ELECTRONIC MEASUREMENTS

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ELECTRONIC MEASUREMENTS

• TR2023
• ELECTRICAL AND ELECTRONIC TECHNOLOGY
• FACULTY OF MANAGEMENT OF TECHNOLOGY
• UNIVERSITI UTARA MALAYSIA

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CONTENTS

1. MULTIMETER
2. OSCILLOSCOPE
3. PROBES
4. SIGNAL GENERATOR

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1. MULTIMETER

• A meter is a measuring instrument. An ammeter
  measures current, a voltmeter measures the
  potential difference (voltage) between two
  points, and an ohmmeter measures resistance.
• A multimeter combines these functions, and
  possibly some additional ones as well, into a
  single instrument.

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Principles - Ammeter

• Before going in to detail about multimeters, it
  is important for you to have a clear idea of how
  meters are connected into circuits. Diagrams A
  and B below show a circuit before and after
  connecting an ammeter

A
B
to measure current, the circuit must be broken
to allow theammeter to be connected in series
ammeters must have a LOW resistance
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Principles - Ammeter (contd)

• To start with, you need to break the circuit so
  that the ammeter can be connected in series.
• All the current flowing in the circuit must pass
  through the ammeter.
• Meters are not supposed to alter the behavior of
  the circuit, or at least not significantly, and
  it follows that an ammeter must have a very LOW
  resistance.

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Principles - Voltmeter

• Diagram C shows the same circuit after connecting
  a voltmeter

A
C
to measure potential difference (voltage), the
circuit is not changed the voltmeter is
connected in parallel
voltmeters must have a HIGH resistance
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Principles - Voltmeter (contd)

• This time, you do not need to break the circuit.
• The voltmeter is connected in parallel between
  the two points where the measurement is to be
  made.
• Since the voltmeter provides a parallel pathway,
  it should take as little current as possible.
• In other words, a voltmeter should have a very
  HIGH resistance.
• Which measurement technique do you think will be
  the more useful?
• In fact, voltage measurements are used much more
  often than current measurements.

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Principles - Voltmeter (contd)

• The processing of electronic signals is usually
  thought of in voltage terms.
• It is an added advantage that a voltage
  measurement is easier to make.
• The original circuit does not need to be changed.
  
• Often, the meter probes are connected simply by
  touching them to the points of interest.

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Principles - Ohmmeter

• An ohmmeter does not function with a circuit
  connected to a power supply. If you want to
  measure the resistance of a particular component,
  you must take it out of the circuit altogether
  and test it separately, as shown in diagram D

A
D
to measure resistance, the component must be
removed from the circuit altogether
ohmmeters work by passing a current through the
component being tested
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Principles Ohmmeter (contd)

• Ohmmeters work by passing a small current through
  the component and measuring the voltage produced.
  
• If you try this with the component connected into
  a circuit with a power supply, the most likely
  result is that the meter will be damaged.
• Most multimeters have a fuse to help protect
  against misuse.

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Digital Multimeter

• Multimeters are designed and mass produced for
  electronics engineers.
• Even the simplest and cheapest types may include
  features which you are not likely to use.
• Digital meters give an output in numbers, usually
  on a liquid crystal display.

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Digital Multimeter (contd)

• The central knob has lots of positions and you
  must choose which one is appropriate for the
  measurement you want to make. If the meter is
  switched to 20 V DC, for example, then 20 V is
  the maximum voltage which can be measured.
• This is sometimes called 20 V fsd, where fsd is
  short for full scale deflection.
• For circuits with power supplies of up to 20 V,
  which includes all the circuits you are likely to
  build, the 20 V DC voltage range is the most
  useful.

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Digital Multimeter (contd)

• Sometimes, you will want to measure smaller
  voltages, and in this case, the 2 V or 200 mV
  ranges are used.
• What does DC mean? DC means direct current. In
  any circuit which operates from a steady voltage
  source, such as a battery, current flow is always
  in the same direction. Every constructional
  project descirbed in Design Electronics works in
  this way.
• AC means alternating current. In an electric lamp
  connected to the domestic mains electricity,
  current flows first one way, then the other. That
  is, the current reverses, or alternates, in
  direction. With UK mains, the current reverses 50
  times per second.

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Digital Multimeter (contd)

• For safety reasons, you must NEVER connect a
  multimeter to the mains supply.

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Digital Multimeter (contd)

• An alternative style of multimeter is the
  autoranging multimeter.
• The central knob has fewer positions and all you
  need to do is to switch it to the quantity you
  want to measure. Once switched to V, the meter
  automatically adjusts its range to give a
  meaningful reading, and the display includes the
  unit of measurement, V or mV. This type of meter
  is more expensive, but obviously much easier to
  use.
• Where are the two meter probes connected? The
  black lead is always connected into the socket
  marked COM, short for COMMON. The red lead is
  connected into the socket labelled V mA. The 10A
  socket is very rarely used.

