Tektronix 3B5, Instruction Manual

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Circuit Description — Type 3B5
Fig. 3-8. Simplified diagram of Trigger Latch Circuit.
the Trigger Latch Circuit is on, the front-panel trigger controls
determine the circuit operation. However, if the unit is not
correctly triggered, the output level of the Bright Baseline
stage is positive enough to forward bias D299. With D299
forward biased, the base of Q54 rises positive and the Trig ­
ger Latch Circuit is shut off. This disconnects the front­
panel trigger controls and the Trigger Mode Logic stage
switches to auto trigger operation. The operation described
here allows the unit to switch to the auto trigger mode,
if not correctly triggered, to present a stable display when ­
ever possible in the Seek Mode of operation.
The following logic sequence takes place each time a
seek command is received:
TABLE 3-1
Trigger Logic Sequence
(Seek Mode)
Visual
Indication
1. Seek mono pulse (produced when
seek command is received) momen­
tarily turns on Trigger Latch Circuit.
SEEK button lights.
II. Trigger Latch Circuit checks for cor­
rect triggering as set by front-panel
controls.
A. Triggered
1. Trigger Latch Circuit remains
on and unit operates as set by
the front-panel controls.
Stable CRT display
presented.
2. If the unit is no longer cor­
rectly triggered after correct
triggering has been estab ­
lished as set by the front panel
controls (e.g., level of trigger
signal changes or external
trigger signal is interrupted)
the unit reverts to auto trig­
gering (see next step).
B. Not triggered
Trigger Latch Circuit shuts off and
Trigger Mode Logic stage
switches to auto triggering.
Auto Trigger P-P Level stage
checks for internal trigger signal.
1. Triggered. If auto trigger
level can be obtained, sweep
rate seeking is performed. Unit
remains in auto trigger mode
until next seek command is
received.
2. Still not triggered. If auto
trigger level cannot be ob ­
tained, Bright Baseline stage
free runs sweep (typically 2 s/
DIV sweep rate).
NOT TRIG'D read­
out and Auto Trig ­
ger light come on.
Auto trigger light
remains on. NOT
TRIG ’D readout
turns off. Stable
CRT display pre ­
sented.
NOT TRIG ’ D read­
out and Auto Trig­
ger light remain
on. Free running
sweep presented.
III. This logic sequence is repeated when
the next seek command is received.
3-12