Tektronix T921 User Manual

TV TRIGGER CIRCUIT
The TV trigger circuit comprises U2126E, 02174,
02176,
U2156B, C, D, and associated circuitry . When the MODE switch is
in
TV,
R2161 is disconnectedfrom +8 volts which allows trigger
signals from the collectors
of
02134
and 02136 to pass through
the TV trigger circuit . (In AUTO and NORM, the +8 volts applied
to R2161 biases U2126E to saturation .)
Transistor U2126E is a high gain feedback amplifier . To
achieve stable triggering on TV signals, the LEVEL control must
be set at a point that will allow the sync pulsesto appear within the
dynamic range of the amplifier .
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The sync separator circuit consists of 02174 and associated
circuitry . It processes sync-positive pulses when the SLOPE
switch is
in the +OUT position and sync-negative pulses in the
-IN position . Transistor 02174 produces large positive-going
pulses from negative-going sync signals at the collector of
U2126E .
In the TV field
mode (SEC/DIV switch set for .1 ms or slower),
02176 is saturated (since base is grounded), and the integrator
(composed of C2174, C2176, R2174, and R2176) is switched into
the circuit (effectively connected to +8 volts through saturated
02176) . The integrator filters
out the horizontal sync pulses,
leaving only the integrated vertical sync pulses,
which triggerthe
TV Schmitt trigger U2156B and U2156C .
VIII .
2
In the TV line mode (SEC/DIV switch set for 50 Ns or faster),
02176 is turned off
(base open),
disconnecting C2174 and C2176
from +8 volts
.
Capacitors C2174 and C2176
no
longer integrate
the pulses, thus allowing both the horizontal and vertical sync
pulses to pass through to the TV Schmitt trigger .
SCHMITT TRIGGERS
The
Schmitttrigger forthe
NORM
and AUTO
triggering
modes
consists of U2156A,U2156B, and associated circuitry . Hysteresis
of this
trigger
circuit is determined by R2152, R2153, and
R2147 .
The Schmitt trigger for the TV triggering mode is U2156B,
U2156C, U2156D, and associated circuitry . Resistors R2154 and
R2178 determine the sensitivity .
When the MODE switch S2150 is in AUTO or NORM, +8 volts is
applied to R2156which causes pin 1 of U2156A to go
HI,
enabling
Schmitt trigger U2156A-U2156B . At the same time, pins 12 and 13
of U2156D are also HI disabling Schmitttrigger U2156C-U2156B .
A trigger signal from U2126D triggers Schmitt trigger U2156A-
U2156B to produce a
logic
trigger signal at pin 6 of U2156B .
When the MODE switch S2150 is in TV, +8 volts is removed
from R2156 and pin 1 of U2156A is LO, disabling Schmitt trigger
U2156A-U2156B . Pins 12 and 13 of U2156D are LO, enabling
Schmitt trigger U2156C and U2156B . The trigger signal from the
TV sync separator triggers Schmitt trigger U2156C-U2156B to
produce a
logic trigger signal at pin 6 of U2156B
.

Tektronix T921 Specifications

General

Brand	Tektronix
Model	T921
Category	Test Equipment
Language	English