(1) Digital Measurement of Electrical Quantities-Concept of digital measurement, Block diagram, Study of digital voltmeter, Frequency meter, Spectrum analyzer, Electronic multimeter. (2) Cathode Ray Oscilloscope-Basic CRO circuit (block diagram), Cathode Ray Tube (CRT) & its components,Applications of CRO in measurement, Lissajous Pattern, Dual trace & dual beam oscilloscopes.

1. ELECTRICAL MEASUREMENT & MEASURING INSTRUMENTS (NEE-302)
Md Irshad Ahmad
irshad.ahmad@jit.edu.in
Electrical Engineering Department
LECTURE
on
UNIT –V
Digital Measurement of Electrical Quantities
&
Cathode Ray Oscilloscope

2. UNIT V
 Digital Measurement of Electrical Quantities-
Concept of digital measurement, Block diagram
 Study of digital voltmeter,
 Frequency meter,
 Spectrum analyzer,
 Electronic multimeter.
Cathode Ray Oscilloscope-
Basic CRO circuit (block diagram), Cathode Ray
Tube (CRT) & its components,Applications of CRO
in measurement,
Lissajous Pattern, Dual trace & dual beam
oscilloscopes.
CONTENTS

3. HOW A CRO LOOKS LIKE

4. INTRODUCTION TO CRO
Versatile electronic instrument giving visual
display of signals
Used for measurement of frequency ,
amplitude, phase etc.
Also used in determining the nature and
characteristics of various components.
 Has a high bandwidth range thus making it a
sensitive and accurate device.
Easier to use as the internal routine act as a
guide to the user.

5. BASIC DIAGRAM

6. ANOTHER SCOOP….

7. WORKING OF A CRO

8. The main parts of Cathode Ray Tube are:
•Electron Gun Assembly
•Focusing Anodes
•Deflection Plates Assembly
•Screen for CRT

14. Multi-Input Oscilloscope
Oscilloscope can have multiple input and display
facilities.

15. The most common one is of - TWO INPUTS
.
Although FOUR and EIGHT inputs are
available for special applications

16. There are primary two types of multi-input
oscilloscope :-
Single Beam Oscilloscope
Dual Beam Oscilloscope
Both of them can be converted into a
further
Number of traces.

17. Dual Trace Oscilloscopes

18. In this oscilloscope two separate
vertical input channels are used.
There are two common operating
modes for the Electronic Switch, called
Alternate and Chop which can be selected
from the instrument’s front panel.
Dual Trace Oscilloscopes

19. BLOCK DIAGRAM

20. Dual Beam Oscilloscope
The Dual Beam Oscilloscope has two
separate electron beams , and therefore
two completely separate vertical
channels.

21. There are two methods used for
generating the two electron beams
within the CRT.
The first method is :-
Double gun tube
Split Beam

22. Multiple input CRO works in
following two modes
alternate mode
chopped mode

23. Alternate mode
In alternate mode the an electronic
switch alternates between sig A & B.

24. Waveform in alternate mode

25. Chopped mode
In the chopped mode the switch free
runs at very high frequency.

26. Waveform of Chopped Mode

27. Lissajous Pattern
When two sinusoidal voltage signals
of equal frequency having some phase
difference are applied to the
deflection plates of CRO, a straight
line or an ellipse appears on the
screen these figures are called
Lissajous figures or Lissajous curves or
pattern.

28. Some examples of lissajous figures……..

29. With the help of lissajous figures we
can measure…….
Frequency
Phase

30. Frequency
Measurement
Fx/Fy = Hp/Vp
Fx = Unknown frequency
Fy = Known frequency
Hp = Points on horizontal tangencies
Vp = Points on vertical tangencies

31. Phase
Measurement
sin Ф = c/b

32. Digital Storage Oscilloscope

33. B
L
O
C
BLOCK DIAGRAM OF DSO

34. Advantage Of DSO
Stores waveform for infinite time
Digitize waveform can be processed in any way
Writing speed is more
Processing of signal is easy

