A Trigger Level Comparator; A Schmitt Trigger And Tv Trigger Circuit; A Sweep Generator And Logic - Tektronix 2236 Instruction Manual

Theory of Operation— 2236 Service
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
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
by the A TRIGGER LEVEL control. Positive or negative
slope triggering is selected by the A TRIGGER SLOPE
switch.
Transistors U460B and U460E compare the wiper volt­
age on the A TRIGGER LEVEL control to the input trigger
signal, and the transistor with the higher base voltage will
conduct more of the available emitter current. The output
collector currents supply emitter current to two transistor
pairs which serve as cross-wired switches that are biased
on or off by the A TRIGGER SLOPE switch. When S464 is
set to the positive slope position, U460C and U460F are
biased on and U460A and U460D are biased off. For the
negative slope position, the transistors reverse states to in­
vert the comparator output polarity.

A Schmitt Trigger and TV Trigger Circuit

This circuitry generates a signal that drives the A Trigger
Logic as a function of the Trigger Level Comparator output
signal and the A TRIGGER Mode switches.
The output signals from the A Trigger Level Comparator
drive Q460 and Q463. These transistors are configured as a
3-10
current mirror that converts the differential output to a sin­
gle-ended current to drive amplifier U480C. Slope Balance
potentiometer R471 corrects for dc offsets between positive
and negative slope. Shunt-feedback amplifier U480C con­
verts a current input to a voltage output to drive the input of
the Schmitt Trigger, U480D, through R469. Positive feed­
back for the Schmitt Trigger is provided by potentiometer
R479, and C479 reduces trigger jitter by increasing positive
feedback at higher frequencies. The setting of R479 deter­
mines the circuit hysteresis.
When TV Field is not selected, the TV Trig Enable line
connected to R402 and R473 is LO. Transistors Q402 and
Q403 are biased off and a LO is placed on one input of
U480A by R474. This LO input will cause U480A to invert
the output from U480D. With Q403 off, a LO will be placed
on one input of U480B by R405 and U480B will also act as
an inverter. The A Trigger signal at the output of U480B is
therefore the same as the input signal to U480A.
When TV Field is selected, the TV Trig Enable line is HI.
The outputs of U480D will determine the conduction states
of Q402 and Q403, and the input of U480A connected to
R473 will be HI. The output of U480A will be LO and U480B
will invert the signal at its other input. Signals at the collec­
tor of Q403 are filtered by C408, R405, and C405 to reject
TV video information and average the TV horizontal-sync
pulses. Setting the trigger-level threshold near the center of
the horizontal-sync-pulse swing establishes the untriggered
level. When the TV vertical-sync block occurs, the output of
the filter applied to U480B pin 7 rises to a level that will
cause the Schmitt Trigger circuit to switch. Precise TV field
synchronization is obtained as a result of this filtering action.
The A Trigger signal output will be the inverse of the filtered
signal appearing at U480B pin 7.

A SWEEP GENERATOR AND LOGIC

The A Sweep Generator and Logic circuitry, shown on
Diagram 4, produces a linear voltage ramp that is amplified
by the Horizontal Amplifier to provide horizontal deflection
of the crt beam. The Sweep Generator circuits also produce
signals that are used to generate correct timing of the crt
unblanking and intensity levels used for viewing the display.
See Figure 3-3 for the block diagram of the A Sweep Gen­
erator and Logic circuitry.
The Sweep Logic circuitry controls the holdoff time,
starts the sweep upon reception of a trigger signal, and ter­
minates the sweep at the proper sweep level. When using
P-P Auto or TV Field triggering, the Sweep Logic circuitry
will cause the Sweep Generator to free run, producing a
baseline trace if a trigger signal is not received within a
predetermined time period.