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7. CIRCUIT DESCRIPTION

7-1. VISION IF amplifier,afc,video demodulator

T h e IF sig n a l fro m th e tu n e r is fe d thro u g h a S AW filte r to th e d iffe re n tia l IF in p u t (pin 4 8 a n d 4 9).

T h e first IF sta ge c on s is ts o f 3 A C -c o u p le d a m p lifie rs w ith a to ta l ga in c o ntro l ra n g e o f o v er 6 6 dB .
T h e re fe re n c e c a rrie r fo r the v id e o d e m o d u la to r is o b ta in e d by a P L L ca rrie r re g e n e ra to r(e lim in a tin g

n o tc h filte r co m p ro m ise s ,a s in re fe re n ce tu n e d c ircu its fo r p as s iv e c arrie r re g e n e ra tio n ).
O n ly a n o sc illa to r co il is n e e d e d (p in 3 ,4 ) th a t c a n b e a lig n e d v ia IIC bu s to th e d o u b le IF fre q u en c y.

T h e A F C in fo rm a tio n is d e riv ed fro m th e V C O c o n tro l vo lta g e o f th e IF -P L L a n d c a n b e re a d via IIC
b u s. B it A F B to g g le s w h e n th e p ic tu re c a rrie r is e xa c tly at th e d e s ire d IF fre q u e n c y (= h a lf th e a lig n e d IF -P LL

fre q u e n cy ). A FA is a c tive in a w in d o w aro u n d th is p o in t.
F o r fa st s e a rc h -tu n in g a pp lica tio n s th is w in d o w c a n b e in cre a s e d b y a fa cto r3 (A F W b it).

7-2. TUNER AGC

T h e au to m a tic g a in c o n tro l(A G C ) c ircu it o p e ra te s o n to p s y n c le ve l a t n e g a tiv e m o du late d s ig n a ls o r o n pe a k

w h ite le ve l a t p o sitiv e m o d u la tio n , se le c te d b y M O D bit.
T h e tu n e r A G C is c o n tro lle d v ia p in 5 4 .

T h e te n e r A G C ta k e o v e r p o in t (TO P ) ca n b e s e t o ve r a w id e ra n ge :0 .8 m Vrm s - 8 0 m Vrm s IF in p u t
s ig n a l a m p litu d e .

T h e tu n e r A G C o u tp u t m a y h a v e to o p e ra te a b o v e V cc o f T D A 8 3 7 4 .
T h e re fo re p in 5 4 is a n o p e n c o lle c to r o u tp u t, th a t c a n o p e ra te fro m 0 .3 u p to V cc + 1 V (a t> 2m A sink cu rre nt)

S o u rc e s e le ct s w itc h

T D A 8 3 7 4 in p u t sw itch ca n se le c t o n e o f th e fo llo w in g so u rce s ;
p in 1 3 fro n t-e n d : C V B S 1in t

p in 1 7 : C V B S 2 e x t
p in 11 ,1 0 : S -V H S Y,C

S e le cte d s ig n a l is a v a ila b le a t th e C V B S o u tp u t p in 3 8 , in ca s e o f Y /C in p u t Y + C a re a d d e d .
It d riv e te le te x t a n d th e T D A 8 3 9 5 S E C A M ad d -o n .

F o r S -V H S a p p lic atio n s , th e Y,C in p u t c a n b e s e le c te d , in d e p e n d e nt o f th e C V B S so u rc e s w itc h .
T D A 8 3 7 4 Y,C in p u ts a re s e le cte d ,w h ile th e s o u rce sw itch o u tp u ts C V B S Iin t o r C V B S 2 ex t o n C V B S o u t.

7-3. Horizontal synchronization and protection

T h e sy n c. s e p e ra to r a d a p ts its s licin g le v e l in th e m id d le b e tw e en to p -s yn c a n d b lac k le v e l o f th e C V B S

s ig n a l. T h e s e p e ra te d s yn c . p u lse s a re fe d to th e firs t p h a se d e te c to r a n d to th e c o in cide n c e d e te cto r.

T h e

-1 lo o p g a in is d ete rm in e d b y th e c o m p o n e n ts a t p in 4 3 (C + R C ).

T h e c o in c id e nc e d e te cto r de te cts w he th e r th e h o riz o n ta l lin e o sc illa to r is sy n ch ro n iz e d to th e in c o m m ing vid e o .
T h e lin e o s cilla to r is a V C O -typ e , ru n nin g a t tw ic e th e lin e fre q u e n cy.

It is ca lib ra ted w ith th e X -ta l o sc illa to r fre q u e n cy o f th e c o lo u r d e co d e r a n d h a s a m a x im u m d e v ia tio n o f 2% o f
th e n o m in a l fre q u e nc y, so n o a lig n m e n t is ne e d e d .

C a lib ra tio n is d o n e a t sta rt up (th e T D A 8 3 7 4 m u s t first k n o w w h a t co lo u r X -ta ls a re c o n n e cte d ,b it

X A ,X B ) a fter sy n ch ro n iz a tio n lo ss (

-1 co in c id e n c e d e te c to r "S yn c L o c ke d " b it S L ).