A Trigger Generator - Tektronix 2215A Instruction Manual

Theory of Operation—2215A Service
The CHOP VERTICAL MODE position grounds the
V Mode T line and places a LO on an input of both U555B
and U555C. The outputs of these two gates will then be LO
and the signal to the Internal Trigger Amplifier will be the
same as for the ADD mode.
Internal Trigger Amplifier
The Internal Trigger Amplifier converts the differential
trigger signals from the Vertical Preamplifiers into a single-
ended signal that drives the X-Axis Amplifier and the A and
B Trigger Generators.
Signal current is applied to the emitters of U350D and
U350E. The collector current of U350D is converted to a
voltage across feedback resistor R357. The opposite-phase
collector current of U350E causes a voltage drop across
R359 which adds to the voltage at the collector of U350C.
This voltage appears at the base of U350A which buffers
and level shifts the signal back to 0 V. The emitter signal of
U350A drives the X-Axis Amplifier, the B Trigger Generator,
and the base of U350B. The emitter signal of U350B in turn
drives the A Trigger Generator whenever CR372 is forward
biased.
A External Trigger Amplifier
The A External Trigger Amplifier buffers signals applied
to the EXT INPUT connector to drive the A Trigger Genera­
tor. Input signal coupling is determined by A EXT COU­
PLING switch S380 which selects AC, DC, or DC -= - 10
coupling.
When S380 is in the AC position, the input signal is ac-
coupled through C376. In the DC position, the input signal is
connected directly to the Amplifier. The DC
attenuates the input signal by a factor of 10 through the
compensated divider composed of R377, R378, C380, and
C381.
The signal is then applied to the gate of Q382A. This
source-follower drives emitter-follower transistor Q384
which lowers the Amplifier output impedance. The two FETs
are a matched pair, and since the gate and source of Q382B
are connected together, Q382B will supply source current
for Q382A such that there will be no voltage drop across the
gate-source junction of Q382A. Protection-diode CR381
clamps the signal at the gate of Q382A to about - 9 V. The
Amplifier output will drive the A Trigger Generator through
forward-biased CR393 whenever the A SOURCE switch is
set to EXT. When the A SOURCE switch is not set to EXT,
the base-emitter junction of Q384 will be reverse biased and
the Amplifier will be disabled.
3-8
Line Trigger Amplifier
The Line Trigger Amplifier supplies a line-frequency trig­
ger signal to the A Trigger Generator when the A SOURCE
switch is in the LINE position.
Transformer T390 in the Power Supply provides a line-
frequency signal through R397 to Q397. Diode CR399 is
forward biased when S392 is in the LINE position, and the
emitter signals of Q397 will drive the A Trigger Generator.

A TRIGGER GENERATOR

The A Trigger Generator, shown on Diagram 3, supplies
trigger signals to the A Sweep Generator. Included in the
A Trigger Generator circuit are the P-P Auto Trigger, Auto
Baseline, and TV Triggering circuitry.
A Trigger Level Circuit
The A Trigger Level Circuit establishes voltages at the
ends of the A TRIGGER LEVEL potentiometer as a function
of the A TRIGGER push button selection and trigger signals
selected by the A SOURCE switch.
In the P-P Auto and TV Field modes, Q413 is off and
CR414 and CR415 are reverse biased. Trigger signals se­
lected by the A SOURCE switch are applied to peak detec­
tors consisting of Q420-Q422 and Q421-Q423. These peak
detectors track dc levels and have a high voltage transfer
efficiency. The positive- and negative-peak signal levels
stored by C414 and C415 are near the peak levels of the
trigger signal. Amplifiers U426A and U426B are configured
as voltage followers with transistors Q428 and Q429 in the
feedback loops. These transistors thermally compensate for
Q420 and Q421 and level shift the amplifier outputs back to
the original dc levels of the input trigger signals. The output
10 position
of U426A will be the positive peak voltage of the input trig­
ger signal and the output of U426B will be the negative peak
voltage. Potentiometers R434 and R435 adjust for dc off­
sets in the trigger circuitry.
In the Norm mode, +8.6 V is applied to the junction of
R411 and R414. Diode CR414 is forward biased, turning on
Q413 which forward biases CR415. Input transistors Q420
and Q421 are then biased off and no trigger signals will
reach the A Trigger Level circuit. The inputs and outputs of
U426A and U426B will then be fixed voltages and indepen­
dent of trigger-signal amplitude.
A Trigger Level Comparator
The A Trigger Level Comparator compares signals se­
lected by the A TRIGGER SOURCE switch to a voltage set