Oscilloscope 0001

8/9/2019 Oscilloscope 0001

1/16

Oscilloscopes

In studying the variou s elech"o nic, electrical networks a nd sys tems, signals which are

functions of tim e, are often encountered. S uch signa ls may be periodi c or non period lC in

nature. The device which al lows, the amplitud e of such signals, to be displaved p rim


2/16

jElectro niC In s trumentat ion

4.2 Ca th o d e Ray Tube (CRT )

The ("thode rilY tube (CRT) i s the heart of th e CR.O. the CRT genera tes the ele ctron

beam , ,h'U'!c ratc s tbe bea m, de flects the beam and al so has a screen wher e beam becom esvisib le ,1" ,1


3/16

such r epulsion forces, an adjustabl e electr ostatic fi eld is created betwee n two cylindrical

anod es, ca lled the focusing anodes .

Key Point: The var iable positive voltagi' on the second anod e is used t o ad just the focus or sharpness of the bright beams pot .

The high positive potential is also given to , the preacc elerating anod es and


4/16

where K x is co nstant of pro port ionalit y.

The def lectio n prod uce d i s usuall y me as ur ed i n em or as n um ber of divisions, on t hescale , in the hor izo nt al d irec tion ..

xThen K x = V x where K x ex pr esse d a s ~ m / vo lt or division / volt , is called hori zontal

se nsitivit y o f th e osc ill osco pe.

Similar ly, the ver tica l deflec tion (y ) pr odu ce d will be proportio nal to the ver tica ldef lec tin g vo ltage, V y, ap pli ed to the y-input .

y ex; V y

I y = Ky VyK y = y / V y an d K y, th e vertical sensitivit y, will be expressed as ~m / vo lt , or

division / vo lt.

Ve rticaldefle ction Vy

platesHorizontaldeflection

plates

Bright spot(e lectron beam )

Fig. 4.2 Arrangement of plates in CRT

4.2.3 Fluorescent Screen

The va lues of ver tica l aJ 'Ldhorizontal . se nsi tiviti es ar e se lec tabl eand adjus tab le thr ough multip os iti ona lswi tch es on the fro nt p ane l th atco ntro ls the ga in o f th e corres pondin gint erna l ampli f ier stage . The b right

spot of the e lec tro n bea m can thustrace (or plot) the x -y re latio nshipbetwee n the two vo ltages, Y x ard V y.

The sc hema tic a rrange ment of th ever tical and the horizo nt al, pl ates,co nt ro lling the pos ition of 'the spo t onthe sc ree n is s how n in the F ig. .4.2 .

The light produce d by th e sc reen does not d isa pp ear immed iately w hen bombard mentby e lec trons ceases , i.e., w hen th e s ignal bec ome s ze ro. T he ti me pe rio d for w hich th e tra ceremains o n th e sc ree n after th e s ignal become s ze ro is k now n as "pers istenc e". T hepers istence may be jS short as a few micf< ?se co nd , or as long as te ns of seconds ~e nmi nutes .

Me dium persis tenc e trace s ar e mostl y used f or ge nera l purpose appl ica tio ns.

Long persiste nce traces a re u sed in the stud y.. of transien ts. Lo ng pe rsistence help s inthe s tud y of transient s s ince the tr ace is st ill seen on th e scree n after the t ransient h asdisappeared. -

Shor t pe rs istenc e is ne eded for extremely high spe ed ph enomena.


5/16

The screen i s coated with a fhlOresce nt material ca lled phosphor whi ch emit -> light

,,,,hen bombarde d by elec trons. There are various phospho rs a\c1J1able whi ch differ m

colour, pe rsistence , ;)nd effic iency.

~ the comm on phosp hor is Will emite, w hich is zinc, ort hosi lica te, ZnO + Si0 2,

with tr aces of man gane se. Thi s produc es the familiar gr eenish trace. O ther usef ul screen

materials include comp ound s of zinc , cadmium , magne sium and silicon.

The kinetic energy of th e electron beam I Sconverted into both light and h eat energ} when it

hits the sc reen . The heal so pr od uced gi\ e s rise to

"phosphor bu rn" whIch j


6/16

The b ase is provided to the CRT th rough whi ch the con nections


7/16

W hen a per iodi ca lly v aryin g vo lta ge say s inu so idal voltage is app lied to the v ter mIna l

of the ~ cop e and i nt ernally ge nerat ed saw toot h vo ltage is applied to the hO rIzo nt al

def lec tion plate s, then saw tooth vo lta ge k ee ps on shift ing the spo t horizontal ly w hile the

ap pl ied vo ltage s hi fts th e spot ve rtica lly pro por tional t o its magni tude. H ence f ina lly du e

to th e effe ct of both th e vo lta ge, a famil iar sinu soida l wave f orm can be o bse rved o n thesc ree n.

