A Trigger Generator - Tektronix 2235A Instruction Manual

A Trigger Bandwidth
With the A COUPL switch in NORM position, the base of
Q409 is pulled to ground by Q374. LF-REJ from U555 is
HI, turning off Q400 (Diagram 5). Output of Q400 is then
low, turning on Q402, which supplies emitter current to
Q405, turning it on. With Q402 turned on, Q405 acts as
an emitter follower which drives the trigger input of U460
and the peak detectors. The voltage at the collector of
Q400 is low enough to keep Q406 and CR406 turned off.
Therefore, the only signal path is through Q405 to the
peak detectors, which passes all frequency com-
ponents of the trigger signal.
In the HF REJ mode of trigger coupling, Q374 is turned
off by S392, turning on Q409 which bypasses high-
frequency components of the trigger signal to ground,
resulting in the attenuation of frequencies above
approximately 40 kHz. Bias conditions on (2402, (2405,
and Q406 (Diagram 5) remain the same as in the NORM
setting.
In the LF REJ mode of trigger coupling, Q409 is turned
off, disconnecting C409 from ground. The LF-REJ line is
now LO, turning on Q400. This turns off Q402, which in
turn, turns off Q405 and CR405. The only signal path is
through Q406 to the trigger peak detectors. Transistor
Q406 and CR406 are turned on by current supplied
through R406. Capacitor C410 and R410 form a high-
pass filter, resulting in the attenuation of frequencies
below approximately 40 kHz.
A 'TRIGGER GEN ERATOR
The A Trigger Generator, shown on Diagram 5, supplies
trigger signals to the A Sweep Generator. Included in the
A Trigger Generator circuit are the P-P Auto Trigger, A
Trigger Level comparator and Schmitt Trigger, 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 func-
tion of the A TRIGGER push button selection and trigger
signals selected by the A & B SOURCE switch.
In the P-P Auto and
TV Field modes, Q413 is off and
CR414 and CR415 are reverse biased. Trigger signals
selected by the A & B SOURCE switch are applied to
peak detectors consisting of (2420-Q422 and
Q421 4 4 2 3 . 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 (2429 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
of U426A will be the positive peak voltage of the input
trigger signal and the output of U426B will be the nega-
tive peak voltage. Potentiometers R434 and R435 adjust
for dc offsets 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 and Q413 is turned on,
which forward biases CR415. Input transistors (2420 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 inde-
pendent of trigger-signal amplitude.
A Trigger Level Comparator and Schmitt
Trigger
Integrated circuit U460 contains the Trigger level
comparator and Schmitt Trigger circuitry. The output volt-
age of the A Trigger amplifier is applied to U460, pin 4.
The other input to the comparator is the wiper voltage on
the A TRIGGER LEVEL control, applied to pin 2 of U460
through zero-offset buffer Q446 and Q450. The resistor
R452 and the voltage at pin 5 of U460 set the emitter cur-
rent for the comparator.
The Trigger Slope is determined by the relative voltages
on U460, pins 7 and 8. If pin 8 is at a higher level than pin
7, the plus output of U460 will change to a HI state when a
positive-going input signal crosses the threshold at pin
2 of U460. With pin 8 more negative than pin 7, the
Schmitt fires on a negative-going input. The voltage at
pin 7 is fixed, while that at pin 8 is selected by the A
TRIGGER SLOPE switch S460 through R459, R461, and
R462.
The sensitivity of the Schmitt Trigger is controlled by the
current at pin 9. The setting of R479 determines the circuit
hysteresis.
The outputs of the Schmitt Trigger are at pins 10 and 12 of
U460. The outputs are at ECL levels and are from emitter
followers internal to U460. Collector voltage to U460 is
supplied through pins 11 and 14. When
7V
Field is not
selected, the TV TRIG-ENABLE line connected to
CR476 and R473
is
LO. Transistors Q473 and Q474 are
biased off which also biases (2487 off. Resistor R476
biases CR467 and CR477 on and the +Out Trigger sig-
nal from pin 10 of U460 passes through the diodes to
U506-6 of the A Sweep Generator.
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