Internal Trigger Am P Lifier; Trigger Source-Switc Hing Circuit; E Xternal Trigger Amplifier; Auto Trigger Circuit - Tektronix 2213 Service Instructions Manual

Internal Trigger Am p lifier
T h e Internal Trigger Am p lifier converts th e
d
ifferential
curre n t input f r om t h e Trigger
P ic koff circuit to α ze r o-
reference d , si n gle-e n de d out put 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 co n necte d via R421 and R422 to common- b ase
transistors U421 E an d U421 D
resp ectively . Transistor
U 421C an d R428 co n stitute an i n verting-feed back am p lifier
t h at co nverts
U 421D collector c urrent to α voltage at th e
collector of U421 C.
T
h is voltage is adde d
i
n
p h ase wit h t h e
voltage dro p across R427 produce d by t h e signal current of
U421 E .
T
h e resulting sum is α single-ende d voltage sig n al
t h at is a p p lie d to t h e base of emitter-follower U421 A . The
emitter-follower stage
p
rovi d es a low-output-impedance
signal source th at
d
rives
b
ot h
t
h e ΧΥ Am p lifier (t hroug h
R 701) a n d emitter-follower U421 B . 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 d e
circuit at t h e cat h o d e of C R 440 w h ere it is available for
selectio n as t h e triggering signal .
Trigger Source-Switch ing Circuit
Trigger signal selection is accom p lis h e d
b y using t h e
SO UR C E switch (5440) to ena ble 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 . With S440 set to ΙΝΤ , t h e i nh ibiting
voltage is removed from R438, causing bot h U421 B a nd
d
io d e C R 440 to be biase d on . The internal trigger signal is
t h en passe d from t h e emitte r of U421 B t h roug h
d
io d e
C R 440 to t h e T r igger
L
evel Comparato r an d Auto Trigger
circuits. Th e SO UR C E switc h preve n ts t h e li n e an d external
triggering signals from reach ing t h e Trigger
L
evel Com-
parator by reve r se biasing
d
iodes C R444 an d
C
R 448
an d also by reverse
b iasing Q414 and C R 418 t h roug h R 417 .
Wh en S440 is set to
L
I N E,
U
421 Β an d
C
R 418 are
b
iased
off t h roug h R 438 an d R 417 respectively, w h ile C R444 is
enable d
b y removal of t h e in h ibiti n g voltage from R446 .
Similarly, wit h 5440 set to ΕΧΤ , th e external trigger signal
is selecte d
b
y
b
iasing off C R444 an d U 421 B
t
h rough
resistors R444 a nd R 438 respectively an d
b
y ena b ling Q414
t h roug h
t h e removal of t h e in h i biting voltage from R417 .
E
xternal Trigger Am p lifier
T h e
E
xternal Trigger Am p lifier provides α means of
triggering t h e instr u ment from an externally s u pp lie d signal
t h at can be a pp lie d to th e ΕΧΤ I NPU T connecto r . Input
cou p ling to t h e Amplifier is selectable by th e t h ree-p osition
ΕΧΤ CO UPL I N G switc h , 5401 . In t h e AC positio n , t h e
do component of t h e exter n al trigger sig n al is bloc k ed by
coupling t h e signal th ro u g h C402 . In t h e DC position, all
components of t h e signal are cou p le d d irectly to t h e gate
of Q41 1A
t h rou g h an i n put divider compose d of R404 and
R408 . Resistors R402 an d R403 fo r m α voltage-divi d er
n
etwor k
t
h at atten u ates t h e signal
b y α factor of 10 w h en-
ever 5401 is set to DC +10 .
T heo ry of O peration -2213 Service
F
ield-effect tra nsistors Q411 Α an d Q411 Β are α match e d
p air . Source-follower Q411A pr ovi d es α
h
igh input
impeda n ce for t h e external trigger signal . Cu rre nt-source
transisto r Q411 Β causes Q411 Α to operate at zero gate-to-
sou r ce
b ias, so th e
d
evice f u nctio n s
w
it h no
d
o offset
b etween t h e in p ut an d output signals . T h e out put signal
from t h e source of Q411A drives t h e
b
ase of emitter
follower
Q414 . T h e emitter-follower stage lowers t h e
output im p e d a n ce of t h e Trigger Am p lifie r an d functions
as
p art of th e Trigger Source Switc h ing circuitry .
Auto Trigger Circuit
Wh en eit h er AU TO or TV F I EL D triggering is selecte d,
t h e Auto Trigger circuit detects
p ositive a n d negative
p
eak s
of th e in p ut trigger signal a n d prod u ces output voltages
t h at set th e T R IGG ER LEVEL control range to wit h in
t h e
p ea k -to-pea k am p litu d e of th e 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 app lie d to both en d s of T R IGG ER
LEVEL p ote n tiometer R455 .
In eit h er A U TO or T V F I EL D, t h e T R IGG ER
M
OD E
switc h (S611) opens t h e Auto Disable signal line to allow
C R 503 and C R 504 to become reverse
b
iased . T h is actio n
isolates th e voltage
d
ivi d er
n
etwork (compose d of R525,
R527, R528, R526, an d Q519) from th e + in puts of
U
507A and
U
507 B . Th e
p
ea k detecto rs (com posed of
Q503 for th e
p
ositive peak a n d Q504 for t h e negative
p
ea k )
become ena b led w h en t h e A uto Disable signal is remove d .
T h e trigger signal is applie d to t h e bases of Q503 an d
Q504 via R444 .
P
ositive trigger signal
p
ea k s
b
ias Q503 into
co nduction, ca u sing its emitter voltage level to rise to t h e
p ea k level of th e trigge r amplitu d e min u s th e
b
ase-to-
emitter voltage dro p .
Ca pacitor C503 c h arges up to t h e positive emitter
voltage level . T h e c h arge is retained between trigger
p
u lses
due to th e long RC time constant of R505 and C503. T h e
com parator voltage is a pp lied 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 h e voltage at one
end of t h e T R IGG ER LEVEL
p otentiometer (13455) .
Transistor Q507
p rovides t h e feedbac k pat h for
U
507A
an d
t h ermally compensates fo r Q503 . T h e
b
ase-to-emitte r
d ro p of Q507 corrects for t h e
d o offset introduced by
Q503, an d potentiometer R511 is adj usted to balance out
d
o offsets introduce d from t h e trigger circuitry .
T h e negative
p ea k d etector operates in t h e same manner
as t h e
p
ositive
p
ea k d etector, with
correspon d i n g
com p o n ents pe rforming t h e i d entical circuit functio n on
th e trigger-signal negative
p
ea k s .