Wh en th e swee p 'lnd signal frequenci es a re eq ual , a s ingl e cy cle appea rs on th e screen .

W hen th e sweep i s lowe r th an the signal, s everal cyc les a ppear on the scr ee n. In such ca se,

the nu mb er of cyc les depend s on th e rati o o f th e two fr equ encies. Wh en th e swe ep is

hIgher th an the s ignal , le ss than one c yc le a pp ear s on th e scre en.

The d ispl ay of spot on the screen app ear s s tationar y onl y wh en th e two f req uencies i.e.

sw eep 'lnd s ignal are s am e o r are inte gral multiple s o f each o ther. For an y o ther

f requ encies th e trace on the screen keeps on driftin g hori zont ally. Thu s fo r th e trace to

app ear statio nary, th e sawt oo th voltage i s sy nchr oni ze d with signal appl ied to the ve rticalin put . Fo r th e ve rti cal input signal , the tri gge rin g puls es a re derived for the

sy nchronizatio n.

There , Ire two imp ortant requirement s o f a swee p ge nerator :

1. The swee p mu st be line ar in natur e, for all s cree n h orizontal deflec tion .

2. To move the spot in one direction onl y, th e swe ep volta ge mu st dr op to zer osudde nly, aft er reac hing it s ma ximum va lue. Ot herwise the re turn swe ep wil ltrace th -e s ignal backw ard s.

/ .; 4 .3.1 Types of Sweep

Th e v arious types o f sweep g enerated in th e os cillo sco pes are ,

1. Recurren t Sweep: Th e sawtooth swe ep i s an a.c. vo ltage a nd it rapidl y a lternatesand h ence the di splay occ urs repetiti ve ly. T he o bse rver sees the la stin & , im ag e by the eye.

This repea ted op eration i s called rec urren t sweep . Thi s is al sd c alled continuous sweep.

2. Single Sweep : Th e wa veform to b e in ves tigat ed p ro duc es a t rigge r sig nal w hich

pr odu ce s a s in gle sw eep .

3. Driven Sweep : The sweep is dri ve n b y the s ignal it se lf in thi s op eration. If the

swee p cyc le star ts af ter the start of signal c yc le, then part of th e cyc le may be los t. Thisposs ib lili ty is remove d b y s tarting the sweep c yc le by the signal , alon gwith th e s tar t of th esignal c yc le.

4. Triggered Sw eep : A recurrent sweep cont inues t o produc e a hor izo ntal line

whe the r input signal i s applied or not. But in a triggered sweep, a sweep i s initi ated by a

tri gger vo ltage w hich i s deri ve d from an incoming signal . Thu s in th e a bse nc e of s ignal ,

th ere is no swe ep and C RT screen remains blank .

The co ntinu ous sw eep use s astable (free running ) multi vihr a10r. It co vers wide

f requ ency range . It can be locked i nto s ynch roni sation b y an input signal it se lf . W heneve r


8/16

,we ep fr equency and input signal frequen cy are same or multipl es of eac h other, theslnchroni sation resu Its.

The trigge red swee p uses monostable multi vibrator. It beco mes on w henever trigge rpulse com es and till then remains off , k eeping th e scre en blank . After specific voltage isrea ched,the multi vibrator switches to off state. Thu s triggered swee p produces the displayfor specific p eriod of time . It produce s the displa y irrespectiv e of the signal frequency.Thu swith tri gge red swee p single s hot eve nts or transient s can be obse rved. Prac ticallyoscillo sopesusing trigge red sweep us es the calibr ation of swee p speed in time perdivision The rec iproca l of the time p eriod selected is the frequency of the sweep .

5. Intensit y Modulation: In m any applic ations, it is necessa ry to mak e the intensity of pmt of the signal more than the oth er part. This is call ed intensity modul ation. It isachieved by applying a.c. signal to the control eletrode of CRT . Thus the display is brig htfor po sitive half cyc le and dark ' for negativ e half cy cle. This is a lso ca lled Z-axis

modulation.

.r Block Diagram of Simple OscilloscopeThe block diagram of os cill oscope is shown in the Fig. 4.5.

