Tektronix 2213 - Page 42

T h e collector cu rrent cha n ges from Q474 are filtere d
b y α netwo rk compose d of C476, C477, R 477, and R 478 .
T h e filter n etwor k re j ects T V vi d eo information a nd
averages the T V h o r izontal-sync pu lses . Setti ng t he trigger-
level t h res h old at near t he ce n te r of t he h o r izo n tal-sync-
pu lse swi ng establis h es th e untriggered level . Wh en t h e TV
vertical-sync b loc k occurs, the o u tput of t he filter r ises to
α level t h at will cause t he Sc h mitt Trigger ci r cu it to switc h .
P recise TV field sy nch ronization is obtai ne d as α r esult of
the filte r ing action .
T h e o utp ut signal from th e filter is a pp lie d to U 480 B
pi n 6 . T he Sch mitt Trigger circ u it res pond s only to t he TV
sync sig n al b ecause p in 7 is h eld L O b y t he ou tput of
U 480C .
Sc
h mitt Trigger Circuit
W it h α L O on U 480 B p in 7, t h e o u tput at pin 3 goes
L O as soon as t h e signal on U 480 B p in 6 reac h es t h e
switc h ing t h res h ol d . T he L O is applie d to U 480A p in 4 and,
toget he r wit h t
he
fixe d L O on p in 5, ca u ses t
h
e out p ut of
U 480 B pin 6
via
R 480 to
reinforce t h e switc h i n g action .
As α res u lt, t h e outpu t signal at U 480A pin 2 switc h es
r ap idly .
Wh
en t he level from
t
h e filter networ k falls
to
t he
L O
t h res h ol d level, t he fee db ac k s up plie d by R480 h ol d
s
t he
Sc
h mitt Trigger switc h ed ΗΙ fo r
α
s h ort time .
T
he amo u nt
of time i n volved preve n ts noise occu r ring exactly at t h e
t h res h ol d level from causi ng false trigge r ing .
Wh en eit h e r A U TO or N O R M triggeri ng is selected,
i npu t p in
6
of U 480 B is hel d LO, and t he
Co
mp arator
o u t pu t signal on U 480 B pin 7 su p p lies t he in p ut to t he
Sc
h mitt Trigger circ u it .
T he output of t h e Sch mitt Trigger ci r cuit is o b tai ned
f r
om
U
480D pins 9
a nd
15 .
T he d ifferential ou tp u t signal
derived
from
U 480D is ap p
lie d
to
α
two-transistor
level-
s h ifting circ u it com p osed of
Q492
an d Q493 .
T
h e level-
s
h
ifting circuit
converts
t
h e E
C L
logic levels
to TT L logic
levels required
for
t
h e Swee p Ge n erator .
Α signal obtai
ne d
from t he collector of
Q493 is
used to d r ive t h e Auto
B
aseline circuit
.
Auto B aseline Circuit
T
he
Auto
Baseline
circuit
(com p osed of U
640A, Q605,
and associated components) is
e
nab le d in b ot h A U TO a n d
T V F I EL D triggering mo d es . T h is circuit p rovides α signal
to t he Sweep Generator circuit (Diagram 5) t h at i n itiates
α sweep if α t r iggeri n g signal is not receive d by t he Sch mitt
Trigger circuit wit h in α perio d of about 100 ms . Α seco n d
ou t put from t h e ci r cuit illumi n ates t h e T R IG'D LE D on
t h e instrument front p a n el w hen t he sweep is triggere d .
T h eory of Operation-2213 Se r vice
Wh en a d equ ate t r igge
r ing
signals are bei ng r eceived,
t h
e
ou t p ut of Q493
is
a p
plied
to
pin
5
of
mo
n osta b le m u lti-
vi br ator U 640A . T h e n egative-goi ng ed ge of t he signal
ca
u ses
p
i n
6
of U 640A to switc h ΗΙ . T he ΗΙ forwar db iases
C
R
615,
an d Q605 is t hen b iase d i n to con d uctio n . W it h
Q605 con d ucti n g, t he Auto Baseli ne signal li n e is hel d L O
to p revent t h e Swee p Generator circuit from free runni ng .
T h e amo u nt of time t h at p i
n
6 of U 640A stays ΗΙ
wit hout receivi n g an in p ut sig n al is d ete
r
mined by timing
com p onents R 614 and C614 . If α trigger signal is not
received in a b out
100
ms, p i
n
6
of
U 640A
will
go L O
a n d
Q605 will be b iase
d off .
T
h
e Auto B
aseli n e signal line t hen
goes ΗΙ t h roug h pull-up r esisto r R 610, and t he Sweep
Gene r ato r free run s to p ro du ce t he baseline trace .
In N O RM t r iggering mo d e, t he Auto Disa b le signal
(+8
.6 V) is a p p lied to t h e b ase circ u it of Q605 via C R 611
an d R 611 . T he signal h ol d s Q605 fo r war d biase d a nd
p r eve n ts t he Sweep Ge ne r ator from free ru nni ng .
T h e ot h er fu n ctio n of t h e A u to B aseli ne circuit is to
illumi n ate t h e T R IG'D LE Dw hen t
h
e sweep is p roperly
triggere d . As long as U 640A pi
n
6
remains
ΗΙ (t
r igge r ing
sig
n
als occur
r ing with t he p rope r
ti
m e),
T
R
IG'D LE D
DS618 will be ill u minate d . T he t r igger mode i n u se d oes
not affect t he o p eratio n of t he T R IG'D LE D .
S
WEEP
G
ENER ATO
R
A N
D L OGIC
T h
e
Sweep Gene r ator an d L ogic circuitry, s h own
on
Diag
r
a
m
5, p
roduces α sawtoot h voltage t h
at
is
am
p lifie d
by t he H o r izontal Am p lifier to p rovi de h o r izontal d eflec-
tion on the c r t . T h is sawtooth voltage (swee p ) is p ro duced
on comma
n
d
from
t
he Swee p L
ogic
circuits
. T
h
e Swee p
Ge ne r ator ci r cuits also produce gate wavefo r
ms
th
at
are
use d
b y
t he Auto Intensity an d Ζ -Axis circuits to esta b lis h
t h e
co
r rect timing of t h e crt unb la nk ing and i n te n sity
levels u sed for viewi ng t he d is p lay . See F ig u re
3-5
for t h
e
Swee p timi ng diagram .
T h
e Swee p Logic circuitry
controls
t he h oldoff time,
starts t
h
e swee p upon
reception
of α trigger signal, and
te
r
mi n ates
t h e sweep
at
t h
e p
roper
swee
p
level
.
Wh e
n
u si n g
A U TO or T V F I EL D t r iggeri n g, t he Sweep L ogic circuitry
will
ca
u se t h e Swee p Generator to free ru n, p ro
d
ucing α
b aseline trace if α trigger signal is n ot received wit h in t he
p r ed etermined time perio d .
M
iller Swee p
Generator
T he M
iller Sweep ci r cuit is com p osed of Q630A, Q630 B ,
0631, an
d
associate
d timing components . T he circ u it
operates to h ol d
t
he c h a r ging cu rre n t to t he timing
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