Tektronix 2213 - Page 41

T h eory of O p eration-2213 Se
r vice
Wh
en
S611
is set to N O RM,
+8 .6
V is applie
d t h roug h
t
h e switc h to R 525 a nd R 517 . Transistor
Q519 is b iased
into sat u ration by t h e p ositive
voltage, a n d both CR 503
an d C R 504 b ecome
forwar d b iase d . T h is action reverse
b iases p ea k detector tra
n sistors Q503 and Q504 to pr eve n t
t he trigger sig
n
al
f r om affecting t he T R IGG ER LEVEL
co
n trol ra n ge .
W it h C R 503 an d C R 504 forwar d biased, t
h e voltage
d ivi d er n etwor k ( R 525, R 527, R 526, a
nd R
528)
sets t he
in pu t voltage to U 507A pin 3 and U
507
B
pin 5 . Α fixed
p ositive ou tput voltage
from
U 507A p
in 1 is app lied to one
en d of R 455, an d α fixe d
negative outpu t voltage from
U 507 B pin 7 is ap plie d to t h e ot
h
er
end of R 455 .
Trigger L evel Com p arator
T he Trigge r L
evel Com p arato r circuit dete r mi
n
es
both
t h e trigger
level
an
d slope at w h ic h α triggering
signal is
prod
uced . Transistors U 460 E and U 460 B
form α com-
parator circuit . It com p ares t h
e
t
r igger signal level applie d
to t he base of U 460 E wit h t
h e reference d o level set b y t h e
TR IGG ER L EVEL
potentiometer ( R 455) and applie d
to t he base of U 460 B .
Slope switc h ing is accom p lis
h
ed
by
controlli
ng t he biasing of transistor p airs U 460A-
U 460D
an
dU 460C- U 460 F .
Wh en AU TO or TV F I EL D
triggering is selected, the
Auto
Trigger circ u it mai n tains α do
level r ange at t h e b ase
of U
460 B t h at is de pend e n t upon
t h e am p litu de of t he
trigger in p ut signal . In t h is i n
stance, t h e Comparator
( U 460
E an d U 460 B ) d etermines t
h
e p
oint on t h e i npu t
trigger
waveform at w h ic h t h e Sch mitt T
r igger circuit will
produ ce an
outpu t .
Wh
en N O RM triggering is selecte d
,
t
he T R IGG ER
LEVEL
potentiomete r ( R455) is set manually
to α do level
t h at will pro
d uce α trigger signal at t
he
o
u tput of t h e
Comparator . If t
h e trigge r sig n al amplitu d e at t
he b ase of
U 460 E is below
t h e reference level, t
h
e Sc
h mitt Trigger
circuit will never
switc h . If t he trigger signal is
a b ove t h e
reference level, t
he
Sc
h mitt Trigger circuit
o u
tput
will
switc h ΗΙ and
remain ΗΙ until eit h er t he trigger
signal is
decrease d or
t h e reference d o level is inc r eased .
T he T R IGG ER S L O
PE switc h (S464) controls t h e
b ias on U 460C an
d U 460 F . Wh en set to t he p ositive slo p e
position,
t he ground is remove d from t he b ottom end of
R 464, an d
t h
e
forwar d bias is t hen d etermined by t
h
e
voltage d
ivi d er formed by R 462 and R 463 . B ot h U 460C
and U 460
F are biase d into con d uction an d carry t
h e signal
cur
r ent from t he Com p arator transistors .
M ovi n g t h e
SL O
PE switc h to t he negative slope p
ositio n g r ound s t h e
b otto m of
R 464 and re d uces t h e b ias level of U 460C and
3- 1 4
U 460 F . T he fixe d b ias
level
on
t he b ases of U 460A and
U 460D is now
h
ig
h er
t
h
a
n t h e bias
on
U 460C and U 460 F
so t h at U 460A an
d U 460D carry t h e signal curre n t from
t he Com p arator
transitors
. T
he collectors of U 460A and
U 460D are cross
connecte d to t h e collectors of U 460 F an d
U 460C, so t h e resulti
n g trigger signal o u t p ut is i n verte d .
Inverting Amplifier an
d T V Trigger Circ u it
Cu r r ent
from one transistor of t h e conducti n g p ai r of
tra
n sistors c h ose n by SL O PE switc h 5464 is applie d to
U 480C pi
n
10 .
Cu rre n t from t he ot h er si d e of t
h
e Com-
parator is ap plie d to p i n 14 at t h e outpu t side of U 480C
t h roug h R 468 . P i n 11 of U 480C is at α
L
O
logic level
exce p t w hen T V
F
I EL D
triggering is e nab led . T h is L O
d oes n ot affect ci
r
c
u it o p e r ation in eit h er A U TO o r N O RM
triggeri n g .
N O R -gate
U 480C is an emitter-cou p le d logic (E C L)
d evice t h at is op erate d in t h e li n ear r egion . In t
h e linear
region,
U 480C acts as α hig h -speed inverting amplifier
.
Common-mode
signals suc h as noise or t h ermal d rift in
t he Comparator
output signal are cancelle d b y U 480C
and associate
d circuitry . T h ese ty p es of offsets equ ally
affect t
h
e o
u t p uts f r om b ot h si d es of t he Comparator .
C
h anging current to p i n 10 of U 480C causes α correspond-
ing voltage
c h ange at U 480C pin 14 . T h e voltage chan ge
at o n e en d
of
R468
is equ al in amount an d o p posite in
d irection to t he
voltage ch ange at t h e ot he r end since t he
same commo n
-mo
d
e sig n al f r om t h e ot h er h alf of t he
Com p arator is applie
d to t h e ot h er en d of R 468 .
Wh en t h e T R IGG ER M OD
E
switc
h
is
set to T V F I EL D,
+8 .6 V is a p p lie d to t
h e
T
V Trig E nable sig n al line . Tran-
sistors Q474 an
d Q476 are b iase d on via R 474, and U 480C
p i n 11 is set
ΗΙ , causing t h e output of U 480C at p in 14 to
be L O .
Cur r ent flowing t
h
ro
u g h R
466
fr om eit h er U 460C or
U 460D ca u ses α
voltage drop t h at esta b lis h es t h e b ias
voltage on t h e base of
Q474 . Current flowi ng t h roug h R 473
an d R 472 p ro
du ces α voltage drop across R 473 t h at esta b -
lis h es t
h
e bias
voltage on t he base of Q476 . T h
e
ci
r cuit
com p
o n ents are selected s uch t h at w h en t h e Compa
r
ator
outp ut voltages f r om both h alves are e qu
al,
t
h e base
voltages to both Q474 an d Q476 will be t
h
e
same . W it h
e q ual
base
voltages, each transistor will con duct an
e q ual
amo
u nt of c ur rent .
Wh
en t h e Comparator outp ut b ecomes un b
alance d , due
to an
input trigger signal, uneq u al b iasing of Q474 an d
Q476 occurs
.
In
response to α c h anging b ias cond ition, t
h
e
collector cu r rents vary p roportio n ally .