35. Analog Storage Oscilloscope

36. CRT: Cathode Ray Tube
CRT: Cathode Ray Tube
CRO: Cathode Ray Oscilloscope CRT+ control & input circuitry
CRT Block diagram:
Electrode system in evacuated glass tube which ends at screen
• Triode section
• Focussing Lens
• Deflection grids
• Post deflection acceleration
• Screen

37. CRT Construction
 .
 .
-2 KV
6.3 V -2.050 KV -2 KV +12 KV
Cathode
Filament
Grid
Glass tube
A1 A2 A3
Vertical
deflection
plate
Horizontal
deflection
plate
Isolation
shield
ResistivehelixElectron beam
Aquadag
Screen
Power Supply
Triode
section
Focussing Deflection Post deflection
acceleration
Electron Gun

38. CRT Construction: Triode section
• A electron beam is generated by a cathode heated by filament
• Consists of a cathode , grid and anode
• Grid is a Nickel cup with a hole in it
• Cathode is Nickel cylinder, with flat oxide coated electron emitting
surface towards the grid hole. Heating is provided by a filament
• Cathode is kept at -2 KV and grid is adjustable in the range –2KV to
-2.05 KV
• Grid cathode potential controls electrons flow rate towards screen,
thus Grid potential is brightness control.

39. CRT Construction: Focussing
-2 KV
A2 A3
Line 2
Equi-potential lines Line 1
A1
Convergent Force
on beam
Divergent Force
direction on beam
Electro static focussing using focussing lens:
Equi-potential lines setup convergent /divergent forces
• Purpose is to focus electron beam at a fine point of screen
• A1 and A3 are grounded while A2 held around -2KV, resulting in equi-
potential lines as shown in figure. Lines in A1 converges electron
beam, while line in A3 diverges electron beam.
• Convergent/ divergent forces are adjusted by potential of A2. i.e
point of focus is shifted. A2 is sometimes referred as focus ring.

40. CRT Construction: Deflection
• With potential across plates, beam deflects to +ve potential.
• Voltage to produce one cm at screen (V/cm) is referred as deflection factor. Deflection by
1 V (cm/V) is termed as deflection sensitivity
• If horizontal/ vertical plates are grounded, beam is not deflected.
• When ac is applied at deflection plates, horizontal/vertical lines are produced at the
screen. Waveform to be displayed is fed to vertical plates while horizontal plates are fed
with a ramp.
• Grounded isolation shield is lie between horizontal/ vertical plates

41. CRT Construction: Screen
• CRT Screen is formed by coating phosphor material to inside of the
screen. When electron beam strikes, electrons in the phosphor
material go to higher energy level and return to original statae while
emitting the visible light (Glow).
• The glow may persist for some time (ms to second) and may be of
colour Red, Blue , Green or White depending on the material.
• Phosphors are insulators. Secondary emission electrons are
collected by a graphite coating “aquadag” around the neck of tube.
• Post deflection acceleration is provided by helix of resistive material
deposited inside of tube between deflection plate and screen with
starting point at ground while ending point at aquadag (12 KV)
• Thus electrons leaving deflection plates finds continuous
acceleration before striking screen

42. CRO Block diagram
4
2
6.3 V
-2 KV
-2.050 KV -2 KV +12 KV
Screen
Power Supply
Vertical
Amplifier
Delay
Line
Input
signal
Trigger
Circuit
Time base
Circuit
Horizontal
Amplifier
Attenuator
Calibration
input

43. CRO : Waveform Display
+2 V
0
-2 V
0 1 2 3 4 ms
t
Input to vertical deflection plate
Input to horizontal deflection plate
+2 V
0
-2 V
1
2
3
4
5
6
7
8
9
Display
• When ac is applied to vertical plates
and horizontal plates are grounded,
then spot on the screen produce a
vertical line by moving up and down
• If ramp (a period of saw tooth wave)
is applied on horizontal plate, spot
moves horizontally along with up and
down movement, thereby producing
a waveform

