Tektronix 434, Инструкция по эксплуатации

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C irc u it D e scrip tio n —434
adjustm ent R 629. The Trigger Level Centering adjustm ent
sets the level at the base o f U 650E so the display is
c o rre c tly triggered when the L E V E L c o n tro l is centered.
The L E V E L c o n tro l varies the base level o f U 650E to select
the p o in t on the trigger signal where triggering occurs.
Diodes C R 615 and CR616 prevent overdrive occurring in
the Slope C om parator. Q 618 is a re la tive ly constant cu rre n t
source fo r U 650D and U650E.
The slope o f the in p u t signal w h ich triggers the Sweep
Generator is determ ined by the SLOPE sw itch S60. When
the SLOPE sw itch is set to +, +5 volts is connected to the
base o f Q 635 through R635 and to the anode o f C R634.
See Fig. 3-9. T his fo rw a rd biases C R 634, w h ich tu rn s on
and pulls the cathode o f CR634 positive also. A t the same
tim e the base-em itter ju n c tio n o f Q 635 is zero biased,
w h ich turns o ff Q 635. W ith Q 635 turned o ff, its c o lle c to r
fa lls to a level o f about +2 volts set by the voltage d ivid e r
R 637-R 638. This back biases C R 635, and the trigger signal
in the c o lle c to r c irc u it o f U 650D is allowed to pass to the
Trigger T D C R 644. The trigger signal in the c o lle c to r c irc u it
o f U650E is conducted by CR634 and blocked fro m
reaching the Trigger T D b y back-biased diode C R 632. The
trigger signal in the c o lle c to r c irc u it o f U650D is a m p lifie d
and inverted w ith respect to the in p u t trigger signal. Since
the o u tp u t pulse fro m the Trigger Generator is derived fro m
the negative-going p o rtio n o f th e signal applied to the
Trigger T D , the sweep is triggered on the positive-going
slope o f the in p u t trigger signal.
When the SLOPE sw itch is set to —, co n d itio n s are
reversed. The base o f Q 635 is returned to ground through
R634 and R 635. See Fig. 3-10. This back biases C R 634 and
fo rw a rd biases the base-em itter ju n c tio n o f Q 635 su f­
fic ie n tly to drive Q 635 in to saturation. The c o lle c to r o f
Q 635 rises to w ith in a b o u t 0.2 v o lt o f the e m itte r, w hich
fo rw a rd biases C R 635. N ow C R 630 is back-biased and
C R632 is fo rw a rd biased, a llo w in g the trigger signal present
in the c o lle c to r c irc u it o f U650E to be applied to the
Trigger T D . The trigger signal in the c o lle c to r c irc u it of
U650E is in phase w ith respect to the in p u t trigger signal,
so the sweep is triggered on the negative-going slope o f the
in p u t trigger signal.
Trigger TD
The Trigger TD stage shapes the o u tp u t o f the Slope
C om parator to provide a trigger pulse w ith a fast leading
edge. T u n n e l diode CR644 is quiescently biased so i t is on
and in its low-voltage stage. The c u rre n t fro m one o f the
transistors in the Slope C om parator stage is diverted
through the Trigger T D stage. As the c u rre n t increases due
to a change in the trigger signal, tunnel diode CR644
switches to its high-voltage state. The tunnel diode remains
3-12
Fig. 3-9. T rig g e r signal p a th f o r p o s itiv e -s lo p e trig g e rin g (s im p lifie d T rig g e r G e n e ra to r d ia g ra m ).