Tektronix 3B5, Instruction Manual

Page: 37 / 187

Circuit Description — Type 3B5
Signal at base of Q74
Condition
CRT display
( + slope)
LEVEL control centered. Sweep
triggered at midpoint of signal.
LEVEL control counterclockwise
from midrange. Sweep trig­
gered on negative portion of
signal.
LEVEL control clockwise from
midrange. Sweep triggered on
positive portion of signal.
Fig. 3-7. Effect of LEVEL control on AC coupled, symmetrical trigger signal at the base of Q74 for small-signal operation (less than +/-5
volts). One kilohertz sine-wave displayed.
Slope Comparator
Q74 and Q84 are connected as a comparator. The com ­
parator is referenced to ground through R94 at the base
of Q84. The emitters of Q74 and Q84 are connected
together through D91-D92 or C92-R92. Signals with ampli ­
tudes higher than about one volt, peak to peak, are DC-
coupled between the emitters of Q74 and Q84 through D91
or D92. Smaller signals are AC-coupled through C92 and
R92. Since there is very little voltage drop in this coupling
between emitters, the voltage at each emitter is essentially
the same. Therefore, the transistor which is forward biased
controls conduction. For example, assume that the trigger
signal applied to the base of Q74 is negative going and it
forward biases Q74. The increased current flow through
R63 and R93 produces a larger voltage drop and the emitters
of both Q74 and Q84 go more negative (less positive). A
more negative voltage at the emitter of Q84 reverse biases
this transistor and the collector voltage goes negative. At
the same time, the collector current of Q74 has increased
and its collector goes more positive. Notice that the signal
current at the collectors of Q74 and Q84 is opposite in
phase. The negative-going signal has been inverted through
Q74 but not through Q84. The sweep can be triggered from
either the negative-going or positive-going slope of the
input signal by producing the trigger pulse from either the
inverted signal at the collector of Q74 or the uninverted
signal at the collector of Q84. This selection is made by
the Slope Multivibrator stage (see Slope Multivibrator dis­
cussion).
The DC level of the trigger signal at the emitter of Q23
can be varied by the LEVEL control, the Auto Trigger P-P
Level stage or external program trigger level. This also
changes the bias level on Q74 and determines the change
in trigger signal necessary to turn Q74 on (or off). For
example, when the LEVEL control (also applies to auto trigger
or external level) is set near midrange, the base of Q74 is
at about zero volts (TRIG LEVEL CENTERING adjustment set
correctly). Therefore, signal changes around the zero-volt
level will turn Q74 on or off (see Fig. 3-7A). If the LEVEL
control is set counterclockwise, a more positive level is
established at the base of Q74 and this shifts the trigger
signal at the base of Q74 to a more positive level (see Fig.
3-7B). Since Q74 still switches
near
zero volts, it now switches
at a more negative point on the trigger signal than when
the LEVEL control was centered. This produces an output
pulse from the Sweep Triggered circuit at an earlier time,
which starts the Sweep Generator sooner. The resultant dis­
play shows more of the positive-going edge of the dis ­
played waveform (positive slope triggering). The effect is
the opposite when the LEVEL control is set clockwise from
midrange (see Fig. 3-7C). The DC level of the waveform
is shifted negative and Q74 switches at a later time. Less
of the positive-going edge is shown on the displayed wave­
form (positive slope triggering). The LEVEL control was used
to change the trigger level for this explanation. However,
the effect will be the same when either the external pro­
gram level or Auto Trigger P-P Level circuit sets the trigger
level.
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