Chapter 14

Electronic
Instruments
Dr.Debashis De
Associate Professor
West Bengal University of Technology
Contents:
● 14-1 Introduction
● 14-2 Components of the Cathode-Ray
Oscilloscope
● 14-3 Cathode-Ray Tube
● 14-4 Time-Base Generators
● 14-5 Measurements Using the
● Cathode-Ray Oscilloscope
● 14-6 Types of Cathode-Ray Oscilloscopes
● 14-7 Sweep Frequency Generator
● 14-8 Function Generator
● 14-9 Sine Wave Generator
● 14-10 Square Wave Generator
● 14-11 AF Signal Generator
Objectives:
● This final chapter discusses the key instruments of
electronic measurement with special emphasis on the most versatile
instrument of electronic measurement—the cathode-ray
oscilloscope (CRO).
● The objective of this book will remain unrealized
without a discussion on the CRO.
● The chapter begins with the details of construction of
the CRO, and proceeds to examine the active and passive mode
input–output waveforms for filter circuits and lead-lag network delay.

● This will be followed by a detailed study of the dual

beam CRO and its uses in op-amp circuit integrator, differentiator,
inverting and non-inverting circuits, comparative waveform study,
and accurate measurement with impeccable visual display.
● In addition to the CRO, the chapter also examines the
sweep frequency generator, the function generator, the sine wave
generator, the
square wave generator and the AF signal generator.
INTRODUCTION:
● The cathode-ray oscilloscope (CRO) is a
multipurpose display instrument used for the observation,
measurement , and analysis of waveforms by plotting
amplitude along y-axis and time along x-axis.
● CRO is generally anx-y plotter; on a single
screen it can display different signals applied to different
channels. It can measure amplitude, frequencies and phase
shi of various signals. Many physical quantities like
temperature, pressure
● and strain can be converted into electrical signals by the use
of transducers, and the signals can be displayed on the CRO.
● A moving luminous spot over the screen
displays the signal. CROs are used to study waveforms, and
other time-varying phenomena from very low to very high
frequencies.
● The central unit of the oscilloscope is the
cathode-ray tube (CRT), and the remaining part of the CRO
consists of the circuitry required to operate the cathode-ray
tube.
Block Diagram of CRO
Block diagram of a cathode-ray
oscilloscope:
COMPONENTS OF THE CATHODE-RAY
OSCILLOSCOPE:

The CRO consists of the following:

● CRT
● Vertical amplifier
● Horizontal amplifier
● Time-base generator
● Triggering circuit
● High and Low voltage supply
● Delay line
CATHODE-RAY TUBE:
● electron gun or electron emitter, the
The
deflecting system and the fluorescent screen are the
three major components of a general purpose CRT. A
detailed diagram of the cathode-ray oscilloscope is given in
Fig. 14-2.
CRT
Electron Gun:

