CRO| Block Diagram of CRO| CRO Probes| Lissajous Figures| Applications| specifications| high frequency CRO considerations

1. Unit – III
Oscilloscopes &
Special Purpose Oscilloscopes

2. Contents
Oscilloscopes
• CRT
• Block Schematic of CRO
• Time Base Circuits
• Lissajous Figures
• CRO Probes
• High Frequency CRO Considerations
• Delay lines
• Applications: Measurement of Time, Period
and Frequency
• Specifications
Special Purpose Oscilloscopes
• Dual Trace
• Dual Beam CROs
• Sampling Oscilloscopes
• Storage Oscilloscopes
• Digital Storage CROs.

3. Introduction
• Oscilloscopes are widely used in Electronic Measurements and Instrumentation systems.
• Unlike other measuring instruments, the signal being measured can be visually seen on
the screen.
• The characteristics of the signals, like amplitude, frequency, phase, time period, duty
cycle, etc., can be measured using oscilloscopes.
• The amplitude of the signals can vary from µV to even a few hundred volts.
• The frequency can range from very low to even MHzs.
• Because of the phosphorescence effect, the electrical signal is converted to a visible form
and the shape of the signal can be seen.

4. CRT(Cathode Ray Tube)
• The CRT is the heart of the CRO.
• It generates the electron beam, accelerates the beam, deflects the beam and also has a
screen where the beam becomes visible as a spot.
• The main parts of CRT are
(i) Electron gun
(ii) Deflection System
(iii) Fluorescent screen
(iv) Glass Tube or envelope
(v) Base

5. Block diagram of CRT

6. Electron Gun
• Electron gun provides a sharply focused electron beam directed towards the fluorescent coated
screen.
• By heating the cathode electrons are emitted and control grid is given negative potential with
respect to cathode, which controls the number of electrons in the beam, going to the screen.
• The momentum of electron(i.e., number x speed) determines the intensity or brightness of light
emitted from the fluorescent screen due to the electron bombardment.
• The beam tends to diverge because electron beam consists of many electrons and they repel each
other.
• To compensate such repulsion forces focusing anodes are used.
• To control the acceleration of electron preaccelerating and accelerating anodes are used with high
positive potential.

7. Deflection System
• When the electron beam is accelerated it passes through the deflection system, with
which beam can be positioned anywhere on the screen.
• The deflection system consists of two pairs of parallel plates, referred to as the
vertical and horizontal deflection plates. One of the plates in each set is
permanently connected to the ground (zero volt), whereas the other plate of each set
is connected to external deflection voltages with terminals Vx and Vy.
• If a +ve voltage is applied to Vx deflection is towards right, if Vx is negative
deflection is towards left, Vy is positive towards upward and Vy is negative towards
downward.

8. Contd…
• The amount of deflection is directly proportional to corresponding applied voltage
X α Vx X = KVx
K is constant of proportionality
• It is usually measured in cm or number of divisions on the scale.
K = X / Vx called horizontal sensitivity
K = Y / Vy called vertical sensitivity

9. Fluorescent Screen
• The time period for which the trace remains on the screen after the signal becomes zero is called
“Persistence”.
• It may be as short as few µSec or as long as minutes.
Medium – general purpose applications
long – study of transients
short – extremely high speed phenomena
• The screen is coated with a fluorescent material called phosphor which emits light when bombarded
by electron.

10. Glass Tube and Base
Glass Tube
• All the components are enclosed in an evacuated glass tube called envelope.
Base
• The base is provided to the CRT through which the connections are made to the
various parts.

11. Features of CRT
CRT Features – Electrostatic CRTs are available in a number of types and sizes to suit
individual requirements. The important features of these tubes are as follows.
• Size refers to the screen diameter. CRTs for oscilloscopes are available in sizes of 1,
2, 3, 5, and 7 inches. 3 inches is most common for portable instruments.
• For example a CRT having a number 5GP1. The first number 5 indicates that it is a
5 inch tube.
• Both round and rectangular CRTs are found in scopes today. The vertical viewing
size is 8 cm and horizontal is 10 cm.

12. Contd…
• The screen is coated with a fluorescent material called phosphor. This material
determines the color and persistence of the trace, both of which are indicated by the
phosphor.
• Various phosphors available which differs in color, persistence and efficiency.
• Common phosphor is Willemite which is zinc orthosilicate Zno + SiO2, with
trace of manganese which produces greenish trace.
• Different types of phosphors are available for different applications.

