The document describes the key components and operation of a cathode ray oscilloscope (CRO). The CRO uses a cathode ray tube (CRT) to display signal waveforms by deflecting an electron beam. The CRT contains an electron gun, deflection plates, and fluorescent screen. It generates the electron beam and accelerates and focuses it to create a visible spot when it hits the screen. The document explains how the electron beam is deflected horizontally and vertically by the input signal to display the signal's amplitude over time. It provides details on various CRO parts and controls as well as techniques for improving its performance with high frequency signals.

1. Unit 1

2. • Cathode ray oscilloscope: most useful lab
instrument for studying waveshapes of
alternating currents & voltages, measurement of
voltage, current, power & frequency (any qty that
involves amplitude & waveform)
• Allows user to see amplitude of electrical signals
as a ftn of time on the screen
• The CRO depends on the movement of an
electron beam which is deflected on the x and y
axis.

4. Parts of CRO
An oscilloscope consists of two parts-
• Cathode ray tube
• Control and input circuitry

5. CRT
• Cathode ray tube is the heart of the
oscilloscope.
• The CRT makes the applied signal visible by
the deflection of a thin beam of electrons.
– Generates the electron beam
– Accelerates the beam to a high velocity,
– deflects the beam to create an image
– Contains a phosphor screen where the electron
beam becomes visible

6. • Low voltage supply: for heater of the electron
gun for generation of electron beam
• High voltage supply: for CRT to accelerate the
electron beam
• Normal voltage supply: for other control ckts
in CRO

7. • Horizontal & vertical deflection plates fitted
btwn electron gun & screen to deflect the
beam acc to input signal
• Elec beam strikes the screen & creates a
visible spot: spot deflected on the screen in
the hor direction (X axis) with constant time
dependant rate
• The signal to be viewed is supplied to the
vertical deflection plates through the ver
amplifier: raises the potential of the input
signal : provide usable deflection of electron
beam

8. • Elec beam deflects in 2 directions: X (hor) & Y
(ver)
• Triggering ckt provided for synchronizing the
two types of deflection : hor deflection starts
at the same point of input ver signal each time
it sweeps

9. • Main parts of CRT are-
– Electron gun assembly: for producing a stream of
electrons
– Deflection plate assembly: for controlling the beam
path
– Fluorescent screen
– Glass envelope: evacuated glass envelope with
phosphor screen giving bright spot when struck by a
high vel electron beam
– Base: connections made to diff components

10. 1. Electron gun assembly
• Consists of
– Indirectly heated cathode
– Control grid surrounding the cathode
– A focusing anode & accelerating anode
• Ftn: to provide a focused elec beam accelerated
towards the phosphor screen
• Cathode: heated -> emits plenty of elec; emitting
surface as small as possible. Rate of emission of
elec depends on the magnitude of cathode
current controlled by control grid
• Beam comes out of control grid through a small
hole & enters pre-accelerating anode (more
positive than cathode: to accelerate the beam in
electric field)

11. • Accelerated beam gets scattered (variations in
energy):
– lead to ill defined spot on screen
– So focussing: by electrostatic lens (focussing anode &
accelerating anode: these concentrate & focus the
beam on screen & accelerate the speed of elec)

12. 2. Deflection plate assembly
• Elec beam passes through the 2 pairs of
deflection plates
– One pair: mounted vertically: deflects beam in hor or
X direction: X plates
– Other pair: mounted horizontaly: deflects beam in ver
or Y direction: Y plates
• Deflect the beam acc to the voltage applied
across them
• Eg: a const pd applied to Y plates: elec beam will
deflect upward if upper plate is positive: deflect
downward if lower plate is positive

13. 3. Screen
• Crystalline mat like phosphor: property of
emitting light when exposed to radiation:
fluorescence (electrical energy to light energy)
– Phosphoroscence: fluorescent mat continue to emit
light even after radiation exposure is cut off
– Length of time during which it occurs: persistence
• End wall of CRT coated with phosphor; when
elec beam strikes the CRT screen, a spot of light
is produced, phosphor absorbs kinetic energy of
bombarding elec & emits energy

14. • Luminance: intensity of light emitted from CRT
screen depends on:
– no of bombarding elec striking the screen/sec
– The energy with which the bombarding elec strike
the screen
– Time the beam strikes a given area of the screen :
sweep speed
– Physical char of phosphor

15. 4. Glass body & base
• CRT assembly protected in a conical evacuated glass
housing
• Inner walls of CRT btwn neck & screen are usually
coated with a conducting material: aquadog :
– coating is electrically connected to accelerating anode
– Coating provided to accelerate the elec beam after
passing btwn deflectig plates & collect elec produced by
secondary emission when elec beam strikes the screen
– Prevents formation of –ve charge on the screen
– Graticule: hor & ver marks are marked on the screen to
provide user a correct measurement

