EMI Unit 3 CRO

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
•OscilloscopesarewidelyusedinElectronicMeasurementsandInstrumentationsystems.
•Unlikeothermeasuringinstruments,thesignalbeingmeasuredcanbevisuallyseenon
thescreen.
•Thecharacteristicsofthesignals,likeamplitude,frequency,phase,timeperiod,duty
cycle,etc.,canbemeasuredusingoscilloscopes.
•TheamplitudeofthesignalscanvaryfromµVtoevenafewhundredvolts.
•ThefrequencycanrangefromverylowtoevenMHzs.
•Becauseofthephosphorescenceeffect,theelectricalsignalisconvertedtoavisibleform
andtheshapeofthesignalcanbeseen.

4. CRT(Cathode Ray Tube)
•TheCRTistheheartoftheCRO.
•Itgeneratestheelectronbeam,acceleratesthebeam,deflectsthebeamandalsohasa
screenwherethebeambecomesvisibleasaspot.
•ThemainpartsofCRTare
(i)Electrongun
(ii)DeflectionSystem
(iii)Fluorescentscreen
(iv)GlassTubeorenvelope
(v)Base

5. Block diagram of CRT

6. Electron Gun
•Electrongunprovidesasharplyfocusedelectronbeamdirectedtowardsthefluorescentcoated
screen.
•Byheatingthecathodeelectronsareemittedandcontrolgridisgivennegativepotentialwith
respecttocathode,whichcontrolsthenumberofelectronsinthebeam,goingtothescreen.
•Themomentumofelectron(i.e.,numberxspeed)determinestheintensityorbrightnessoflight
emittedfromthefluorescentscreenduetotheelectronbombardment.
•Thebeamtendstodivergebecauseelectronbeamconsistsofmanyelectronsandtheyrepeleach
other.
•Tocompensatesuchrepulsionforcesfocusinganodesareused.
•Tocontroltheaccelerationofelectronpreacceleratingandacceleratinganodesareusedwithhigh
positivepotential.

7. Deflection System
•Whentheelectronbeamisaccelerateditpassesthroughthedeflectionsystem,with
whichbeamcanbepositionedanywhereonthescreen.
•Thedeflectionsystemconsistsoftwopairsofparallelplates,referredtoasthe
verticalandhorizontaldeflectionplates.Oneoftheplatesineachsetis
permanentlyconnectedtotheground(zerovolt),whereastheotherplateofeachset
isconnectedtoexternaldeflectionvoltageswithterminalsVxandVy.
•Ifa+vevoltageisappliedtoVxdeflectionistowardsright,ifVxisnegative
deflectionistowardsleft,VyispositivetowardsupwardandVyisnegativetowards
downward.

8. Contd…
•Theamountofdeflectionisdirectlyproportionaltocorrespondingappliedvoltage
XαVx X=KVx
Kisconstantofproportionality
•Itisusuallymeasuredincmornumberofdivisionsonthescale.
K=X/Vxcalledhorizontalsensitivity
K=Y/Vycalledverticalsensitivity

9. Fluorescent Screen
•Thetimeperiodforwhichthetraceremainsonthescreenafterthesignalbecomeszeroiscalled
“Persistence”.
•ItmaybeasshortasfewµSecoraslongasminutes.
Medium–generalpurposeapplications
long–studyoftransients
short–extremelyhighspeedphenomena
•Thescreeniscoatedwithafluorescentmaterialcalledphosphorwhichemitslightwhenbombarded
byelectron.

10. Glass Tube and Base
GlassTube
•Allthecomponentsareenclosedinanevacuatedglasstubecalledenvelope.
Base
•ThebaseisprovidedtotheCRTthroughwhichtheconnectionsaremadetothe
variousparts.

11. Features of CRT
CRTFeatures–ElectrostaticCRTsareavailableinanumberoftypesandsizestosuit
individualrequirements.Theimportantfeaturesofthesetubesareasfollows.
•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
Theblocksofageneral-purposeOscilloscope,isasfollows:
1.CRT
2.Vertical amplifier
3.Delay line
4.Time base
5.Horizontal amplifier
6.Trigger circuit
7.Power Supply

18. Contd…
ThemajorBlockDiagramofOscilloscopeisshowninFig.7.4

19. Contd…
Thefunctionofthevariousblocksareasfollows.
1.CRT
Thisisthecathoderaytubewhichemitselectronsthatstrikesthephosphor
screeninternallytoprovideavisualdisplayofsignal.
2.VerticalAmplifier
This is a wide band amplifier used to amplify signals in the vertical section.

