Tektronix T921 User Manual

Circuit Description-T921/T922/T922R
TRIGGER CIRCUIT
DESCRIPTION
(For T921 Below SN
B010130 and T922 Below SN
B010170)
Digital logic devices are used to perform some of the
functions in this instrument . LO and HI designations
are used in this
circuit
desciption to indicate the state of the digital circuit .
HI indicates the more positive of the two levers . The
specific voltages
which constitute a LO and HI logic state may vary
between individual devices .
SOURCE Switch
The SOURCE switch, S2100, selects trigger
signals from INT,
LINE, X-Y, EXT, and_ EXT
.
10
INT . Signal from the trigger pickoff circuit in
the vertical
amplifier is connected to the trigger
input amplifier U2126B .
LINE . A sample of
the line voltage, obtained from the power
transformer, is connected to
the trigger input amplifier U2126B .
EXT . External
trigger signals applied to J2110 external trigger
(X) input are connected to the trigger input amplifier,
U2126B, via
input buffer amplifier 02114-02116 and coupling
capacitor
C2123 . Diode CR2116 protects
02114 from large negative-going
trigger signals . (The
02114 gate-drain junction provides protec-
tion from
large positive-going signals by limiting the 02114 gate
voltage to
about +8 volts .) Transistor 02116 provides a relatively
constant current source for 02114 . Resistor
R2111 sets the
external trigger sensitivity and converts the voltage
signal to a
current signal .
EXT
MODE Switch
REV . BJUL
1977
INPUT AND
SWITCHING

NORM . Connects +8
volts to R2223 in the sweep generator
circuit .
Allows the sweep to run only when a suitable
triggering
signal
is present .
10 .
Resistors R2119 and 82120, with R2111, form a current
divider to attenuate
the external trigger signal by a factor of 10 .
Diodes CR2115 and CR2186 form a current switch that
prevents
external trigger signals from coupling into the
horizontal
amplifier in EXT and _EXT .
10
X-Y . Signals from the X (EXT) input are routed to
the horizontal
amplifier via 02114, S2110, and CR2186 . The SOURCE
switch
connects +8 volts to 82184, forward
biasing CR2186 and
providing current to the horizontal
amplifier
to
center the crt
display . Also, +8 volts is applied to CR2182 and R2134 to
switch
the sweep into the NORM mode and to disable
the trigger circuit .
The MODE switch S2150 selects three triggering modes :
AUTO, NORM, and TV .
AUTO . Allows the sweep to
free run in the absence of
triggering signal . See
sweep circuit description for details .
TV . The trigger signal is applied to the
TV sync separator
circuit (U2126E, 02174, 02176)
and the normal trigger generator
circuit is disabled .
In this mode, the SLOPE switch is used to
supply
the sync separator with a signal of the proper polarity
. For
SEC/DIVsettings of .1 ms/divor slower, the sweep
is triggered by
vertical sync pulses which occur at
a field rate . For SEC/DIV
settings of 50 Ns and faster, the
sweep is triggered by the TV
horizontal line signals .
TRIGGER INPUT AMPLIFIER
The trigger input, amplifier consists
of U2126A, U2126B,
02124, and associated
circuitry . Resistors R2127 and 82128 set
the amplifier input at -4
volts
.
The inverting configuration and
feedback from
the emitter of 02124 forman inverting operational
amplifier with a null point at the base of U2126B . Any current
into
the null
point produces a voltage at the output proportional
to the
feedback resistor 82123 . Diode CR2124
prevents the emitter of
02124 from going below ground and
reversing the voltage across
C2132 .
TRIGGER LEVEL COMPARATOR
Differential
amplifier 02134-02136 functions as a comparator
.
TheLEVEL control 82138 selects the point on
the waveform that
starts
a sweep . Capacitor C2132 couples the trigger
signal to the
comparator . As the trigger signal at
the base of 02134 passes
through the same voltage level
as the base of 02136 (set by
LEVEL control), the signal at the
collector of U2126D passes
through the threshold (about
1 volt) of Schmitt trigger U2156A-
U2156B producing a logic
trigger signal . The trigger signals at the
collectors of 02134 and
02136 are of opposite polarity . This
allows the SLOPE
switch S2140 to invert the signals applied to
the
TV trigger input
amplifier U2126E and the normal trigger input
amplifier
U2126D, C . When the SLOPE switch is
in the +OUT
position, the output at the collector of
U2126D is in phase with the
trigger source signal . Transistors U2126D
and U2126C convert
the current signal from the
collector of 02134 or 02136 to a
voltage signal
for triggering the Schmitt trigger .

Tektronix T921 Specifications

General

Brand	Tektronix
Model	T921
Category	Test Equipment
Language	English