Tektronix 2213 - Page 40

Internal Trigger Am p lifier
T he Internal Trigger Am p
lifier converts t he d ifferential
curren t input f r om t
he Trigger P ic koff circuit to α ze
r
o-
referenced , si
n gle-e n de d out p ut for use b y t h e Trigge r L
evel
Comparator
. Differential signals from t
h
e P ic k off Am
p lifier
ci r cuit are con necte d via R421 and R422 to common-
b ase
transistors U421 E an d U421 D res p
ectively . Transistor
U 421C an d R428 co n stitute an i n
verting-fee d bac k am p lifier
t h at co n verts U 421D
collector current to α voltage at t h e
collector of U421 C .
T h is voltage is a dd ed i n ph ase wit
h
t
h e
voltage dro p
across R427 produce d by t he signal current of
U421E . T
h e resulting sum is α single-ende d voltage sig
n
al
t
h
at is a pp
lie d to t h e base of emitter-follower U421 A . The
emitter-follower stage p r ovi d es a low-output-impedance
signal
source t h at d rives b oth t h e ΧΥ Am
p lifier (t hr oug h
R 701)
a nd emitter-follower U421B . T
h
e out
p
ut
signal from
U421 B
is ap p lie d to t h e Trigge r Source Switc h ing
Dio de
circuit at t he cat hode of CR440 w h ere it is
available for
selectio n as t he triggering signal .
Trigger Source-Switch ing Circuit
Trigger
signal selection is accom p lis h e d b y
using t he
SO UR C
E switch (5440) to ena b le o n e of t
h
ree
triggering
signal p at
h
s
(internal, external, or line) to t
h
e
Trigger
L
evel
Comparator circuit . Wit h S440 set to ΙΝΤ , t
h
e
i nh ibiting
voltage is removed from R438, causing bot h U421B
a nd
d io d
e
C
R 440 to be biase d on . T h e internal
trigger signal is
t
h en passed from t he emitte r of U421 B t
h roug h d io de
CR440 to t he T r igger L evel Comparato
r an d Auto Trigger
circuits . T he SO UR C
E switch preve n ts t h e li n e and external
triggering signals
from reach ing t h e Trigger L evel Com-
parator by reve r
se biasing d iodes CR444 and C R448
an d also by
reverse b iasing Q414 and CR418 t h roug
h
R417
.
Wh en S440 is set to L I
N E, U 421 Β an d C R418 are b iased
off t h roug h R438
an
d
R417 respectively, w h ile CR444 is
enable d b y
removal of t h e in h ibiti n g voltage fr om R446 .
Similarly, with 5440 set to ΕΧΤ , the external
trigger signal
is
selecte d by b iasing off C R 444 an d U 421 B
t h roug h
resistors R444 a nd R438 respectively and
b y ena b ling Q414
t h roug h t he removal of t h e in h i b iting voltage
from
R417
.
E xternal Trigger Am p lifier
T
h
e
E xternal Trigger Am p lifier provides α means
of
triggering
t he instr u ment from an externally s u pp lie d signal
t h at
can
be
a pp lie d to t he ΕΧΤ I NPU T connecto r . Input
cou p ling to t h e Amplifier is selectable by
t he t h ree- p osition
ΕΧΤCO UPL I NG switch , 5401 . In
t he
AC
positio n , t h e
do component of t he exter
n
al
trigger sig n al is blocked by
coupling t h e signal t h ro u g
h
C402 . In
t he DC position, all
components of t he signal are cou p le d d
irectly to t h e gate
of Q411A
t h rough an i n put divider compose
d of R404 and
R408 .
Resistors R402 and R403 fo
r m
α
voltage-divi d er
n etwor k t h at atten u ates t he signal b y α
factor of 10 w h en-
ever 5401
is set to
DC
+10 .
T
h eo r y of O p eratio n -2213 Service
F ield-effect tra n sistors Q411 Α and Q411Β are α matc h
e
d
p air
. Source-follower Q411A pr ovi d es α h ig h input
impeda nce
for t he external trigger signal . C u rre n t-source
transisto
r Q411Β causes Q411 Α to op erate at zero gate-to-
sou r ce b ias, so
t he device f unction s w it h no d o offset
b etween t
h e in put and out p ut signals . T h e out p ut signal
from t
h e source of Q411A drives t he b ase of emitter
follower Q414
.
T
he emitter-follower stage lowers t he
output im p e da
n
ce of
t h e Trigger Am p lifie r and functions
as p art of t h e
Trigger Source Switc h ing circuitry .
Auto
Trigger Circuit
Wh en eit h er A UTO or TV F I EL D triggering is
selecte d ,
t h e Auto Trigger circuit detects p
ositive and negative pea k s
of t he in put
trigger signal a nd pro duces out p ut voltages
t h at set t he TR IGG ER
LEVEL control range to with in
t h e p eak -to-pea
k am p litu d e of t he triggering signal . T h e
pea
k detectors are d isable d w h en S611 is set to N O RM, an d
fixe d voltage levels are a pp lie d to bot h end s of T R
IGG ER
LEVEL poten tiometer R455 .
In eit h er A U TO or T
V F I
EL
D, t he T R IGG ER M OD E
switch (S611)
opens t h e Auto Disable signal line to allow
C R 503 and C
R 504 to become reverse b iase d . T h is actio n
isolates
the voltage d ivi d er n etwor k (compose d of R525,
R527, R528,
R526, an d Q519) from t he
+
in puts of
U 507A and
U 507 B . T he pea k detecto r s (com p osed of
Q503
for t he p ositive pea k a n d Q504 for t he negative peak )
become
ena b led w h en t h e A u to Disable signal is remove d .
T h e
trigger signal is applie d to t h e bases of Q503 and
Q504 via R444
. P ositive trigger signal p ea k s b ias Q503 into
co nd uction, ca
u sing its emitter voltage level to rise to t h e
peak level
of t he trigge r amplitu de min u s the b ase-to-
emitter
voltage dro p .
Ca p acitor C503 c h arges up
to t h e positive emitter
voltage level . T h e ch arge is
retaine d between trigger pu lses
due to t h e long RC
time constant of R505 and C503 . T h e
com parator voltage is a pp
lie d to U 507A p in 3 w h ic h is α
voltage follower an d level s h ifter t
h
at sets
t he voltage at one
end
of t h e T R IGG ER LEVEL p
otentiometer (13455) .
Transistor Q507 p rovides t he feedbac
k path for U 507A
and t
h ermally compensates fo r Q503 . T
he b ase-to-emitte r
drop of Q507 corrects for t h e do
offset introduce d by
Q503, an
d potentiometer R511 is a d j usted to
balance out
d o
offsets intro d uce d from t h e trigger circuitry
.
T h e negative pea
k detector operates in t he same manner
as t he p ositive
peak d etector, wit h correspon d i
ng
com ponents pe
r forming t h e i d entical circuit functio n on
t he trigger-signal
negative p eak s .