16. Focusing device
• Beam coming out of accelerating anode has a tendency to
spread from the axis: because of mutual repulsion btwn
electrons
• To bring beam to sharp focus at screen: focusing device
• Electrostatic focusing in CRO: Electrostatic lens consists of
3 anodes (middle anode at lower potential than other 2)

17. Electrostatic focusing in CRO
• In fig, 2 anodes & its
electrostatic lines &
equipotential
surfaces
• A pot diff is kept btwn
the electrodes: elec
field generated
• Spreading of elec field caused (bcos of repulsion btwn
electric field lines): equipotentials would bulge at the
centre of the anodes
• Electrons move in direction opp to that of elec field lines
• Equipot surfaces are perpendicular to elec field lines :
force on elec exerted in direction normal to the equipot
surface

18. • Fig: elec entering at
the centre line of the
two anodes
experience no force
• Elec displaced from
centre: experience
force normal to the
direction of equipot
surface & deflects
• Elec with vel v1, at angle θ1 enters & experiences force : vel
increases to v2. (force on elec only in direction normal to
the equipot surface: only normal component v1N increases
to v2N; tangential comp v1T remains same )
• v1T = v1 sinθ1; v2T = v2 sinθ2
As v1T = v2T , so v2/v1 = sinθ1/sinθ2
• Equipot surface acts as concave lens: electrostatic lens

19. ES deflection of moving elec in CRT
• Principle: Force exp by elec when kept in uniform elec field
• Consider : elec with initial vel u (along X-axis at O btwn plates
A & B); length of A =B= l ; dist btwn A & B = d; pot diff across
plates= V
• Period during which an elec remains in region btwn A & B : t=
l /u
• No initial vel along Y axis, but acceleration =
• Vel of elec along Y axis after time t

20. • After leaving the plate region, elec travels in a
strght line (as no field acts). If this line is
extended backward, it intercepts the X axis at
the centre of the plates at x= l /2
• S: dist along X axis from this point x; deflection
y determined by similar triangles
• Va= accelerating voltage;
• Vd= deflecting voltage
• Substitute u2 in y

21. • So, For a fixed accelerating voltage Va, the
deflection of elec beam on the screen is
directly prop to the deflection voltage Vd: CRT
is linear voltage indicating device (image
follows variations in Vd in linear manner)
• Deflection sensitivity: vertical defl of the beam
on screen per unit deflecting voltage
• Deflection factor: 1/ Deflection sensitivity

22. Post Deflection Acceleration
• After electrons pass beyond the deflection plates, they
may or may not experience additional acceleration.
• This depends primarily upon on the max freq to be
applied to CRT.
• For good sensitivity Ea should be low below 4 kV but
reduces brightness, which can be seriously impaired at
high frequencies.
• Below 10 MHz ,monoaccelerator may be used.
• If signals of frequencies higher than 10 MHz are to
displayed, post deflection acceleration tubes (PDA) or
post accelerators is necessary to increase the
brightness of the trace which otherwise would be dim.

23. Basic controls
• No of controls reqd in CRO to facilitate proper ftning
• Intensity control provided for adjusting brightness of
spot on screen: by varying voltage btwn 1st and 2nd
anodes
• Hor & ver position controls: for moving the beam on ay
part of screen : by applying dc voltage to defl plates
1. Ver defl system
2. Hor defl system
3. Position control (knobs to move the spot hor: left/
right or ver: up/down)
4. Intensity control: by grid potential (ie the amount of
elec leaving the cathode; large number= brighter
spot)

24. 5. Focus control: focusing elec beam by varying pot of
middle anode (ES lens)
6. Astigmatism: Additional Focus control
7. Banking ckt: hor defl plates have sawtooth sweep
voltage ( this moves the spot on the screen during
sweep period) : retrace time in sawtooth waveform
should be zero; if not blank it by supplying high –ve
voltage to the grid during retrace.
8. Calibration ckt: an oscillator for generating known
& fixed voltage in square waveform

25. Time base generator
• Time base: axis used to represent time so that
variations of qty like voltage, current can be plotted
wrt time
• For movement of spot, voltage (linearly varying
with time) is applied to one defl plates pair. This
voltage sweeps elec beam across screen: sweep
voltage (shape is sawtooth / ramp)
• The ckt which develops this linearly time varying
voltage : Time base generator/ sweep ckt

26. • Fig: voltage starts from some initial value ,
increases linearly with time to a max value & then
returns to initial value
• Time req to return to initial value: Restoration time
/ flyback time (Tr)
• Time of linear portion: sweep time (Ts)
• (Tr) << (Ts)
• Idealized sweep: Tr zero & exact linear sweep;
Practically: deviation from linearity
1. Slope or sweep speed error:
– Ratio: difference in slope at the beginning & end of
sweep/ initial value or slope