● In the electron gun of the CRT, electrons are emitted,

converted into a sharp beam and focused upon the fluorescent
screen.
● The electron beam consists of an indirectly heated
cathode, a control grid, an accelerating electrode and a focusing
anode.
● The electrodes are connected to the base pins. The
cathode emitting the electrons is surrounded by a control grid
with a fine hole at its centre.
● The accelerated electron beam passes through the
fine hole.
● The negative voltage at the control grid controls the
flow of electrons in the electron beam, and consequently, the
brightness of the spot on the CRO screen is controlled.
● Working of Cathode Ray Tube
● As the cathode is heated, it produces a
large number of electrons.
● These electrons pass through the
control grid on their way to the screen.
● The control grid controls the amount
of current flow as in standard vacuum
tubes. If negative potential on the
control grid is high, fewer electrons will
pass through it. Hence the electron
beam will produce a dim spot of light
on striking the screen. Reverse will
happen when the negative potential on
the control grid is reduced.
● Therefore, the intensity of the light spot on
the screen can be controlled by changing
the negative potential on the control grid.
● A er leaving the control grid, the electron
beam comes under the influence of focusing
and accelerating anodes.
● Since, the two anodes are at high positive
potential, therefore, they produce a field
which acts as electrostatic lens to converge
the electron beam at a point on the screen.
● A er leaving the accelerating anode,
the electron beam comes under the
influence of vertical and horizontal
deflection plates.
● When no voltage is applied to these
deflection plates, the electron beam
produces a spot of light at the centre
as shown by  point O in fig below on
the screen.
● If the voltage is applied to the vertical
deflection plates only, the electron
beam and so as the spot of light  will
be deflected upwards i.e. point O1.
Ans if the potential on the plates is
reversed, the spot of light will be
deflected downwards i.e. point O2.
● Similarly, the spot of light can be
deflected horizontally by applying
voltage across the horizontal
deflection plates.
 Deflection Systems:
● To deflect the beam horizontally, an alternating voltage is
applied to the horizontal deflecting plates and the spot on the screen
horizontally, as shown in Fig. 14-3(b).
● The electrons will focus at point X2. By changing the polarity
of voltage, the beam will focus at point X1. Thus, the horizontal
movement is controlled along X1OX2 line.
Deflection Systems:
● Electrostatic deflection of an electron beam
is used in a general purpose oscilloscope. The
deflecting system consists of a pair of horizontal and
vertical deflecting plates.
● Let us consider two parallel vertical
deflecting plates P1 and P2. The beam is focused at point
O on the screen in the absence of a deflecting plate voltage.

● If a positive voltage is applied to plate P1

with respect to plate P2, the negatively charged electrons
are attracted towards the positive plate P1, and these
electrons will come to focus at point Y1 on the fluorescent
screen.
Deflection Systems:
The deflection is proportional to the deflecting voltage
between the plates. If the polarity of the deflecting voltage is
reversed, the spot appears at the point Y2, as shown in Fig. 14-3(a).
Spot Beam Deflection Sensitivity:
Electrostatic Deflection:
Electrostatic Deflection:
Electrostatic Deflection:
Electrostatic Deflection:
Fluorescent Screen:
● Phosphor is used as screen material on the
inner surface of a CRT. Phosphor absorbs the energy
of the incident electrons. The spot of light is
produced on the screen where the electron beam hits.