20. Contd…
3.DelayLine
Itisusedtodelaythesignalforsometimeintheverticalsections.
4.TimeBase
Itisusedtogeneratethesawtoothvoltagerequiredtodeflectthebeamin
thehorizontalsection.

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 —veHigh Voltage (HV) supply and a +ve
Low Voltage (LV) supply. Two voltages are generated in the CRO.
•The +vevoltage supply is from + 300 to 400 V.
•The —vehigh 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—veHV Supply
•Theaccelerating anodesand 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 theattenuator.

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…
•Figure7.13showsaDelayLineCircuit.Figure7.14indicatesthe
amplitudeofthesignalw.r.ttimeandtherelativepositionofthesweep
generatoroutputsignal.

32. Contd…
•Thediagramshowsthatwhenthedelaylineisnotused,theinitialpartof
thesignalislostandonlypartofthesignalisdisplayed.
•Tocounteractthisdisadvantagethesignalisnotapplieddirectlytothe
verticalplatesbutispassedthroughadelaylinecircuit,asshowninFig.
7.13.
•Thisgivestimeforthesweeptostartatthehorizontalplatesbeforethe
signalhasreachedtheverticalplates.

33. Contd…
•ThetriggerpulseisPickedoffatatimet
0afterthesignalhaspassed
throughthemainamplifierthenthesignalisdelayedbyx1nswhile
sweeptakesy1nstoreach.
•Delaytimex1ishigherthanthey1time.
•Thesweepgeneratordeliversthesweeptimetothehorizontal
amplifierandthesweepstartsattheHDPattimet
0+80ns.
•Hencethesweepstartswellintime,sincethesignalarrivesattheVDPat
timet
0+200ns.

34. Sweep or Time BaseGenerator
•AcontinuoussweepCROusingaUJTasatimebasegeneratorisshown
inFig.7.8.TheUJTisusedtoproducethesweep.

35. Contd…
•Whenthepowerisfirstapplied,theUJTisoffandtheC
Tchanges
exponentiallythroughR
T.
•TheUJTemittervoltageV
ErisestowardsV
BBandwhenV
Ereachesthe
peakvoltageV
P,asshowninFig.7.9,theemittertobase‘1’(B
1)diode
becomesforwardbiasedandtheUJTtriggersON.
•Thisprovidesalowresistancedischargepathandthecapacitor
dischargesrapidly.TheemittervoltageV
Ereachestheminimumvalue
rapidlyandtheUJTgoesOFF.Thecapacitorrechargesandthecycle
repeats.

36. Contd…
•Toimprovesweeplinearity,twoseparatevoltagesuppliesareused,alow
voltagesupplyforUJTandahighvoltagesupplyfortheR
TC
Tcircuit.
•R
TisusedforcontinuouscontroloffrequencywithinarangeandC
Tis
variedorchangedinstepsforrangechanging.Theyaresometimescalled
astimingresistorandtimingcapacitorrespectively.
•Thesyncpulseenablesthesweepfrequencytobeexactlyequaltothe
inputsignalfrequency,sothatthesignalislockedonthescreenanddoes
notdrift.

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
•TriggeredSweepCRO–Thecontinuoussweepisoflimitedusein
displayingperiodicsignalsofconstantfrequencyandamplitude.When
attemptingtodisplayvoiceormusicsignals,thepatternfallsinandout
ofsyncasthefrequencyandamplitudeofthemusicvariesresultingin
anunstabledisplay.
•Atriggeredsweepcandisplaysuchsignals,andthoseofshortduration,
e.g.narrowpulses.Intriggeredmode,theinputsignalisusedtogenerate
substantialpulsesthattriggerthesweep.Thusensuringthatthesweepis
alwaysinstepwiththesignalthatdrivesit.
•

39. Contd…

40. Contd…
•AsshowninFig.7.10,resistanceR
3andR
4formavoltagedividersuch
thatthevoltageV
Datthecathodeofthediodeisbelowthepeakvoltage
V
PforUJTconduction.
•Whenthecircuitisswitchedon,theUJTisinthenonconductingstage,
andC
TchargesexponentiallythroughR
TtowardsV
BBuntilthediode
becomesforwardbiasedandconducts,thecapacitorvoltagenever
reachesthepeakvoltagerequiredforUJTconductionbutisclampedat
V
D.

41. Contd…
•Ifnowa–vepulseofsufficientamplitudeisappliedtothebaseandthe
peakvoltageV
Pismomentarilylowered,theUJTfires.
•Asaresult,theC
TdischargesrapidlythroughtheUJTuntilthe
maintainingvoltageoftheUJTisreached;atthispointtheUJTswitches
offandtheC
TchargestowardsV
BB,untilitisclampedagainat
V
D.Figure7.11showstheoutputwaveform.