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Analogue Multimeter

• An analogue meter moves a needle along a scale.
  Switched range analogue multimeters are very
  cheap but are difficult for beginners to read
  accurately, especially on resistance scales. The
  meter movement is delicate and dropping the meter
  is likely to damage it!
• Each type of meter has its advantages. Used as a
  voltmeter, a digital meter is usually better
  because its resistance is much higher, 1 MO or
  10 MO , compared to 200 kO for a analogue
  multimeter on a similar range. On the other hand,
  it is easier to follow a slowly changing voltage
  by watching the needle on an analogue display.

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Analogue Multimeter (contd)
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Analogue Multimeter (contd)

• Used as an ammeter, an analogue multimeter has a
  very low resistance and is very sensitive, with
  scales down to 50 µA. More expensive digital
  multimeters can equal or better this performance.
• Most modern multimeters are digital and
  traditional analogue types are destined to become
  obsolete.

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Example 1 voltage measurements
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Example 2 voltage measurements
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Example 3 resistance measurements
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Example 4 current measurements
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2. Oscilloscope

• 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.
• 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.

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The Interface

• An oscilloscope is an impressive piece of kit.
• The diagrams show a Hameg HM 203-6 and a
  Tektronix model 475A portable analogue
  oscilloscope, a popular instrument in UK schools.
  Your oscilloscope may look different but will
  have similar controls.
• 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
  specially helpful. Following the systematic
  description below will give you a clear idea of
  what an oscilloscope is and what it can do.

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The Display

• The function of an oscilloscope is extremely
  simple it draws a V/t graph, a graph of voltage
  against time, voltage on the vertical or Y-axis,
  and time on the horizontal or X-axis.
• As you can see, 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.

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The Display (contd)

• Many of the controls of the oscilloscope allow
  you to change the vertical or horizontal scales
  of the V/t graph, so that you can display a clear
  picture of the signal you want to investigate.
• 'Dual trace' oscilloscopes display two V/t graphs
  at the same time, so that simultaneous signals
  from different parts of an electronic system can
  be compared.

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Working Principles
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Working Principles (contd)

• 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.
  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 ot left, but the spot is 'blanked out'
  so that nothing appears on the screen.

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Working Principles (contd)

• In this way, the time base generates the X-axis
  of the V/t 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
  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 Chapter
  5, the capacitor blocks DC signals but allows AC
  signals to pass.

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Working Principles (contd)

• The Y-amplifier is linked in turn to a pair of
  Y-plates so that it provides the Y-axis of the
  the V/t 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.

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Working Principles (contd)

• 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.

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Connectors
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3. LOGIC PROBES

• Logic probes, as shown in figure opposite, are
  extremely simple and useful devices that are
  designed to help you detect the logic state of an
  IC.
• Logic probes can show you immediately whether a
  specific point in the circuit is low, high, open,
  or pulsing.

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Logic Probe

• A high is indicated when the light at the end of
  the probe is lit and a low is indicated when the
  light is extinguished.
• Some probes have a feature that detects and
  displays high-speed transient pulses as small as
  5 nanoseconds wide.
• These probes are usually connected directly to
  the power supply of the device being tested,
  although a few also have internal batteries.

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Logic Probe (contd)

• Since most IC failures show up as a point in the
  circuit stuck either at a high or low level,
  these probes provide a quick, inexpensive way for
  you to locate the fault.
• They can also display that single, short-duration
  pulse that is so hard to catch on an
  oscilloscope.

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Characteristics

• The ideal logic probe will have the following
  characteristics
• 1. Be able to detect a steady logic level
• 2. Be able to detect a train of logic levels
• 3. Be able to detect an open circuit
• 4. Be able to detect a high-speed transient
  pulse
• 5. Have over voltage protection
• 6. Be small, light, and easy to handle
• 7. Have a high input impedance to protect
  against
• circuit loading

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Logic Pulser

• Another extremely useful device for
  troubleshooting logic circuits is the logic
  pulser.
• It is similar in shape to the logic probe and is
  designed to inject a logic pulse into the circuit
  under test.
• Logic pursers are generally used in conjunction
  with a logic clip or a logic probe to help you
  trace the pulse through the circuit under test or
  verify the proper operation of an IC.

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Logic Pulser (contd)

• Some logic pursers have a feature that allows a
  single pulse injection or a train of pulses.
• Logic pursers are usually powered by an external
  dc power supply but may, in some cases, be
  connected directly to the power supply of the
  device under test.