● The bombarding electrons striking the

screen, release secondary emission electrons. These
electrons are collected or trapped by an aqueous
solution of graphite called “Aquadag” which is
connected to the second anode.
● Collection of the secondary electrons is
necessary to keep the screen in a state of electrical
equilibrium.
● The type of phosphor used, determines
the color of the light spot. The brightest available
phosphor isotope, P31, produces yellow–green light
with relative luminance of 99.99%.
● The screen of the CRT is coated with
a natural or synthetic phosphor that
emits visible light when the electron
beam impinges on it.
● The phosphor tends to absorb the kinetic
energy of the impinging electron beam
and re-emits the same absorbed energy in
the visible spectrum (400 – 700 nm).
● This phenomenon of certain materials
to emit energy in the visible range
a er absorbing energy or being
stimulated by radiation is referred to as
Fluorescence.
● Fluorescent materials sometimes also possess a
secondary property called Phosphorescence
wherein they continue to emit light even a er the
source of excitation is removed.
● The length of time during which ‘a erglow’ or
Phosphorescence occurs is popularly referred to
as Persistence of the phosphor.
● Luminance is another property of the phosphor that
gives an idea about the intensity of light emitted
from the phosphor.
● Some of the standard phosphor types include
materials including Zn2SiO4:Mn [Green, 528 nm],
● ZnS:Cu(Ag)(B*) [Blue-Green, 543 nm],
Zn8:BeSi5O19:Mn [Yellow, 602 nm] etc. The type of
● phosphor controls the type of signal whether fast or
slow can be seen on the CRT screen
Display waveform on the screen:
Figure 14-5(a) shows a sine wave applied to vertical deflecting
plates and a repetitive ramp or saw-tooth applied to the horizontal plates.
● The ramp waveform at the horizontal plates causes the
electron beam to be deflected horizontally across the screen.
● If the waveforms are perfectly synchronized then the exact
sine wave applied to the vertical display appears on the CRO display screen.
 Triangular waveform:
● Similarly the display of the triangular waveform is as shown in Fig. 14-5(b).
TIME-BASE GENERATORS:
● The CRO is used to display a waveform that varies as a function
of time. If the wave form is to be accurately reproduced, the beam should
have a constant horizontal velocity.
● As the beam velocity is a function of the deflecting voltage, the
deflecting voltage must increase linearly with time.
● A voltage with such characteristics is called a ramp voltage. If
the voltage decreases rapidly to zero—with the waveform repeatedly
produced, as shown in Fig. 14-6—we observe a pattern which is generally
called a saw-tooth waveform.
● The time taken to return to its initial value is known as flyback
or return time.
Simple saw-tooth generator &
associated waveforms:
● The circuit shown in Fig. 14-7(a) is a simple sweep circuit,
in which the capacitor C charges through the resistor R.
● The capacitor discharges periodically through the transistor T1,
which causes the waveform shown in Fig. 14-7(b) to appear across the
capacitor.
● The signal voltage, Vi which must be applied to the base of the
transistor to turn it ON for short time intervals is also shown in
Fig. 14-7(b).
Trigger (Sync.) Circuit:
A sample of the input waveform is fed to a trigger circuit
which produces a trigger pulse at some selected point on the
input waveform. This trigger pulse is used to start the time
base generator which then starts the horizontal sweep of
CRT spot from left hand side of the screen.
Time base (Sweep) Generator:
This produces a saw – tooth waveform that is used as
horizontal deflection voltage of CRT. The rate of rise of
positive going part of sawtooth waveform is controlled by
TIME/DIV knob. The sawtooth voltage is fed to the horizontal
amplifier if the switch is in INTERNAL position. If the switch
is in EXT. position, an external horizontal input can be
applied to the horizontal amplifier.
Vertical Amplifier:
The input signal is applied to vertical amplifier. The
gain of this amplifier can be controlled by VOLT/DIV
knob. Output of this amplifier is applied to the delay
line.
Delay Line:
The delay Line retards the arrival of the input
waveform at the vertical deflection plates until the
trigger and time base circuits start the sweep of the
beam. The delay line produces a delay of 0.25