44. Upper trigger level
Lower trigger level
V1
V2
 (VCC – 1)V
 -(VEE = -1)V
T
+
-
R5
R6
V2 -VEE
VCC
Sync
input
Schmitt Trigger
R7 C2
Q1
R1
R3
R2
IB2
Q2
R4 C1
VBE
VB1
I1
IE1
S1
V1
VCC
CRO time base: Horizontal sweep
generator
Ramp generator
Sweep time
control switch
(time base)
-V

45. Oscilloscope: X-Y and Z display
• Simple figures occur for waveforms of same frequency waveform, while for
different frequencies quite complex figures are formed. For stationary figures
there must be exact ratio of frequencies
Vertical
input
Horizontal
input
Vertical input : sine wave
Horizontal input: No input
Vertical input : No input
Horizontal input: sine wave
Vertical input : Sine wave
Horizontal input: In-phase Sine
• When time base is disconnected, and input waveforms are applied on
horizontal and vertical amplifiers, resulting display are called lissajou
figures and depends on relationship of two waveforms.

46. Oscilloscope: Lissajou figures
Vertical
input
Horizontal
input
Vertical input : sine wave
Horizontal input: anti phase sine wave
Vertical input : No input
Horizontal input: sine wave
with 900 phaseshift
Vertical input : Sine wave
Horizontal input: Sine wave with
phase difference between 0-900
Vertical
Input(f1)
Horizontal
Input(f2)
f1:f2=2:1 f1:f2=3:2

47. Oscilloscope: Z-axis modulation
• CRO have a intensity modulation input termed as Z-axis modulation
• The input wave actually modulates the grid input of oscilloscope.
This dims or blank out the traces.
• For a lissajou figure ‘circle’ when z-axis modulation is applied, gaps
are formed. Ratio of modulating frequency (fm) to deflecting plate
signal frequency (fp) is equal to number gaps in the circle.
• When fm:fp is an exact quantity, gaps in circle will bestable
Vertical
input
Horizontal
input
fm:fp :: 3:1 fm:fp :: 8:1

48. Oscilloscope specifications and
performance
 Sensitivity: defines the amplitude that can be displayed on screen
• Typically sensitivity ranges from 2 mV/div to 10 V/div.
• Using the probe measuring sensitivity can be increased.
 Voltage Measurement Accuracy:
• Accuracy of V/div sensitivity is typically 3%.
• Reading accuracy is typically 5% per division.
• For peak to peak voltage in 5 div, reading accuracy is 5%/5= 1%.
 Overall measurement accuracy becomes 1%+ 3%= 4%
 Frequency Response:
• Highest and lowest frequency of waveform that may be displayed
 with no more than 3 dB attenuation
• For CRO upper cutoff frequency (fH) having negligible
effect on displayed waveform, signal frequency should not exceed fH/10
4
8

49. Oscilloscope specifications and
performance
 Time Base Accuracy:
•Accuracy of V/div sensitivity is typically 5%.
•Reading accuracy of time base is typically 5% per division.
•For peak to peak voltage in 5 div, reading accuracy is
5%/5= 1%. Overall measurement accuracy becomes
1%+ 5%= 6%
 Rise Time Measurement:
•is rise time imposed on oscilloscope on an input pulse
wave.
 • tro=0.35/fH

50. Delay time based
Oscilloscopes
• In normal CRO, the input signal is used to trigger the time base,
while vertical plates are fed with some delay. This facilitates study of
leading/ lagging edge of a pulse type waveform. However this delay
time may not be high enough to cover full signal.
• In Delay time based CRO, a variable delay is introduced in time base.
• Un-blanking pulse of delayed time base (DTB) is added with that of
main time base (MTB). This increases the intensity of CRT in the
duration of DTB ramp, and highlights the portion of waveform.
• The part of waveform highlighted, can be displayed on CRT by
switching the DTB ramp to horizontal plate amplifier.
• Using alternate mode selection of MTB and DTB, the waveform
with highlighted portion and magnified waveform of the portion
can be displayed simultaneously on the screen.