Inputsignal

Timebase

generator

- veHigh

voltageLow

voltage

P owersupply

Fig . 4.5 Bas ic blo c k d iagram o f C.R.O

The variou s block s o tblock diagram of simpl e oscillo scope ar e as f ollows:

4.4.1 CRT

Thi s is the cath ode ray tub e which i s the he art of CR .O. It is' used to emit th erlectrons requir ed to strik e the phosphor screen to produ ce the spot for the vis ual displayofth e s ignals.


9/16

Electron ic Instrumentation

'j 4.4.2 Vertical AmplifierThe input signals are generally not strong to provide the measurabl e deflection on th e

screen. Hence the ver tical amplifier . stage is used Jo amplif y the input signal s. T h e

ampli f ier stages used are generally wide band amplifiers so as to pass faithful ly the entireband of frequencies to be measured. --'

Sim ilarly it contai ns the attenuat or stages as we ll. The attenuators are used whe n very

high voltage signa ls are to be exa mined, to bring t he signals within the proper range o f

operat ion.

FET input amplifierr- - -----------I

Phase I

inverter :I

I

I

J-- - --- - --- - - -

Fig. 4.6 Vertical amplifier

It con sists of se vera l stages with overal l fixed senslt ivity. The amplifi er can be

designed for s tability and required bandwidth very easily due to the fi xed gain .

The input stage co lrtsists of an attenuator f ollowed by FET sour ce follow er. It has vel'

high input imp edan ce required to isolate the amplifier from the attenuator .

It is f ollowe d by BJT emitt er follower t o match th e out put imp edan ce of FET o utput

W ith input of phase inver ter.

The phase inverter prov ides two antiph ase o utput si gnal s which are required to

opera te the push pull output amplifier.

The push pull operation has adva ntages like be tter hum voltage canc ellation , even

harm onic suppre ssion especially large 2 nd harmonic, great er power output p er tube and

reduced number of defocusing and no nlinear effects .

cd4.4.3 Delay LineThe delay line is used to dela y the signal for some time in th e verticClI sections. Wh en

the delay line is not used, the part of the signal gets lost. Thus the inpu t signal is not

appli ed directly to the vertic al plates but is d elClyed bv some time using a dela y line

cu-cuit as shown in the Fig. 4.7.


10/16

Inp utsignal

Fig . 4.7 Delay l ine c ircuit

Key Point : As the signa l is d elayed , the swee p genera t or out put gets enough t ime to reacht o the hori zontal plat es before signal r eaches the verti~al plates.

If the trigger puls e is pick ed off at a time t = to after the signal ha s passed throughthe main amplifi er then signal is dela yed by XI nano seco nds whi le sweep ta k es YI

nano seconds to reach. The design of del ay line is such th at the delay tim e XI is higherthan th e tim e YI' Generally XI is 200. nsec w hile tl ;1.eYI is 80 ns, thus the swee p starts wellin time and no part of the signal is lost .

There are two t ypes of delay lines used in CR .O. which ar e:

i) Lumped param eter dela y line

ii) Distributed parameter dela y line

4.4.3.1 Lumped Parameter Delay Line

Lumped parameter dela y line con sists of num ber of

t:=]Ll2Ll2 cascaded ' symme trical LC network s called T sections . Each

; ' _ ~ o . ,e"ion i' capable of d e layin g the 'ign a J b y 3 '0 6 " ,ec, __ Such a T filter section i s shown in th e Fig. 4.8 .

The T section acts as a low-pa ss filter having cut of f Fig. 4 .8 T-filter secti .on frequenc y as,

I f, = nJ~C IIf Vi con sists of frequencies much less than the cut -off f~equenc y, output signal Vo will

be a f aithful reproduction of Yt but delayed b y the tim e,

I to =~ =)IT I


11/16

A practica l de lay line circuit in CR .O . is dri ven b y pushpull amplifier and i s show n i n

the Fi g. 4.9.

Fig. 4.9 Prictical delay line

4.4.3.2 Distributed Parameter Delay Line

It i s basicall y a transmission line constructed with a wound helic al coil on a m andr eland extruded insulation between it. It is speciall y manufactured co-axial cable w ith h ig hindu ctance per unit length. The cons truc tion of such line is shown in the Fi g. 4.10 .

The ind uctance can be increased by wind ing th e he lica l inner condu ctor, o n

ferromagne tic core . This increases the characteristics impedance Zo and d ela y tim e. Typic alpClrameters' for helica l, dis tributed parameter delay line are Zo = 1000 D and t d = 180nsec / m . The coaxial dela y line i s advantageous as :

i) It does not require careful adjus tment a s lumped parame ter.

ii) It requires le ss space.