microsecond so that the leading edge of the input
waveform can be viewed even though it was used to
trigger the sweep.
Horizontal Amplifier:
This amplifies the saw – tooth voltage. As it includes a
phase inverter two outputs are produced. Positive going
sawtooth and negative going sawtooth are applied to
right – hand and left – hand horizontal deflection plates
of CRT.
Blanking Circuit:
The blanking circuit is necessary to eliminate the retrace
that would occur when the spot on CRT screen moves
from right side to left side” This retrace can cause
confusion if it is not eliminate. The blanking voltage is
produced by sweep generator. Hence a high negative
voltage is applied to the control grid during retrace
period or a high positive voltage is applied to cathode in
CRT.
Time-base generator using UJT:
● The continuous sweep CRO uses the UJT as a time-base
generator. When power is first applied to the UJT, it is in the OFF
state and CT changes exponentially through RT .
● The UJT emitter voltage VE rises towards VBB and VE reaches
the plate voltage VP.
● The emitter-to-base diode becomes forward biased and
the UJT triggers ON. This provides a low resistance discharge path
and the capacitor discharges rapidly.
● When the emitter voltage VE reaches the minimum value
rapidly, the UJT goes OFF. The capacitor recharges and the cycles
repeat. To improve the sweep linearity, two
separate voltage supplies are used; a low voltage
supply for the UJT and a high voltage supply for the
RTCT circuit. This circuit is as shown in Fig. 14-7(c).
RT is used for continuous control of
frequency within a range and CT is varied or
changed in steps. They are sometimes known as
timing resistor and timing capacitor.
 Oscilloscope Amplifiers:
● The purpose of an oscilloscope is to produce a faithful
representation of the signals applied to its input terminals.
● Considerable attention has to be paid to the design of these
amplifiers for this purpose. The oscillographic amplifiers can be classified
into two major categories.
(i) AC-coupled amplifiers
(ii) DC-coupled amplifiers
● The low-cost oscilloscopes generally use ac-coupled amplifiers.
The ac amplifiers, used in oscilloscopes, are required for laboratory
purposes. The dc-coupled amplifiers are quite expensive. They
offer the advantage of responding to dc voltages, so it is possible to
measure dc voltages as pure signals
and ac signals superimposed upon the dc signals.
● DC-coupled amplifiers have another advantage. They eliminate
the problems of low-frequency phase shi and waveform distortion while
observing low-frequency pulse train.
● The amplifiers can be classified according to bandwidth use also:
(i) Narrow-bandwidth amplifiers
(ii) Broad-bandwidth amplifiers
Vertical Amplifiers:
● Vertical amplifiers determines the sensitivity and
bandwidth of an oscilloscope. Sensitivity, which is expressed in
terms of V/cm of vertical deflection at the mid-band frequency.
● The gain of the vertical amplifier determines the
smallest signal that the oscilloscope can satisfactorily measure by
reproducing it on the CRT screen.
● The sensitivity of an oscilloscope is directly
proportional to the gain of the vertical amplifier. So, as the gain
increases the sensitivity also increases.
● The vertical sensitivity measures how much the
electron beam will be deflected for a specified input signal. The
CRT screen is covered with a plastic grid pattern called a graticule.
● The spacing between the grids lines is typically 10
mm. Vertical sensitivity is generally expressed in volts per division.
● The vertical sensitivity of an oscilloscope measures
the smallest deflection factor that can be selected with the rotary
switch.
 Frequency response:
● The bandwidth of an oscilloscope detects the range of frequencies that
can be accurately reproduced on the CRT screen. The greater the bandwidth, the wider
is the range of observed frequencies.
● The bandwidth of an oscilloscope is the range of frequencies over which
the gain of the vertical amplifier stays within 3 db of the mid-band frequency gain, as
shown in Fig. 14-8.
● Rise time is defined as the time required for the edge to rise from
10–90% of its maximum amplitude. An approximate relation is given as follows:
MEASUREMENTS USING THE CATHODE-RAY OSCILLOSCOPE:
1) Measurement of Frequency:
MEASUREMENTS USING THE CATHODE-RAY OSCILLOSCOPE:
● 2) Measurement of Phase:

