CATHODE RAY OSCILLOSCOPE

(CRO)

Rahul Katiyar
Assistant Professor
Banasthali University
CRT photograph
 CRT SIMPLIFIED…..
y plates x plates
electron gun
produces a beam
of electrons anode light produced on
the screen by
electron beam

heater
supply

a p.d. across the y a p.d. across the x

-plates
+ deflects the plates deflects the
H.T. supply
trace vertically phosphor
trace horizontally
screen
Electrostatic Electromagnetic
Deflection Deflection
Beam Deflection More
angle is less
Fast speed Slow
Very large BW Less than 20 KHz
possible Dependent on e/m
Deflection ratio
sensitivity is
independent of
electron
parameters Used where area of
Used where area of sweep are large and
sensitivity
weep are small and
requirements are
image is formed at
high
the center of
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
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.
 Electrostatic 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.
 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.
An electrically deflected CRT has a final
anode voltage of 2 KV and parallel
deflecting plate 1.5 cm long and 5 mm
apart. If the screen is 50 cm from the
center of the deflecting plates, find (a)
Beam speed (b) Deflection Sensitivity ©
Deflection factor of CRT.
Block diagram of a CRO
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).
Sawtooth 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 R1 .
 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.
 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 the CRO consists of
the circuitry required to operate the cathode-ray tube.
 What is the purpose of an
oscilloscope
The purpose of an oscilloscope is to measure
a voltage that changes with time and show it
in a graphical format

1) Here is the oscilloscope in

our lab
-Notice the X-Y axes
2) Here is our alternating voltage
signal from before
Cables
We will use three
types of connecters
in this lab.
BNC
Banana
Mini-Grabber
Triggering
It is required to tell the scope when to
display the signal.
Electric signals change much faster than
we can observe, so we must tell the
Oscilloscope when to refresh the display.
We accomplish this by setting a Triggering
Level.
Triggering

Without Triggering With Triggering

Triggering
We want to tell the oscilliscope when it is
the best time for it to “refresh” the display
In our wave below, we tell the scope to
“trigger” or ‘capture’ the signal when it is
going upward AND hits 2.0Volts

SO, ‘trigger’ condition is:

When we’re

Going up!

AND
George Washington University
When at 2.0 Volts on our waveform!
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:
 Theelectron 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.
 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.
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%.
 CRT Features
Size: Screen Diameter (3 inch)

Front of CRT: Faceplate (flat for screen sizes 80

mm * 100 mm and slightly curved for larger
displays.

Operating voltages

Graticule: Grid of lines that serves as scale when making time

and amplitude measurements. They are generally laid out in a
8 * 10 pattern with a 8 major divisions along the vertical axis
and 10 major divisions along the horizontal axis.
Spacing between grid lines: 10 mm.
Screen: Light produced by screen does not disappear
immediately when bombardment by electron ceases i.e. signal
becomes zero. The time period for which the trace remains on
the screen after the signal becomes zero is called
“PERSISTENCE”.
Phosphor: Medium persistence [ P1, P2, P11, P31] - (general
purpose lab applications)
Long persistence phosphor [P 19, P26]- Transient waveforms
display - Radar
Short persistence phosphor [P 13] – High speed waveforms –
Sampling CRO
Aquadog : The bombarding electrons, striking the CRT screen,
release secondary emission electrons. These secondary
electrons are collected by an aqueous solution of graphite called
aquadog. It is coated with the internal walls on CRT.
 CRO CONTROLS
Intensity control – Controlled by varying the cathode grid
negative potential.
Focus control – Done by adjusting the focusing anode
potential.
Position control – Small DC potential is applied to the
deflecting plates to keep the position of the beam at the
center of the screen. For accurate positioning at the middle
of screen, “Astigmatism” is used.
Synchronizing control – Lock the display of the signal
Calibration control – Uses a square waveform calibrating
voltage.
 CRO SPECIFICATIONS
 Sensitivity
 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:
Vertical Amplifiers:
 Determines the sensitivity and bandwidth of an oscilloscope.

 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.

 Vertical sensitivity is generally expressed in volts per division.

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
Dual Trace Oscilloscope
Dual Beam Oscilloscope
 Dual Beam
Dual Trace CRO CRO
Two electron guns
Used single electron
beam to display two which passes through
traces. two completely
Cannot capture two
separate VDP in order
to display two signals.
fast transients.
Used to capture two
Low cost (Widely used)
fast transients.
Single VDP & Single VA
Cost- Very high
It cannot used switch
(Research Labs)
quickly between Double VA and Double
traces, so
Vertical deflection
simultaneous display
plates.
becomes difficult.
Display two traces
Brightness and focus
simultaneously.
are not controlled
 Sampling CROs are capable of operating at Very high frequencies
(Upto 15 GHz).
 Only repetitive waveforms can be investigated.
 Transient waveforms cannot be investigated by sampling CROs.
 The resolution of the final image depends upon the size of steps of
staircase generator. Smaller the size of steps, larger the no. of
samples and higher the image resolution.
 Used to examine very fast signals.
 Used to respond & store rapid bits of information and presents them
in a continuous display.
Digital Storage Oscilloscope
 DSO - Features
High Accuracy & Storage capacity.
High bandwidth and High speed
Simple user interface.
Easy comparison of stored and live signals
Retain the image of the waveform on the screen
for a practically unlimited span of time.
Producer better and more contrast waveform
images.
Easy operation.
Used in research and medical fields.
Only drawback: Low bandwidth bcoz of low speed
of ADC.