27. 2. Displacement error:
– Ratio: max difference btwn actual sweep volt & linear
sweep volt passing through the beginning & end points
of sweep / max value attained by sweep voltage
– Define non-linearity of time base signal
3. Transmission error:
– Ratio: difference btwn max amplitude of I/P & O/P
signals to max amplitude of signal

28. Types of TBG
1. Volt TBG: generate a voltage linearly varying with
time (ES defl: CRO)
2. Current TBG: generate a current linearly varying
with time & this current is caused to flow through
inductors or defl coils (EM defl: TV)
• Other categorization:
– Triggered : generates a cycle & then comes back to its
stable state; the pulse rate depends on freq of applied
trigger
– Free running: switches back & forth btwn 2 levels & freq
depends on ckt parameters
– Synchronized: freq of oscillation determined by synch
voltage

29. Types of sweeps
• 1. free-running/ recurrent
– Sawtooth waveform is repetitive: new sweep stared
immediately after the previous sweep is terminated & ckt is
not initiated by any external signal
• 2. triggered / single
– Used when waveform to be observed is not periodic: so
desirable to keep sweep ckt inoperative & initiate the sweep
by waveform
– The spot is swept once across the screen in response to a
trigger signal: used for examination of transient or one time
signal
• 3. Driven
– Used where sweep is recurrent but triggered by signal under
investigation
• 4. Non-sawtooth
– Used for applications like comparison of 2 freq or for
determining the phase shift btwn 2 voltages and the sweep
freq vary with type of oscilloscope

30. Multiple Trace Oscilloscope
• In analysis of elec ckts & systems : view behavior of
2 or more voltages simultaneously: use 2
oscilloscopes? (cost, precise synchronization in
triggering, comparison tough)
• Soln: special CRO: uses 2 separate elec guns & defl
systems in same CRT: 2 diff elec beam displayed
simultaneously
– Dual Beam Oscilloscope
– Dual Trace Oscilloscope

31. Dual Beam Oscilloscope
• Display 2 simultaneous non-current signals
• Eg: I/P & O/P of a pulse shaping ckt both exhibited on
same screen
• In Dual beam CRO, 2 channels can have common time
base system or independent
• Common: Only one beam synchronized at a time: both
inputs must have same freq; Independent: allows diff
sweep rates for 2 beams
• 2 methods to obtain separate beams:
– The O/P of a single electron gun mechanically split into 2
separate beams: Split-beam technique (drawback: 2 displays
have diff brightness)
– 2 separate elec gun (allows brightness & focus of each beam
to be controlled ; drawback: bulkier)

33. Dual Trace Oscilloscope
• Single elec gun : produces dual trace display by
electronic switching of 2 separate i/p signals.
• 2 separate vertical i/p channels with separate
attenuator & preamplifiers: amplitude of each
signal independently controlled
• O/ps of preamplifier supplied to elec switch :
passes one signal at a time into main vertical
amplifier of oscilloscope
• Operating modes:
– Alternate
– Chop mode

36. • Alternate :
– Elec sw feeds each signal alternately to vertical amplifier. It
adds a diff dc component to each signal which directs the
beam alternately to upper or lower half of screen
– Switching done at the start of each new sweep of TBG:
channel A signal on one sweep, channel B signal on next
• Chop mode
– Elec sw free runs at high freq (100-500khz) ; independent of
the freq of TBG.
– Small segments from btoh channels are connected
alternately to main ver amp at a fast chopping rate
– chopping rate >> hor sweep rate: continuous display for
each channel (<< : continuity lost so btr use alternate mode)
• Added mode: single image displayed by addition of
signals from both channels (polarity inversion sw in
channels allow A+B, A-B, etc. )

37. • When the signal to be displayed is of a very high frequency
,the electron beam does not get sufficient time to pick up
the instantaneous level of the signal.
• Also at high frequencies the numbers of electrons striking
the screen in a given time and the intensity of the beam is
reduced.
• Instead of one set of deflection plates,a series of vertical
deflection plates are used.
• The plates are so shaped and spaced that an electron
travelling along the CRT receives from each set of plates an
additional deflecting force in proper time sequence.
High frequency crt or travelling wave
type crt

38. • This synchronisation is achieved by making the signal travel
from one plate to the next at the same speed as the transit
time of the electrons.
• The signal is applied to each pair of plates,and as the
electron beam travels the signal also travels through the
delay lines.
• The time delays are so arranged that the same electrons
are deflected by the input signal.
High frequency crt or travelling wave
type crt