13. Contd…
• The phosphor of the oscilloscope is designated as follows.
P1 – Green medium
P2 – Blue green medium
P5 – Blue very short
P11 – Blue short
• These designations are combined in the tube type number. Hence 5GP1
is a 5 inch tube with a medium persistence green trace.

14. Contd…
• The CRT requires a heater voltage of 6.3 volts ac or dc at 600 mA.
• Several dc voltages are listed below. The voltages vary with the type of
tube used.
• Negative grid (control) voltage – 14 V to – 200 V.
• Positive anode no. 1 (focusing anode) – 100 V to – 1100 V
• Positive anode no. 2 (accelerating anode) 600 V to 6000 V
• Positive anode no. 3 (accelerating anode) 200 V to 20000 V in some cases.

15. Contd…
• Either ac or dc voltage will deflect the beam.
• The distance through which the spot moves on the screen is proportional
to the dc, or peak ac amplitude.
• The deflection sensitivity of the tube is usually stated as the dc voltage (or
peak ac voltage) required for each cm of deflection of the spot on the
screen.

16. Contd…
• The viewing screen is the glass face plate, the inside wall of which is
coated with phosphor.
• The viewing screen is a rectangular screen having graticules marked on it.
The standard size used nowadays is 8 cm x 10 cm (8 cm on the vertical
and 10 cm on horizontal).
• Each centimeter on the graticule corresponds to one division (div).
• The standard phosphor color used nowadays is blue.

17. Block Schematic of CRO
The blocks of a general-purpose Oscilloscope, is as follows:
1. CRT
2. Vertical amplifier
3. Delay line
4. Time base
5. Horizontal amplifier
6. Trigger circuit
7. Power Supply

18. Contd…
The major Block Diagram of Oscilloscope is shown in Fig. 7.4

19. Contd…
The function of the various blocks are as follows.
1. CRT
This is the cathode ray tube which emits electrons that strikes the phosphor
screen internally to provide a visual display of signal.
2. Vertical Amplifier
This is a wide band amplifier used to amplify signals in the vertical section.

20. Contd…
3. Delay Line
It is used to delay the signal for some time in the vertical sections.
4. Time Base
It is used to generate the sawtooth voltage required to deflect the beam in
the horizontal section.

21. Contd…
5. Horizontal Amplifier
This is used to amplify the sawtooth voltage before it is applied to
horizontal deflection plates.
6. Trigger Circuit
This is used to convert the incoming signal into trigger pulses so that the
input signal and the sweep frequency can be synchronized.

22. Contd…
7. Power Supply
• There are two power supplies, a —ve High Voltage (HV) supply and a +ve
Low Voltage (LV) supply. Two voltages are generated in the CRO.
• The +ve voltage supply is from + 300 to 400 V.
• The —ve high voltage supply is from — 1000 to — 1500 V. This voltage
is passed through a bleeder resistor at a few mA.
• The intermediate voltages are obtained from the bleeder resistor for
intensity, focus and positioning controls.

23. Advantages of using —ve HV Supply
• The accelerating anodes and the deflection plates are close to ground. The
ground potential protects the operator from HV shocks when making
connections to the plates.
• The deflection voltages are measured w.r.t ground, therefore HV blocking
or coupling capacitor are not needed, but low voltage rating capacitors can
be used for connecting the HV supply to the vertical and horizontal
amplifiers.
• Less insulation is needed between positioning controls.

24. Vertical Amplifier
• Vertical Amplifier – The sensitivity (gain) and frequency bandwidth
(B.W.) response characteristics of the oscilloscope are mainly determined
by the vertical amplifier. Since the gain B.W. product is constant, to obtain
a greater sensitivity the B.W. is narrowed, or vice-versa.
• Some oscilloscopes give two alternatives, switching to a wide bandwidth
position, and switching to a high sensitivity position.

25. Block Diagram of a Vertical Amplifier
• The block diagram of a vertical amplifier is shown in Fig. 7.7.

26. Contd…
• The input signals are generally not strong to provide the measurable
deflection on the screen.
• Hence, vertical amplifier stage is used to amplify the input signals.
• The amplifier stages used are generally wideband amplifiers so as to pass
faithfully the entire band of frequencies to be measured and attenuator
stages are used for high voltage signals to be examined.

27. Contd…
• The vertical amplifier consists of several stages, with fixed overall
sensitivity or gain expressed in V/divs.
• The advantage of fixed gain is that the amplifier can be more easily
designed to meet the requirements of stability and B.W.
• The vertical amplifier is kept within its signal handling capability by
proper selection of the input attenuator switch.
• The first element of the pre-amplifier is the input stage, often consisting
of a FET source follower whose high input impedance isolates the
amplifier from the attenuator.

