CRO and FG.ppt

Electronic
Instruments
Chapter 14
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

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 an x-y plotter; on a
single screen it can display different signals applied to
different channels. It can measure amplitude,
frequencies and phase shift 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

Block diagram of a cathode-ray
oscilloscope:

COMPONENTS OF THE CATHODE-RAY
OSCILLOSCOPE:
The CRO consists of the following:
 (i) CRT
 (ii) Vertical amplifier
 (iii) Delay line
 (iv) Horizontal amplifier
 (v) Time-base generator
 (vi) Triggering circuit
 (vii) Power supply

CATHODE-RAY TUBE:
 The electron gun or electron emitter, 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.

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.

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

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).

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.

Spot Beam Deflection Sensitivity:

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

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).

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 shift 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:

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 after 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

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.

 The approximation may be further improved
by employing a six-level diode loading circuit, as
shown in Fig. 14-20(a).

 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).

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.

CRO and FG.ppt

More Related Content

What's hot (20)

Similar to CRO and FG.ppt (20)

Recently uploaded (20)

CRO and FG.ppt