● 3 Measurement of Phase Using Lissajous

Figures:
Measurement of Phase Using Lissajous Figures:
Measurement of Phase Using Lissajous Figures:
Measurement of Phase Using Lissajous Figures:
Measurement of Phase Using Lissajous Figures:
 TYPES OF THE CATHODE-RAY OSCILLOSCOPES:
● The categorization of CROs is done on the basis of whether
they are digital or analog. Digital CROs can be further classified as
storage oscilloscopes.
● 1. Analog CRO: In an analog CRO, the amplitude, phase and frequency
are measured from the displayed waveform, through direct manual
reading.
● 2. Digital CRO: A digital CRO offers digital read-out of signal
information, i.e., the time, voltage or frequency along with signal
display. It consists of an electronic counter along with the main body of
the CRO.
● 3. Storage CRO: A storage CRO retains the display up to a substantial
amount of time a er the first trace has appeared on the screen. The
storage CRO is also useful for the display of waveforms of low-
frequency signals.
● 4. Dual-Beam CRO: In the dual-beam CRO two electron beams fall on
a single CRT. The dual-gun CRT generates two different beams.
● These two beams produce two spots of light on
the CRT screen which make the simultaneous observation of two
different signal waveforms possible. The comparison of input and its
corresponding output becomes easier using the dual-beam CRO.
SWEEP FREQUENCY GENERATOR:
● A sweep frequency generator
is a signal generator which can
automatically vary its frequency
smoothly and continuously over an
entire frequency range. Figure 14-15
shows the basic block diagram of a
sweep frequency generator.
● The sweep frequency
generator has the ramp generator and
the voltage-tuned oscillator as its basic
components.
Applications of the Sweep Frequency Generator:
FUNCTION GENERATOR:
● The basic components of a function generator are:
● (i) Integrator
● (ii) Schmitt trigger circuit
● (iii) Sine wave converter
● (iv) Attenuator
SINE WAVE GENERATOR:
● A sine wave is produced by converting a triangular wave,
applying proper circuits. The triangular wave is produced by employing an
integrator and a Schmitt trigger circuit.
● This triangular wave is then converted to a sine wave using the
diode loading circuit ,as shown in Fig. 14-19. Resistors R1 and R2 behave as
the voltage divider. When VR2 exceeds V1, the diode D1 becomes forward-biased.
● There is more attenuation of the output voltage levels above
V1 than levels below V1. With the presence of the diode D1 and resistor R3 in
the circuit, the output voltage rises less steeply.
● The output voltage falls below V1 and the diode stops conducting, as
it is in reverse-bias. The circuit behaves as a simple voltage-divider circuit. This
is also true for the negative half-cycle of the input Vi . If R3 is carefully
chosen to be the same as R4 , the negative and the positive cycles of the output
voltage will be the same. The output is an approximate sine wave.
 SINE WAVE GENERATOR:
● The approximation may be further
improved by employing a six-level diode
loading circuit, as shown in Fig. 14-20(a).
SINE WAVE GENERATOR:
● The circuit is adjusted by comparing a 1 kHz sine
wave and the output of the triangular/sine wave converter on a
dual-track CRO. R1, R2, R3 and the peak amplitude of Ei are adjusted
in sequence for the best sinusoidal shape.
CIRCUIT DIAGRAM OF SINE WAVE GENERATOR:
 SQUARE WAVE GENERATOR
● A square wave can be most easily obtained from an
operational amplifier astable multi-vibrator. An astable multi-
vibrator has no stable state—the output oscillates continuously
between high and low states.
● In Fig. 14-21, the block comprising the op-amp,
resistors R2 and R3 constitutes a Schmitt trigger circuit. The capacitor
C1 gets charged through the resistor R1. When the voltage of the capacitor
reaches the upper trigger point of the Schmitt trigger circuit, the
output of the op-amp switches to output low. This is because the
Schmitt trigger is a non-inverting type. Now, when the op-amp
output is low, the capacitor C1 starts getting discharged.
 SQUARE WAVE GENERATOR:
● As the capacitor discharges and the capacitor
voltage reaches the lower trigger point of the Schmitt trigger,
the output of the op-amp switches back to the output high
state.
● The capacitor charges through the resistor
again and the next cycle begins. The process is repetitive
and produces a square wave at the output.
● The frequency of the output square wave
depends on the time taken by the capacitor to get charged
and discharged when the capacitor voltage varies from UTP
(upper trigger point) and LTP (lower trigger point).
AF SIGNAL GENERATOR:
POINTS TO REMEMBER:
● 1. CRO is used to study waveforms.
● 2. CRT is the main component of a CRO.
● 3. Prosperous P31 is used for the fluorescent screen of a CRO.
● 4. A CRO has the following components:
● (a) Electron gun
● (b) Deflecting system
● (c) Florescent screen
● 5. Lissajous figures are used to measure frequency and phase
of the waves under study.
● 6. A time-base generator produces saw-tooth voltage.
● 7. An oscilloscope amplifier is used to provide a faithful
representation of input signal applied to its input terminals.
IMPORTANT FORMULAE:
 analog oscilloscope digital oscilloscope

It reads the analog voltage and

Direcly reads analog voltage
converts it into digital form before
and displays it on screen. being displayed on the screen.

Do not require ADC,

Requires ADC, microprocessor and
microprocessor and acquisition memory
acquision memory

Can only analyze signal in Can analyze signal in real time as well
as can analyze previously acquired
real time as there is no large samples of data with facility of
storage memory available. storage available.

Can analyze high frequency transients

Can not analyze high due to advanced DSP algorithms
frequency sharp rise time available and ported on
microprocessor which can operate on
transients
stored samples of input voltage.