Tektronix OPTION 10 Service Supplement Manual page 5

464 and 466 Opt ion 1 0
When t he VAR cont rol is rot at ed out of its calibrat ed
det ent , t he B and C out put t ransist ors in U3290 begin t o
conduct by am ount s det erm ined by t he posit ion of t he
VAR cont rol . This causes t wo event s :
For VOLTS /DIV posit ions above 5 m V , frequency.
com pensat ed volt age dividers ( at t enuat ors ) are swit ched
i nt o t he circuit t o produce t he vert ical deflect ion fact ors
indicat ed on t he front panel . Each channel has 2X , 4X ,
1 0X , and 1 00X at t enuat ors which are used in com bina
t ions . The at t enuat ion rat ios are const ant at all frequencies
wit hin t he bandwidt h of t he inst rum ent . The input at t enua
t ors m aint ain t he sam e input charact erist ics ( 1 M2 and
about 20 pF ) for each set t ing of t he VOLTS / DIV con
t rol . Each at t enuat or has an adj ust able series capacit or t o
provide correct at t enuat ion at high frequencies and an ad
j ust able
shunt capacit or
t o
provide
correct input
capacit ance.
1 . The signal current in t he A and D out put t ransist ors
is reduced by t he am ount of current flowing in t he
B and Cout put t ransist ors .
(
ni
2. Out put t ransist ors A and C and out put t ransist ors B
and D conduct current s of opposit e polarit ies . The
out put of t ransist or C is sum m ed wit h t he out put of
t ransist or A t o reduce t he signal current available at
pins 5 and 6 , and t he out put current of t ransist or B is
sum m ed wit h t he out put current of t ransist or D t o
reduce t he signal current available at pins 8 and 9 .
C
NOTE
Each at t enuat or is a hybri d encapsulat ed plug - in
assem bly. Individual com ponent s in t he at t enuat or
cannot be replaced . The at t enuat or m ust be serviced
as a unit .
The com ponent values select ed for t he variable funct ion
provide a variable at t enuat ion rat io of about 2.5 t o 1 .
Channel 1 Variable Balance adj ust m ent R3484 is adj ust ed
so t hat t he display does not shift when rot at ing t he VAR
cont rol . When t he Channel 1 VAR cont rol is out of its
calibrat ed det ent t he Channel 1 UNCAL indicat or i llum i
nat es . The com ponent s connect ed bet ween pins 2 and
3 of U3290 provide high - frequency com pensat ion .
n
Source Follower. The Channel 1 signal from t he input
at t enuat or reaches Source Follower Q3297A via R42 , 242 ,
and R45 . Resist or R39 provides t he input resist ance and
R 45 funct ions as a dam ping resist or . Q3297B is a const ant
current source for Q3297A . Toget her , Q3297A and B pro
vide high input im pedance for t he at t enuat ors and t he cur
rent drive needed for t he First Am plifier .
C
Transist ors Q31 75 and Q3375 are com m on - base am
plifiers t hat convert t he out put current signals from U3290
int o volt age signals for t he Second Am plifier . Gain adj ust
m ent R3482 set s t he overall gain of t he Channel 1 Vert ical
Pream plifier by adj ust ing t he signal volt age t o t he bases of
Q31 55 and Q3355 .
n
In t he event t hat excessively -high -am plit ude signals are
applied t o Q3297A , t hey will be lim it ed by CR3298 and
t he gat e - source j unct ion of Q3297A , If t he negat ive signal
am plit ude forward - biases CR3298 , t he gat e of Q3297A is
clam ped t o about -8 ,7 V. Excessive posit ive signal am pli
t ude will forward -bias t he gat e - source j unct ion of Q3297A.
When gat e current flows , t he gat e volt age will st op i n
creasing . Gat e current is lim it ed t o a safe value by t he high
resist ance of R42 .
-
Second Am plifier . Transist ors Q31 55 and Q3355 , wit h
Q3344 and Q3346 in t he Vert ical Swit ching circuit (Di a
gram 1 8 shows t he Vert ical Swit ching Logic ) , form push
pull
cascode
am plifiers .
Volt age- variable capacit ors
CR3263 , CR3264 , and t herm ist or RT3267 com pensat e t he
high -frequency gain for varying am bient t em perat ure . As
t em perat ure increases , t he resist ance of RT3267 decreases
and t he reverse- bias on
CR3263 and CR3264 decreases .
The capacit ance of CR3263
and CR3264 increases as
reverse- bias decreases . The result ant increase in capacit y at
higher
t em perat ures
provides addit ional
high - frequency
peaking t o count eract t he effect s of increased t em perat ure
on t he am plifier's bandwidt h .
C
First Am plifier .
The First Am plifier st age is an i nt e
grat ed
em it t er - coupled , push - pull , cascode
am plifier ,
U3290 . The input signal on pins 1 3 and 1 6 is convert ed
from a single - ended t o a push - pull signal by a paraphase
am plifier . It is t hen fed t o com m on - base st age Q31 75 and
Q3375 . The CH 1 VAR VOLTS/ DIV cont rol connect s t o
pin 1 1 of 43290 and provides variable vert ical deflect ion .
Wit h t he VAR cont rol in its calibrat ed det ent ( wiper at
ground ) , t he A and D out put t ransist ors in U3290 are
conduct ing. The B and C out put t ransist ors are reverse
biased . Thus t he signal current available t o t he following
am pli fi er st age is t he collect or current flowing in out put
t ransist ors A and D.
The push - pull signals at t he em it t ers of Q31 55 and
Q3355 are convert ed t o a single -ended signal by Q31 50 ,
Q3250 ,
Q3045 , and Q31 45 . The current
signal from
Q31 45 is convert ed t o a volt age signal by com m on - base
am plifier Q3038 and applied t o t he bases of Q31 25 and
Q3028 . Q3028 provides t he out put signal at t he CH 1
VERT SIGNAL
OUT connect or on t he
inst rum ent 's
rear panel . The out put signal at t he em it t er of Q31 25
serves as t he t rigger signal source in t he CH 1 posit ion of
1
3
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