39. 1. The vertical amplifier must be designed both for high B.W.
and high sensitivity or gain. Making the vertical amplifier
a fixed gain amplifier simplifies the design. The input to
the amplifier is brought to the required level by means of
an attenuator circuit. the final stages is the push-pull
stage.
2. The LF CRT is replaced by an HF CRT.
3. A probe is used to connect the signals,e.g. a high Z
passive probe acts like a compensated attenuator.
4. By using a triggered sweep,for fast rising signals,and by
the use of delay lines between the vertical plates,for
improvement of HF characteristics.
Characteristics of a HF CRO(HF
improvement in a CRO)

40. Sampling Oscilloscope
• Display repetitive signals (with freq higher than HF osc)
• Signal reconstructed from sequential samples of its
waveform
• Samples obtained at slightly diff points along successive
cycles of signal waveform: repetitive waveform so that
sampling can be done
• Sampling pulse turns on osc measuring ckt for a brief time:
ver posn of beam controlled by resulting voltage observed at
that point
• Following pulse samples the waveform in its next cycle at
slightly diff point
• Hor posn: stepped fwd very slightly & ver posn: determined
by new voltage
• Bandwidth of amplifier reqd to handle HF waveforms can be
much lower than the freq of waveform

41. • Repetitive i/p waveform applied to sampling gate
• Sampling pulse bias the diodes of balanced sampling
gate in fwd direction: connects gate i/p capacitance to
test point
• Ver amplifier amplifies capacitor voltage & applied to
ver defl plates
• Signal delayed in ver amp: sweep triggering initiated by
i/p signal

43. Storage oscilloscope
• The purpose of storage oscilloscope is to
freeze images or store them for later analysis.
These types of oscilloscopes are useful for
single shot events such as transients, glitches.
• Types of storage oscilloscopes are:
1.Analog Storage oscilloscope
2.Digital Storage oscilloscope

44. Analog storage osc
• Secondary elect emission to build up & store
electrostatic charges on surface of an insulated
target
• Used for
• Real-time observation of events that occur only
once
• Displaying waveform of a very low freq signal
• Types:
1.Variable persistence storage.
2.Bistable Storage oscilloscope

45. Secondary elec emission
• When beam of high vel elec suddenly strike a
metallic surface, kinetic energy transfers to
electrons & atoms they strike
• Some Primary elec (bombarding elec) collide
directly with free elec on metal surface & knock
them out : Emission of elec called secondary
emission (secondary electrons)
• No of sec elec depends on:
– No of primary elec
– Energy of primary elec
– Angle of incidence of elec at surface
– Type of emitting material

46. • Evacuated glass envelope with emitter E, collecting
anode A, & a source of elec
• Anode +ve wrt E
• When elec emitted by elec source S (primary elec)
strike emitting surface E at high speed, elec knock
out from E: sec elec (attracted towards the anode) :
constitute a flow of current in external ckt

47. Digital Storage Osc
• The input signal is converted into a digital form and
stored in memory. It is then converted back into analog
signal, reconstructed and presented to CRT display.
• The logic control provides the synchronous operation
of the oscilloscope. its functions include:
1)To receive trigger pulses.
2)To determine sampling rate of ADC.
3)Controlling entry of data into store.
4)Controlling the release of data stored into DAC.
5)Controlling DAC by determining its speed and release
of data of the CRT.

49. -In high frequency and pulse applications,the
input capacitance of the oscilloscope begins to
load the circuit.
-The effect of probe is to increase the input
resistance of the oscilloscope.
Oscilloscope probes

50. 1)Direct /Passive Probes
 It is simplest of all the probes.
 Uses shielded co-axial cable.
 Avoids stray pick-ups which may create
problems when low level signals are being
measured.
 Usually used for low frequency or low
impedance circuits.
Types of probes

51.  Using the shielded probe, the shunt
capacitance of the probe and cable is added to
the input impedance and capacity of the
scope and acts to lower the response of the
oscilloscope to high impedance and high
frequency circuits.
 External high impedance probes are used to
increase the input resistance and reduce the
effective input capacitance of an oscilloscope.
Types of probes

52. 2)Active Probes:-
Block diagram of FET probe
Types of probes

53.  Passive probe is mostly used voltage probe.
 It is apparent that low capacitive loading can
be obtained at the expense of considerable
attenuation.
 These problems can be overcome by using
active (FET) probe.
Types of probes

54. 3)CURRENT PROBES-
TYPES of probes

55.  The arrangement of figure which have a core
that may be slid open to allow the current
carrying conductor to be inserted.
 This works on principle of transformer,with
one winding of the transformer being the
measured wire.
 The probes using this principle are used for
a.c. measurements only.
TYPES OF PROBES

56.  Oscilloscopes are designed for voltage,but can
be used to measure current using current
probe.
 The current probe has set of jaws which
encloses the wire that the measured current is
flowing through.
 No connection is required.
TYPES of probes