Outerconductor

.:74.4.4 Trigger Circuit

It is necessary th at horizon tal d efle ct ion s tarts at the same point of the input vertic alsigna l, each time i t sweeps. He nce to synchronize horizonta l deflection with vertic aldef lection a sy nchronizing or triggeri ng circuit is used . It converts the incoming signal int o

~e triggering pulses, which are used for the s ynchronization .

The time base generator is used to generate the sawtoo th voltage , required t o deflectthe beam in the horizo nt al sec tion. This vo ltage def lects the spot at a co nstant timedependent rate. T hu s the x -axis ' on t he screen ca n be represen ted as time, w hic h, help s todisplay a nd ana lyse the time varying sig nals .


12/16

J Electronic Instrumentation J 4.6 Supply Voltages for CRT

The va rious vol tages ar e n ecess ar y to va rious co mp onents of CRT, f or its p roper

operat ion. T he C RT wi th var io us suppl y vo ltage leve ls a t va rious point s is s hown in th eFig. 4. 16.

Intensi tycontrol

To othercircuits

High voltagesuppl y

Low voltag esup ply

It h ,lS bee n m enti oned ear lier that th e tim e dom ain os cillo scopes req uire a sweep

ge nerato r th at is lin ear with tim e for th e x -a xis di spla y. The motion of spot on th e scree n

fro m extre m e lef t to ex tre m e right is ca lled sweep .

The g~nerator which ge nerates a wavefo rm which is respo nsib le f or th e m ove m ent of

spot lm screen horizontally is called tim e base genera tor or sw eep ge nera tor . The sweepcircu its alo ng w ith th e d ispl ay gati ng fu nctions are ca lled time bases .

The l in ear sweep mo ves th e sp ot f rom l ef t to, rig ht while th e m ove m ent of spot fro m

right t o lef t is not visibl e. Th is portion of wav ef orm ge nerat ed b y tim e base is ca lled

f lyback or retrace . Durin g thi s time , th e ca th ode ray tub e is b lanke d.

The t im e base ge nerator al so contr ols th e rate at w hic h th e spo t m oves; across t hesc ree n, this rate is t o be a dju sted fr om fro nt panel co nt rol .

4.7 .1 Why Sweep Generator is Called Time Base Generator?

1\ 11 th e t ime de pend ent wave f orm s nee d x-ax is to be calib rat ed as time axis. T he swee pge nerato r pr odu ces the movement of spot on sc ree n such that i t act s a s a tim e axis o r tim e

bdse fo r th e wavefor m s to be di spla ye d. Hence th e sw ee p gen erator is visa ca lled tim e

base gene rator ,

The two f ront pa nel col11r ols w hi ch ar e us ed t o co nt rol rate a nd d ur ation of ti m e base

wavefo rm are i) Tim e/ di vision and ii) T im e v ariable contr ol.


13/16

Key Point : T he tr igger clr C llit ensur es t ha t the hor iz ont al sweep s ta r t s a t th e ,a llle POlll t of the ue r tica l input signa l.

4.7 .2 Requirements of T ime Base

The time base requirem ents are :1. Sweep time variations from 10 nsec to 5 sec per divi sion.

2. Time accurac y better than 3 %.

3. Linearit y better than 1% across the cathod e ray tub e .

.t. Te n time~ ex pans ion in the hori zontal amplifi er whi ch al1ow~ I nscc pe r divisiondisplays f or ver y high speed tran sients .

J. The speed of the spot should be c onstant acro ss the entire screen.

6 The spot should be in visibl e whil e tracin g from right to l ef t illld should be

visible onl y from l eft to right.

4.7.3 Basic Principle of Time Base Gene rator

The 'basic sweep generator uses the char ging characteristics of a capacitor to generate

IineiH risetim e voltag es. Lmearly incr easing ramp which be comes ze ro wi thin very short

dur ation of time ensures that the spot i s visible (r om left to right Jnd in vis ible from right

to le ft.

Closed

~open5 , ,

I

I Closed

S2 ' : Open II I.r--;--: ;----

~

I I :V I I I

o II

Sw e ep Flyoack

Whe n swit ch S1 is closed, S2 is open and capacitor char ges to produce linear ramp iltthe output. The sweep rate can be controll ed by chan ging the value of capac itor or

ch


14/16

Another method of s tud ying two voltages simultaneousl y on th e screen is to u

special cathode ray tube having two separate electron guns generating two se parate beami

Each electron beam has its own vertical deflection plate s.

