Tektronix 434, Instruction Manual

Page: 47 / 222

Circuit Description—434
Generator c irc u its tim e to in itia te a sweep before the
vertical signal reaches the vertical d e fle c tio n plates o f the
C RT. T his allow s the in stru m e n t to display the leading edge
o f the signal orig in a tin g the trigger pulse w hen using
internal triggering.
In p u t A m p lifie r
Q 5 0 5 a n d Q 5 4 5 are connected as common-base
am plifiers to provide a lo w in p u t impedance to p ro p e rly
te rm in a te the Delay Line. I t also provides isolation between
the Delay Line and the fo llo w in g stages. Q 520 and Q 550
lim it the m axim u m d ynam ic swing o f the a m p lifie r by
tu rn in g on and co n ducting when excessive am ounts o f
signal are present at the a m p lifie r in p u t. When closed, the
B EA M F IN D E R sw itch S50 fo rw a rd biases CR521 and
C R551, thereby setting a m ore positive level at the bases o f
Q 520 and Q 550. This more positive level makes Q 520 and
Q 550 c o n d u c t on sm aller-than-norm al overdrive signals,
causing the overall d yn a m ic swing o f the V e rtica l O u tp u t
A m p lifie r to be lim ite d w ith in the CRT graticule area.
Q 515 and Q 555 are e m itte r fo llo w e rs to provide low o u t­
p u t impedance.
O u tp u t A m p lifie r
Q 525 and Q 565 com prise an e m itter-coupled push-pull
a m p lifie r th a t drives the o u tp u t pow er a m plifiers Q 530 and
Q 580. R 568, R 571, R572 and R574 set the gain o f the
stage by c o n tro llin g the signal degeneration between the
em itters o f Q 525 and Q 565. R572 is a th e rm is to r w hich
reduces in resistance w ith increases in am bient tem perature.
This increases the gain o f the stage, to c o u n te ra ct the reduc­
tio n in overall a m p lifie r gain th a t occurs when the am b ie n t
tem perature increases. Variable capacitor C572 and the
series RC n e tw o rk C570 and R 570 provide high-frequency
com pensation to o p tim iz e a m p lifie r frequency response.
L533 and L584 are high-frequency peaking adjustm ents to
provide a d d ition a l a m p lifie r com pensation.
T R IG G E R P IC KO FF A N D G E N E R A TO R
General
The Trigger P ic k o ff c irc u it selects and am plifies the
internal trigger signal to the level necessary to drive the
Trigger Generator c irc u it. In p u t signal fo r the Trigger P ick­
o ff c irc u it is e ith e r a sample o f the signal applied to
Channel 1, o r a sample o f the com posite vertical signal fro m
the Channel S w itch c irc u it.
The Trigger Generator c irc u it produces trigger pulses to
start the Sweep Generator c irc u it. These trigger pulses are
derived e ith e r fro m the internal trigger signal fro m the
vertical d e fle c tio n system, an external signal connected to
the E X T T R IG in p u t con n e cto r, or a sample o f the line
voltage applied to the in stru m e n t. C o n tro ls are provided in
th is c irc u it to select trigger level, slope, coupling, and
source. Fig. 3-8 shows a detailed b lo ck diagram o f the
Trigger P ic k o ff & G enerator c irc u it. A schematic o f this
c irc u it is shown on diagram 5 at the back o f this manual.
Trigger P ic k o ff
D iode Gate. The Diode Gates, consisting o f tw o diodes
each, can be th o u g h t o f as switches w h ich a llo w either o f
the tw o internal trigger signals to be coupled to the Trigger
P ic k o ff A m p lifie r. CR280 and CR285 c o n tro l the Channel
1 signal, and C R 270 and C R 275 c o n tro l the com posite
s ig n a l. These diodes are in tu rn co n tro lle d by the
T R IG G E R SOURCE switches.
F or CH 1 trigger o peration, +15 vo lts is applied to the
anode o f C R270 th ro u g h COMP Source sw itch S72B. R270
is fo rw a rd biased and pulls the c o lle cto r c irc u it o f Q280
positive, w h ich back biases and turns o ff C R275. C R285
remains fo rw a rd biased, and couples the CH 1 trigger signal
to the base o f Q 275. In the COMP mode o f trigger opera­
tio n , + 15 volts is applied to the anode o f C R 280 through
COMP Source sw itch S72B and R282. C R 280 is now
fo rw a rd biased, p u llin g the cathode o f C R280 positive,
w h ich back biases and tu rn s o ff C R 285. C R 275 remains
fo rw a rd biased and couples the a m p lifie d COMP trigger
signal present in the co lle c to r c irc u it o f Q 280 to the base o f
Q 275. R274 and R284 set the DC level at the c o lle c to r of
Q 275 to zero volts fo r no-signal, ze ro -vo lt in p u t level
co n d itio n s.
Q 275 and Q 278 com prise an e m itter-coupled com para­
to r a m p lifie r. Q 275 by itself is a feedback a m p lifie r, w ith
R 285 pro vid in g feedback fro m the c o lle c to r to the base of
Q 275. The DC level at the base o f Q 278 determ ines the DC
level at the base o f Q 275.
Trigger Source
The T R IG G E R SOURCE switches S 72A and S72B select
the source o f the trigger signal. F our trigger sources are
available: CH 1, COMP, external, and line. E XT A T T E N
sw itch S70 provides 13 tim es atte n u a tio n fo r the external
trigger signal. The internal trigger signals (CH 1 and COMP)
are obtained fro m the Trigger P ic k o ff c irc u it discussed
above.
The line trigger signal is obtained fro m the secondary of
tra n sfo rm e r T i l 15 in the Power Supply P rim ary c irc u it.
This sample o f the line frequency, about .7 v o lt RMS, is
coupled to the Trigger Generator c irc u it in the L IN E mode
o f ope ra tio n , The T R IG G E R C O U P LIN G switches should
n o t be in the LF REJ mode o f operation when using th is
trigger source, as the signal w ill be blocked by the LF reject
c irc u it.
3-10