Trigger Generator - Tektronix 2205 Service Manual

Theory of Operation—2205 Service
When the SOURCE switch is set to EXT=Z
position, the external signal is buffered by Q370 and
applied to the Z-Axis
modulation.
The line trigger from the power supply is applied
to Q370 when the SOURCE switches are set to EXT
and LINE positions.

TRIGGER GENERATOR

The Trigger Generator, shown on Diagram 3,
supplies trigger signals to the Sweep Generator.
Included in the Trigger Generator circuit are the P-P
Auto Trigger, Norm Trigger, and TV Triggering
circuitry.
Trigger Level Circuit
The Trigger Level Circuit establishes voltages at
the ends of the TRIGGER LEVEL potentiometer as a
function of the TRIGGER Mode switch selection and
trigger signals selected be the SOURCE switch. In
the P-P Auto and TV Field mode, U415E is off and
CR341 is reverse biased. Diode CR417 is forward
biased turning U415A off. Trigger signals selected by
the SOURCE switch are applied to peak detectors
consisting of U415B-Q415 and U380E-Q465. These
peak detectors track dc levels and have a high
voltage transfer efficiency, The positive-peak and
negative-peak signal levels stored by C418 and
C431 are near the peak levels of the trigger signal.
Amplifiers U425A and U425B are configured as
voltage followers with transistors U415D and Q490 in
the feedback loops. These transistors thermally
compensate for U415B and Q465 and level shift the
amplifier outputs back to the original dc levels of the
input trigger signals. The output of U425B will be the
positive peak voltage of the input trigger signal and
the output of U425A will be the negative peak
voltage. Potentiometers R445 and R446 adjust for dc
offsets in the trigger circuitry.
In the Norm mode, +8.6 V is applied to the Junc­
tion of R417 and R432, turning on U415E and forward
biasing CR431. Diode CR417 is reverse biased
turning U415A on. input transistors U415B and Q465
are then biased off and no trigger signals will reach
the Trigger Level circuit. The inputs and outputs of
U42SA and U425B will then be fixed voltages and
independent of trigger-signal amplitude.
3 -8
amplifier for intensity
Trigger Level Comparator
The Trigger Level Comparator compares signals
selected by the TRIGGER SOURCE switch to a
voltage set by the TRIGGER LEVEL control. Positive
or negative slope triggering is selected by the
TRIGGER SLOPE switch.
Transistors U460B and U460E compare the wiper
voltage on the TRIGGER LEVEL control to the input
1 rigger 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 (U460A-U460F and
U460C-4600D) which serve as cross-wired switches
that are biased on or off by the TRIGGER SLOPE
switch. When S460 is set to the positive slope
position, U460A and U460D are biased on and
U460C and U460F are biased off. For the negative
slope position, the transistors reverse states to
invert the comparator output polarity.
Schmitt Trigger and TV Trigger Circuit
This circuitry generates a signal that drives the
Trigger Logic as a function of the Trigger Level
Comparator output signal and the TRIGGER MODE
switches.
The output signals from the Trigger Level
Comparator drive Q400 and Q401. These transistors
are configured as a current mirror that converts the
differential output to a single-ended current to drive
amplifier U480C. Potentiometer R481 corrects for dc
offsets between positive and negative slope. Shunt-
feedback amplifier U480C converts a current input to
a voltage output to drive the input of the Schmitt
Trigger, U480D, through R485- Positive feedback for
the Schmitt Trigger is provided by potentiometer
R489, and C489 reduces trigger Jitter increasing
positive feedback at higher frequencies. The setting
of R489 determines the circuit hysteresis.
When TV FIELD is not selected, the TVF tine is
HI, and is buffered by Q487. Transistors Q488 and
Q489 are biased off and a LO is placed on one input
of U480A by R492-R493. This LO input will cause
U480A to invert the output from U4BOO. With Q489
off. a LO will be placed on one input of U480B by
R495 and U480B will also act as an inverter. The
Trigger signal at the output of U480B is therefore the
same as the input signal to U480A.
When TV FIELD is selected, the TVF line is LO,
and is buffered by Q847. The outputs of U480D will
determine the conduction states of Q488 and Q489,
and the input of U480A connected to R492 wiii be HI.
The output of U480A wifi be LO and U4808 will invert
signal at its other input. Signals at the collector of
Q489 are filtered by C495, R495, and C496 to reject