltages applied to two sets of plates fixed in the tube.

The horizontal deflection plates, or X-plates produce side to side movement. As you can see, they are linked to a system block called the time base. This produces a sawtooth waveform with a Vt graph which looks like this:

Figure 35.   Sawtooth waveform.
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During the rising phase of the sawtooth, the spot is driven at a uniform rate from left to right across the front of the screen. During the falling phase, the electron beam returns rapidly from right to left, but the spot is 'blanked out' so that nothing appears on the screen.

In this way, the time base generates the X-axis of the Vt graph.

The slope of the rising phase varies with the frequency of the sawtooth and can be adjusted, using the TIME/DIV control, to change the scale of the X-axis.
Dividing the oscilloscope screen into squares allows the horizontal scale to be expressed in seconds, milliseconds or microseconds per division (s/DIV, ms/DIV, μs/DIV).
     Alternatively, if the squares are 1 cm apart, the scale may be given as s/cm, ms/cm or μs/cm.



The signal to be displayed is connected to the CH I input. The AC/DC switch is usually kept in the DC position (switch closed) so that there is a direct connection to the Y-amplifier. In the AC position (switch open) a capacitor is placed in the signal path. As will be explained in Capacitors, the capacitor blocks
d.c.
In a d.c., or direct current, circuit, current always flows in the same direction.
d.c.
signals but allows
a.c.
In an a.c., or alternating current, circuit, current flows first in one direction, then in the other.
a.c.
signals to pass.

The Y-amplifier is linked in turn to a pair of Y-plates so that it provides the Y-axis of the Vt graph.
The overall gain of the Y-amplifier can be adjusted, using the VOLTS/DIV control, so that the resulting display is neither too small or too large, but fits the screen and can be seen clearly.
     The vertical scale is usually given in V/DIV or mV/DIV.



The trigger circuit is used to delay the time base waveform so that the same section of the input signal is displayed on the screen each time the spot moves across. The effect of this is to give a stable picture on the oscilloscope screen, making it easier to measure and interpret the signal.

Changing the scales of the X-axis and Y-axis allows many different signals to be displayed.
Sometimes, it is also useful to be able to change the positions of the axes.
This is possible using the X-POS and Y-POS controls.
     For example, with no signal applied, the normal trace is a straight line across the centre of the screen. Adjusting Y-POS allows the zero level on the Y-axis to be changed, moving the whole trace up or down on the screen to give an effective display of signals like pulse waveforms which do not alternate between positive and negative values.

Summary


The step-through animation below summarizes the prototype boards constructed in this unit.

Figure 36.   Using an oscilloscope.
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Parts list:

Quantity Item
oscilloscope
multimeter
1 oscilloscope lead BNC/4 mm plugs
1 oscilloscope lead BNC/BNC
red and black 4 mm leads
crocodile clips
1 prototype board (breadboard)
+9 V, 0 V, –9 V d.c. power supply
1 1 kΩ, 4.7 kΩ, 10 kΩ, 470 kΩ 0.5 W carbon film resistors
2 47 μF 25 V radial electrolytic capacitor
1 741 op-amp
1 miniature electret microphone
link wires
wire stripper
pliers
side cutters

Well done!
Try again!