28. Contd…
• This FET input stage is followed by a BJT emitter follower, to match the
medium impedance of FET output with the low impedance input of the
phase inverter.
• This phase inverter provides two antiphase output signals which are
required to operate the push-pull output amplifier.
• The push-pull output stage delivers equal signal voltages of opposite
polarity to the vertical plates of the CRT.

29. Contd…
• The advantages of push-pull operation in CRO are similar to those
obtained from push-pull operation in other applications; better hum
voltage cancellation from the source or power supply (i.e. dc), even
harmonic suppression, especially the large 2nd harmonic is cancelled out,
and greater power output per tube as a result of even harmonic
cancellation.
• In addition, a number of defocusing and non-linear effects are reduced,
because neither plate is at ground potential.

30. Delay Line
• If both vertical and horizontal signals arrives at the same time to the
corresponding deflection plates, then only we will get the exact
waveform.
• But vertical signal arrive much early compared to the horizontal signal.
• For this reason, the vertical signal at the output of the vertical amplifier
should be delayed with the help of delay line.
• The delay time is almost equal to 200nSec.

31. Contd…
• Figure 7.13 shows a Delay Line Circuit. Figure 7.14 indicates the
amplitude of the signal w.r.t time and the relative position of the sweep
generator output signal.

32. Contd…
• The diagram shows that when the delay line is not used, the initial part of
the signal is lost and only part of the signal is displayed.
• To counteract this disadvantage the signal is not applied directly to the
vertical plates but is passed through a delay line circuit, as shown in Fig.
7.13.
• This gives time for the sweep to start at the horizontal plates before the
signal has reached the vertical plates.

33. Contd…
• The trigger pulse is Picked off at a time t0 after the signal has passed
through the main amplifier then the signal is delayed by x1ns while
sweep takes y1ns to reach.
• Delay time x1 is higher than the y1 time.
• The sweep generator delivers the sweep time to the horizontal
amplifier and the sweep starts at the HDP at time t0 + 80 ns.
• Hence the sweep starts well in time, since the signal arrives at the VDP at
time t0 + 200 ns.

34. Sweep or Time Base Generator
• A continuous sweep CRO using a UJT as a time base generator is shown
in Fig. 7.8. The UJT is used to produce the sweep.

35. Contd…
• When the power is first applied, the UJT is off and the CT changes
exponentially through RT.
• The UJT emitter voltage VE rises towards VBB and when VE reaches the
peak voltage VP, as shown in Fig. 7.9, the emitter to base ‘1’ (B1) diode
becomes forward biased and the UJT triggers ON.
• This provides a low resistance discharge path and the capacitor
discharges rapidly. The emitter voltage VE reaches the minimum value
rapidly and the UJT goes OFF. The capacitor recharges and the cycle
repeats.

36. Contd…
• To improve sweep linearity, two separate voltage supplies are used, a low
voltage supply for UJT and a high voltage supply for the RTCT circuit.
• RT is used for continuous control of frequency within a range and CT is
varied or changed in steps for range changing. They are sometimes called
as timing resistor and timing capacitor respectively.
• The sync pulse enables the sweep frequency to be exactly equal to the
input signal frequency, so that the signal is locked on the screen and does
not drift.

37. Contd…
• Time base generator is used to generate sawtooth voltage required to
deflect the beam in the horizontal section.
• This voltage deflects the spot at a constant time dependent rate.
• Thus, X axis on the screen is represented as time, which helps to display
and analyze the time varying signals.

38. Triggered Sweep CRO
• Triggered Sweep CRO – The continuous sweep is of limited use in
displaying periodic signals of constant frequency and amplitude. When
attempting to display voice or music signals, the pattern falls in and out
of sync as the frequency and amplitude of the music varies resulting in
an unstable display.
• A triggered sweep can display such signals, and those of short duration,
e.g. narrow pulses. In triggered mode, the input signal is used to generate
substantial pulses that trigger the sweep. Thus ensuring that the sweep is
always in step with the signal that drives it.

39. Contd…

40. Contd…
• As shown in Fig. 7.10, resistance R3 and R4 form a voltage divider such
that the voltage VD at the cathode of the diode is below the peak voltage
VP for UJT conduction.
• When the circuit is switched on, the UJT is in the nonconducting stage,
and CT charges exponentially through RT towards VBB until the diode
becomes forward biased and conducts, the capacitor voltage never
reaches the peak voltage required for UJT conduction but is clamped at
VD.