51. Delay time based Oscilloscopes
MainTime
Base MTB
Un-blanking
circuit
Voltage
comparator
Delayed Time
Base DTB
Un-blanking
circuit
Summing
Horizontal deflection
amplifier
Triggerlevel
control
ToCRT grid
Ramp output
+V
-V
+V
-V
MTB
Un-blanking
pulse
MTB ramp
waveform
DC trigger level
DTB ramp
waveform
DTB
Un-blanking
pulse
summed
Un-blanking
pulse
summed
Un-blanking
pulse
td
DTB
time

52. Digital Multimeters
 Digital multimeters (DMMs) are often (inaccurately) referred
to as digital voltmeters or DVMs
 at their heart is an analogue-to-digital converter (ADC)
A simplified block diagram

53. Measuring alternating quantities
 Measuring alternating quantities
 moving coil meters respond to both positive and negative voltages,
each producing deflections in opposite directions
 a symmetrical alternating waveform will produce zero deflection
(the mean value of the waveform)
 therefore we use a rectifier to produce a unidirectional signal
 meter then displays the average value of the waveform
 meters are often calibrated to directly display r.m.s. of sine waves
 all readings are multiplied by 1.11 – the form factor for a sine wave
 as a result waveforms of other forms will give incorrect readings
 for example when measuring a square wave (for which the form
factor is 1.0, the meter will read 11% too high)

54. Analogue Ammeters and
Voltmeters
 Most modern analogue
ammeters are based on
moving-coil meters
 see Chapter 4 of textbook
 Meters are characterised by their full-scale deflection (f.s.d.) and
their effective resistance (RM)
 typical meters produce a f.s.d. for a current of 50 A – 1 mA
 typical meters have an RM between a few ohms and a few
kilohms

55. Concept of Digital Measurement
 The digital techniques used in Electronic
Instrumentation enrich the user with high accuracy
measurement.

56.  Scheme besides digital system provides high input impedance
to ensure less loading effect on the input circuit. The numerical
readout of the digital system allows the worker to perform
measurement with zero parallax error unlike analog system.

57. What is Voltmeter
 A Voltmeter is an instrument used for measuring the
electrical potential difference between two points in an
electrical circuit.
 There are generally seven types of voltmeter
1) Analog Voltmeter
2) Digital Voltmeter
3) Electrostatic Voltmeter
4) Oscilloscope Voltmeter
5) Micro-Voltmeter
6) Vacuum Tube Voltmeter
7) Solenoid Voltmeter

58. What’s so special being
“Digital”?
 General purpose analog voltmeters may have an accuracy
of a few percent of full scale, and are used with voltages
from a fraction of a volt to several thousand volts. Digital
voltmeters can be made with high accuracy, typically
better than 1%.

59. Digital Voltmeter (DVM)

60. Introduction
 Digital Voltmeter is an instrument which use to measured the
voltage & display the measured voltage using LCDs or LEDs to
display the result in a floating point format. They are an instrument of
choice for voltage measurements in all kinds of situations.
 Digital voltmeters usually have scales that are 0-0.3v, 0-3v, 0-30v, 0-
300v.
 Digital voltmeter is essentially an analog to digital converter (A to D)
with a digital display

61. Working
 INTEGRATOR:-The integrator stabilizes the voltage as a first step in
measuring it. It takes a brief time sample, integrates it and outputs a
proportional voltage. Integrating a time sample makes the voltmeter
more immune to noise in the signal.
 ANALOG/DIGITAL CONVERTOR:-The analog/digital converter
or ADC, is the heart of the voltmeter. It is a clocked circuit that takes
an input voltage sample and outputs a number representing the voltage
value. Since it's clocked, you get a steady stream of numbers that
change when the input voltage changes.

63. Advantages of Digital Voltmeter
 Higher accuracy and resolution.
 Greater speed.
 No parallax.
 Reduced human error.
 Compatibility with other digital equipment
for further processing and recording.

64. Application
 Digital voltmeter mostly used in electronics
laboratory & in industry for measure the voltage
between two points with accuracy.

65. Thank You