But th e two beams ar e deflected horizontall y by the comm on set of horizon tal plate\ The tim e base circuit ma y be same or different . Such an osc illosc ope is called Dual Beam

Oscilloscope .

The block diagram of du al beam oscill oscope is shown in the Fig . 4.24.

Scree n

J

The osc illosco pe has t"vo ver tical def lection p lates and t.wo separa te channels A and B

f O! the two separat e input signals. Each ch annel consists of a preclm plif icr 'lIld an

a tten ua tor.

A delay line, main vertical ampl ifier and a set of vertical de flection plates toge therf orms a single chann el.

There is a singl e set of horizontal plates and single tim e base c ircuit.

The sweep gen erator drives the hori zon tal ampl if ier whi ch intur n drives the pbtes.The' horizontal plate s sweep both the beams across the screen a t the same ra te.

The sweep generator can be triggered int ernally by the channel A signal or .channel Bsignal.

Similarly it' can also be triggere d from an external signal or line frequency s ignal . This

is poss ible w ith the h elp of trigger select or swi tch, a fron t panel control.

Such an oscillo scope may have se parate timebase circui t for sepa rate cha nnel.

This allows diff eren t swee p rates for th e two channels but increas es the si ze andweight of th e oscillo sco pe.


15/16

HOrizontalplate s

Horizon talplate s

Fig . 4.25 Dual beam eRO with separate time bases

4 .9 .1 Multiple Beam Oscilloscopes

Multipl e beam osci JJosc ope has a single tub e but several beam producing systems

inside . Eac h system has separ ate vertic al defl ecting pair of pl ates and generally ( l common

time base sys tem.

The trigge ring can be done internaJJ y using eith .er of the multi ple inputs or externally

by an external signal or line voltages .

The comparison of two or more voltages is very m uch ,necessary i n the anal ysis and

study of many electronic circuits and systems . This is poss ible by using more than one

oscillo scope but in such a case i t is diff icult to trigge r the sweep of each o scill osc ope

precisely Clt the same time. A c omm on and less cos tly method to solve this problem is to

use dual trace or m ulti trace osci JJoscopes. In this met hod, the same electron bCC l!11IS used

to generate two t races w hich ca n be deflected f rom two independent ve rtica l sour ces. The

.( \ methods are u sed to generate two independent tra ces which cHe alternclt e sweep

mL 'tllodund other is chop me thod.

The block d iClgram of dual trace osc iJJoscope is s hown in the Fig. 4.26.ITher e al:e two separa te ver tical input channels A and B . A separate preamplifier and

-attenuator stClge ex ists for eac h chann el) Hence amp litllde of each input can be i ndi viduaJJ y

controll ed. Af ter prea mplif ier stage, both the signals are fed to a n electro nic switch. The

switch hcl S an clbility to pass o ne channel at a tim e via de lay line to the ve rtica l ampl ifier .

The tim e base c ircuit uses a trigger se lector sw itch 52 which allows the circuit to betrigger ed on either A or B chann el, on line freq uency or on an external signa l. The

hO rILOntaJclmplif ier is f ed f rom the sweep ge nera tor or the B cha nnel via sw itch 5! and 51.The X-Y mod e mean;?, the osc iJJosco pe operates f rom cha nnel A as the vertical signal dnd


16/16

the ch'llln el B as the horizo ntal signal. Thus in this m ode very acc urate X-Y measurements

can be do ne.

P rea mplifier

anda lten uator

Chan nel B Preamp lifier

anda ltenuator

B - AExt. trigger c:

line\ J 3 2

Tng ge r selectorswitch

De lay Aline

Dependin g on th e selection of front panel control s several mod es of operation can beselected such a s chann el A only, chann el B only, c hannel A and B as se parate traces ,

signals A + B, A - B, B ~ A or - (A + B) as single trace.

Let us stud y the two mod es, alternat e swee p and chop mod e of ope ration.

~ . Wh en the display mode selector i s in the alternate mod e the electronic swi tchcl alternat ely connects the vertical amplifi er to channel A a nd to channel B. fnitially eac h

vertic al amplifier i s adju sted with the help of att enuat or and position c ontrol such that th etwo images are positioned separatel y on th e screen. An electron ic swit ch is controlled b yusing a tog gle flip-f lop. The switchin g takes place a t the start of each new swee p. Theswitchin g rate of an electronic switch . is synchroni sed to the sweep rate so that CRT spot

trac es chann el A signal on onesweep and ch annel B signal on

the next su cceedin g sweep. Thustwo chann els are alternatelyconnected to the vert iCi lI

Documents

Oscilloscope 0001