41. Contd…
• If now a –ve pulse of sufficient amplitude is applied to the base and the
peak voltage VP is momentarily lowered, the UJT fires.
• As a result, the CT discharges rapidly through the UJT until the
maintaining voltage of the UJT is reached; at this point the UJT switches
off and the CT charges towards VBB, until it is clamped again at
VD. Figure 7.11 shows the output waveform.

42. Horizontal Amplifier
• The sawtooth voltage produced by the time base generator may not be of
sufficient strength.
• Hence, before giving it to horizontal deflection plates, it is amplified
using the horizontal amplifier.

43. Power Supply
• Power supply block provides the voltages required by CRT to generate
and accelerate an electron beam and voltages required by other circuits
of the oscilloscope like horizontal and vertical amplifiers etc.,
• High voltage is used for reducing shocks.
• Low voltage is required for the heater of the electron gun, which emits
the electrons and which is also used by the other circuits of CRO.

44. Lissajous Figures
• One of the applications of the CRO is to determine phase , frequency,
amplitude and other characteristics of waveforms.
• These characteristics can be determined by Lissajous patterns or figures.
• Such figures are obtained by applying simultaneously two different
signals to horizontal and vertical deflection plates of CRO with EXT
mode and sawtooth generator is not applied to X- plate.
Let x(t) = A cos(ωt - α), y(t) = B cos(ωt - β)
• The amplitudes are A, B, & the phases are α, β, and δ ≡ α – β

45. Contd…
Some special cases:
• δ = ±π/2 ⇒ An ellipse: (x/A)2 + (y/B)2 = 1
If A = B, this is a circle i.e., x2 + y2 = A2
• δ = 0 ⇒ A straight line y = (B/A) x
• δ = ±π ⇒ A straight line y = -(B/A) x
• Except for special cases, the general path is an ellipse.

46. Contd…
• Paths in the XY planes for A = B and various δ

47. Frequency Measurement
• Lissajous figures for the measurement of frequency depends on ratio of
frequencies.
• Known frequency is obtained from variable frequency oscillator.

48. Contd…

49. Contd…

50. CRO Probes
• The probe is used with the oscilloscope to connect the test circuit to the
oscilloscope.
• While connecting the test circuit, the probe does not alter, load or disturbs the
circuit and signal conditions to be analyzed.
• To achieve this, the probes should have high impedance.
• The probe bandwidth should be as high as possible.
• Generally, it is at least 10 times the bandwidth of the oscilloscope.

51. CRO Probes Block Diagram

52. Contd…
• The probe tip is the signal sensing circuitry. It may be passive or active.
• If passive, it consists of the resistors and capacitors while if active, it consists
of active components like FET source follower circuit.
• The coaxial cable is used to connect the probe head to the termination
circuitry.

53. Various CRO Probes
1. Direct Probes (1:1 Probe)
• The simplest types of probe is the test lead.
• Test leads are simply convenient lengths of wire for connecting the CRO
input to the point of observation. At the CRO end, they usually terminate
with lugs, banana tips or other tips to fit the input jacks of the scope, and at
the other end have a crocodile clip or any other convenient means for
connection to the electronic circuit.

54. 1. Direct Probes (1:1 Probe)
• Since a CRO has high input impedance and high sensitivity, the test leads
should be shielded to avoid hum pickup, unless the scope is connected to low
impedance high level circuits.
• Although the input impedances of most CROs are relatively very high
compared to the circuits where they are connected, it is often desirable to
increase their impedance to avoid loading of the circuits or causing unstable
effects.

55. 1. Direct Probes (1:1 Probe)
• The input capacitance of the scope, plus the stray capacitance of the test
leads, may be just enough to cause a sensitive circuit to break into Isolation
Probe oscillation when the CRO is connected.
• This effect can be prevented by an isolation probe made by placing a carbon
resistor in series with the test lead.

56. 1. Direct Probes (1:1 Probe)

57. 1. Direct Probes (1:1 Probe)
• A slight reduction in the amplitude of the waveform and a slight change
in the waveshape occurs with this probe. To avoid this possibility, a
high impedance compensated probe, called a low capacitance probe or
a 10 : 1 probe, is used.

58. 2. High Impedance Probes(10X)
• This is also called as Passive voltage probe.
• Main function is to increase the impedance and reduce the effective
input capacitance of an oscilloscope.
Fig. High impedance 10X probe

59. Contd…
• The arrangement of various elements can be further simplified by
representing it in a bridge network as shown below.

60. Contd…
• Thus the points A & B will be equipotential and the probe acts as a
potential divider consisting of Rin & R1 across the input.
• Thus the attenuation factor is (R1+Rin)/Rin = 10 and hence the probe
is called as 10X probe.
• Thus the effective input resistance increases by 10 and the input
capacitance decreases by a factor 10.
• Only limitation of the probe is signal attenuation.