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Logic Pulser (contd)

• Figure on the left below shows a typical logic
  pulser. Figure on the right shows a logic pulser
  (right) used with a logic probe (left).

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4. SIGNAL GENERATOR
SIGNAL GENERATOR

• The signal generator is a device used to generate
  a variety of electrical signal waveforms that are
  used as inputs to various electronic circuits
  during testing and/or development activities.
• Useful piece of equipment in the signal generator
  family is function generator.

Signal Generator
Function Generator
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Signal Generator (contd)
SIGNAL GENERATOR

• It contains an electronic oscillator, an
  electronic circuit that is capable of creating a
  repetitive waveform.
• The most common waveform is a sine wave, but
  sawtooth, step (pulse), square, and triangular
  waveform oscillators are commonly available as
  are arbitrary waveform generators (AWGs).
• If the oscillator operates above the audio
  frequency range (gt20KHz), the signal generator
  will often include some sort of modulation
  including one or more of amplitude modulation
  (AM), frequency modulation (FM), or phase
  modulation (PM) as well as a second oscillator
  that provides an audio frequency modulation
  waveform.

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Signal Generated
SIGNAL GENERATOR
A bandlimited sawtooth wave pictured in the time
domain (top) and frequency domain (bottom). The
fundamental is at 220 Hz.
Sine and cosine wave
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Function Generator
SIGNAL GENERATOR

• A function generator is a piece of electronic
  test equipment used to generate repetitive
  waveforms.
• These waveforms can then be injected into a
  device under test and analyzed as they progress
  through the device, confirming the proper
  operation of the device or pinpointing a fault in
  the device.
• Function generators usually generate a triangle
  waveform as their basic output.

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Function Generator (contd)
SIGNAL GENERATOR

• The triangle is generated by repeatedly charging
  and discharging a capacitor from a constant
  current source.
• This produces a linearly-ascending or descending
  voltage ramp.
• As the output voltage reaches upper and lower
  limits, the charging and discharging is reversed,
  producing the linear triangle wave.
• By varying the current and the size of the
  capacitor, different frequencies may be obtained.

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Function Generator (contd)
SIGNAL GENERATOR

• Most function generators also contain a diode
  shaping circuit that can convert the triangle
  wave into a reasonably-accurate sine wave.
• Function generators, like most signal generators,
  may also contain an attenuator, various means of
  modulating the output waveform, and often contain
  the ability to automatically and repetitively
  "sweep" the frequency of the output waveform
  between two operator-determined limits.
• This capability makes it very easy to evaluate
  the frequency response of a given electronic
  circuit.

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Schematic Diagram for Function Generator
SIGNAL GENERATOR
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Function Generator (FG) Principle
SIGNAL GENERATOR

• Built around a single 8038 waveform generator IC,
  this circuit produces sine, square or triangle
  waves from 20Hz to 200kHz in four switched
  ranges.
• There are both high and low level outputs which
  may be adjusted with the level control.
• All of the waveform generation is produced by
  IC1.
• This versatile IC even has a sweep input, but is
  not used in this circuit.

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FG Principle (contd)
SIGNAL GENERATOR

• The IC contains an internal square wave
  oscillator, the frequency of which is controlled
  by timing capacitors C1 - C4 and the 10k
  potentiometer.
• The tolerance of the capacitors should be 10 or
  better for stability.
• The square wave is differentiated to produce a
  triangular wave, which in turn is shaped to
  produce a sine wave.
• All this is done internally, with a minimum of
  external components.
• The purity of the sine wave is adjusted by the
  two 100k preset resistors.

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FG Principle (contd)
SIGNAL GENERATOR

• The wave shape switch is a single pole 3 way
  rotary switch, the wiper arm selects the wave
  shape and is connected to a 10k potentiometer
  which controls the amplitude of all waveforms.
• IC2 is an LF351 op-amp wired as a standard direct
  coupled non-inverting buffer, providing isolation
  between the waveform generator, and also
  increasing output current.
• The 2.2k and 47 ohm resistors form the output
  attenuator. At the high output, the maximum
  amplitude is about 8V pk-pk with the square wave.

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FG Principle (contd)
SIGNAL GENERATOR

• The maximum for the triangle and sine waves is
  around 6V and 4V respectively.
• The low amplitude controls is useful for testing
  amplifiers, as amplitudes of 20mV and 50mV are
  easily achievable.

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Type of Signal
SIGNAL GENERATOR
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Summary
SIGNAL GENERATOR

• This week we have looked at the operation of
• MULTIMETER
• OSCILLOSCOPE
• PROBES
• SIGNAL GENERATOR