42. Horizontal Amplifier
•Thesawtoothvoltageproducedbythetimebasegeneratormaynotbeof
sufficientstrength.
•Hence,beforegivingittohorizontaldeflectionplates,itisamplified
usingthehorizontalamplifier.

43. Power Supply
•PowersupplyblockprovidesthevoltagesrequiredbyCRTtogenerate
andaccelerateanelectronbeamandvoltagesrequiredbyothercircuits
oftheoscilloscopelikehorizontalandverticalamplifiersetc.,
•Highvoltageisusedforreducingshocks.
•Lowvoltageisrequiredfortheheateroftheelectrongun,whichemits
theelectronsandwhichisalsousedbytheothercircuitsofCRO.

44. Lissajous Figures
•OneoftheapplicationsoftheCROistodeterminephase,frequency,
amplitudeandothercharacteristicsofwaveforms.
•ThesecharacteristicscanbedeterminedbyLissajouspatternsorfigures.
•Suchfiguresareobtainedbyapplyingsimultaneouslytwodifferent
signalstohorizontalandverticaldeflectionplatesofCROwithEXT
modeandsawtoothgeneratorisnotappliedtoX-plate.
Letx(t)=Acos(ωt-α),y(t)=Bcos(ωt-β)
•TheamplitudesareA,B,&thephasesareα,β,andδ≡α–β

45. Contd…
Somespecialcases:
•δ=±π/2⇒Anellipse:(x/A)
2
+(y/B)
2
=1
IfA=B,thisisacirclei.e.,x
2
+y
2
=A
2
•δ=0⇒Astraightliney=(B/A)x
•δ=±π⇒Astraightliney=-(B/A)x
•Exceptforspecialcases,thegeneralpathisanellipse.

46. Contd…
•PathsintheXYplanesforA=Bandvariousδ

47. Frequency Measurement
•Lissajousfiguresforthemeasurementoffrequencydependsonratioof
frequencies.
•Knownfrequencyisobtainedfromvariablefrequencyoscillator.

48. Contd…

49. Contd…

50. CRO Probes
•Theprobeisusedwiththeoscilloscopetoconnectthetestcircuittothe
oscilloscope.
•Whileconnectingthetestcircuit,theprobedoesnotalter,loadordisturbsthe
circuitandsignalconditionstobeanalyzed.
•Toachievethis,theprobesshouldhavehighimpedance.
•Theprobebandwidthshouldbeashighaspossible.
•Generally,itisatleast10timesthebandwidthoftheoscilloscope.

51. CRO Probes Block Diagram

52. Contd…
•Theprobetipisthesignalsensingcircuitry.Itmaybepassiveoractive.
•Ifpassive,itconsistsoftheresistorsandcapacitorswhileifactive,itconsists
ofactivecomponentslikeFETsourcefollowercircuit.
•Thecoaxialcableisusedtoconnecttheprobeheadtothetermination
circuitry.

53. Various CRO Probes
1.DirectProbes(1:1Probe)
•Thesimplesttypesofprobeisthetestlead.
•TestleadsaresimplyconvenientlengthsofwireforconnectingtheCRO
inputtothepointofobservation.AttheCROend,theyusuallyterminate
withlugs,bananatipsorothertipstofittheinputjacksofthescope,andat
theotherendhaveacrocodilecliporanyotherconvenientmeansfor
connectiontotheelectroniccircuit.

54. 1.DirectProbes(1:1Probe)
•SinceaCROhashighinputimpedanceandhighsensitivity,thetestleads
shouldbeshieldedtoavoidhumpickup,unlessthescopeisconnectedtolow
impedancehighlevelcircuits.
•AlthoughtheinputimpedancesofmostCROsarerelativelyveryhigh
comparedtothecircuitswheretheyareconnected,itisoftendesirableto
increasetheirimpedancetoavoidloadingofthecircuitsorcausingunstable
effects.

55. 1.DirectProbes(1:1Probe)
•Theinputcapacitanceofthescope,plusthestraycapacitanceofthetest
leads,maybejustenoughtocauseasensitivecircuittobreakintoIsolation
ProbeoscillationwhentheCROisconnected.
•Thiseffectcanbepreventedbyanisolationprobemadebyplacingacarbon
resistorinserieswiththetestlead.

56. 1.DirectProbes(1:1Probe)

57. 1.DirectProbes(1:1Probe)
•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.
•Graticuleis 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/E
d
•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.