61. 3.High Voltage Probes
• These probes are used to measure very high voltages in the range of
Kilovolts.
• It provides the voltage division ratio of 1000 to 1 or more also.
• The probe head is made up of high impact strength thermoplastic
material which provides security against electric shock hazards to the
user.

62. Contd…

63. Contd…
Limitations
• Large Isolation
• Bigger size
• Unsuitable for high frequencies and closely packed components
• High voltage capability is affected by increase in temperature and
shunting capacity of input circuit.
• By adjusting R6, 1000:1 attenuation is possible.

64. 4. Detector Probes
• It is mainly used for the response analysis of the tuned circuits, where
the shape of the curve is important than the frequency.
• It consists of a diode detector which is used for rectifying the
modulated input.

65. Contd…
• C2 bypasses the RF component so that low frequency signal can be fed
to CRO.
• While testing the circuits operating at frequencies beyond the vertical
amplifier capability, a demodulator probe is used.
• If input is modulated then the output is the envelope of the waveform.

66. 5. Active Probe
• For connecting fast raising and high frequency signals active probes are
used.
• These are useful for small signal measurements as their attenuation
factor is small.

67. Contd…
• FET is used as an active element to amplify the signals.
• The voltage gain factor of FET source follower is unity but it provides a
power gain due to which input impedance increases.
• The output impedance of FET source follower is low, thus eliminates
the loading effect.
• BJT emitter follower is used for connecting the coaxial cable to the
CRO for solving improper impedance matching issues.

68. Applications of CRO
• It is used to measure ac as well as dc voltages and currents.
• In laboratory, to measure frequency, period, phase relationship between
signals and to study periodic as well as non periodic signals.
• It is used to determine modulation characteristics and to detect standing
waves in transmission lines.
• In RADAR applications, it is used for giving visual representation of target
such as aero plane, ships etc.,
• In industries, to study the responses of various transducers which measure
temperature, pressure etc.,

69. Contd…
• In medical applications, it is used to display the ECGs which are useful
for diagnosis of heart.
• In radio applications, it is used to trace and measure a signal throughout
the RF, IF and AF channels of radio & television receivers.
• CROs are also used for testing the active devices such as vacuum tubes,
Integrated circuits etc.,

70. Specifications of CRO
• The accuracy of vertical amplifier is + 3% and accuracy of horizontal
amplifier is +5%.
• Max. input voltage is 400V.
• Sensitivity of vertical amplifier is 5mV/div. to 20V/div.
• Sensitivity of horizontal amplifier is 100mV/div. to 0.5V/div.
• Bandwidth is 100MHz.
• Sweep magnification is X10.

71. Contd…
• Time accuracy is +3%.
• Chop Frequency is 500KHz. / channel.
• Time Base is 20ns/div to 0.5s/div.
• Vertical deflection channels 1 and 2 can be varied from 0.2mV/div to
5V/div in 11 decade steps.
• Graticule is defined as scale marking provided on the CRT screen.
• Its value is given as 8X10 divisions of 8mm each.

72. Contd…
• Input impedance
a) trigger circuit is 1M ohm/ 30 PF
b) horizontal amplifier 1M ohm/ 50PF
c) vertical amplifier 1M ohm/ 40PF

73. High Frequency CRO Considerations
• In an ordinary CRO, there is only one pair of VDPs.
• When an high frequency signal is applied to the vertical amplifier, the
electron beam does not get sufficient time to pick up the instantaneous level
of the signal.
• Also, at high frequencies the number of electrons striking the screen in the
given time and the intensity of the beam is reduced as the deflection voltage
changes.

74. Contd…
• Hence, instead of one VDP, a series of VDPs are used.
• Electron beam should be accelerated to very high velocity.
• When beam is accelerated with high velocity, high energy is available to
transmit the electron beam to the screen, and hence increases the brightness
or intensity

75. Contd…
• Deflection is given as

76. Contd…
• Deflection sensitivity is given as (S)
S= D/Ed
• As frequency is high, gain bandwidth product is constant.
• When gain is high, bandwidth is low and hence, this is the drawback of
high frequency CRO.

77. Measurement of Voltage, Period and Frequency
Voltage Measurements

78. Contd…
Period and Frequency Measurements

79. Contd…
• For the waveform observed in the oscilloscope is shown below. If the vertical
attenuation is set to 0.5 V/Div, determine the peak to peak amplitude of the
signal. If the time /div control is 2microseconds/div, determine the frequency
of the signal.